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Affect of Eye Pupil on Binocular Aperture

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#1 EdZ

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Posted 11 August 2004 - 02:45 PM

Affect of Eye Pupil on Binocular Aperture

This information has been discussed before, elsewhere in other forums and also here in the CN binocular forum. However, the question keeps coming up and it is certainly worthwhile to document all of this in one complete discussion. The question answered here is this: What happens when your eye pupil is smaller than the binocular exit pupil? The implications are not at all intuitive, and certainly without explanation may not be clearly understood. There has been much prior debate over this issue, in which I have participated. Finally research, as always, provides some answers. Hopefully, this discussion will answer this question for you.

An important aspect to consider when choosing any binocular is the maximum dilation of your own eye pupils. Your own eyes may have a significant impact on the effective performance of a binocular. Depending on your desired use, failing to take maximum dilated eye pupil into consideration before you make your purchase may lead to an inappropriate choice. Exit pupil larger than eye pupil results in not realizing the maximum potential of the binocular.

There are several simple methods to determine the size of you eye pupils. I had my optometrist measure mine. You can get a close approximation in a dark room by looking into a mirror and using a low level light and an easy to read scale or have someone do it for you.

The average adult has eye pupils that can dilate to about 5mm, although this figure can vary widely from one individual to another. Maximum eye pupil dilation usually diminishes with age. It is usually stated that youth have the capability to reach a maximum dilation of about 7mm. The need to measure your own eye pupil dilation will soon become apparent. Personally, at the time of this writing I am 51 years old and my pupils reach a maximum dark-adapted dilation of 6.5mm. I know another avid binocular astronomer at the age of 49 has a maximum dilation of only 4mm.

In general most binoculars have large exit pupils. Exit pupil, the beam of light that exits the eyepiece, is determined by dividing the diameter of the objective lens (the entrance pupil) by magnification. For example, a 10x50 binocular has a 50/10 = 5mm exit pupil and a 7x50 binocular has a 50/7 = 7.14mm exit pupil.

Generally, it is stated that a larger exit pupil provides a brighter image to the eye. Those who desire the brightest low light image will opt to purchase a binocular with the largest possible exit pupil. However, if the exit pupil of the binocular is larger then the maximum dilated pupil of your eye, all the light in the exit pupil beam will not enter your eye and the effective performance of the binocular will be reduced. To avoid this you simply need to more carefully match the binocular exit pupil to your eye pupil.

NOTE: Brightness as discussed here is that which is provided to the eye by the exit pupil. Leave no doubt in your mind that a higher quality optic of the same size (and sometimes of even a smaller size) may appear brighter than a lesser quality optic . Given all other measurements being equal, optical glass, coatings and baffels within a higher quality instrument will provide a brighter image to the eye. Now back to exit pupil.

What are the affects on image brightness and resolution when eye pupil is smaller than exit pupil?

At this point, I will tell you I have researched about two dozen various textbooks for a discussion of this affect. There is a general lack of discussion in the texts I have referred too as relates to the application of optical theory to the question raised above. The usual text discussion states objective diameter is the determinant of maximum resolution and maximum light gathering and that exit pupil is the determinant for brightness of the image delivered to the eye.

Many texts discuss diffraction and resolution. While some leave the reader groping for complete explanation on which to base conclusions, a few others give a clear explanation. When it comes to resolution, they all have one thing in common. They all say resolution is dependent only on aperture. And as relates to brightness, all state that a larger exit pupil delivers a brighter image. Few if any texts discuss the affects on resolution and brightness of image when eye pupil is smaller than exit pupil. It is up to the reader to develop a clear understanding of these concepts and apply the information correctly to arrive at accurate conclusions. What nearly all of these texts fail to mention is this; in order that the full performance of the objective lens be delivered to the eye, the entire exit pupil light beam must pass through the eye pupil. I have provided reference to three excellent texts that provide sufficient information to understand this concept.

Although this discussion has far greater implications for the astronomer choosing a binocular, it has relevance to the telescope user as well. It was shown above that exit pupil varies inversely as magnification varies. Low magnifications provide large exit pupils and higher magnifications result in smaller exit pupils. Telescope users know very well that a brighter image can be achieved by using a lower powered eyepiece. Optimum resolution is achieved at relatively high magnifications where exit pupil is very small, much smaller than dark-adapted eye pupil. At usual telescope magnifications for optimum resolution, there is little need to discuss exit pupil being larger than eye pupil. The situation where telescope users need to be concerned about exit pupil larger than eye pupil is when using very low magnifications for wide-field viewing or maximum low light observing of extended objects.

For example, using a 100mm scope at 20 power would result in a 5mm exit pupil. Using an 8” scope (203mm) at about 50 power would result in a 4mm exit pupil. When I use my 6” (150mm) refractor (focal length 1200mm = f8) with a 40mm eyepiece, I can get 30 power with an exit pupil of 6mm and about a 1.6° field of view. I also own a 6” reflector (focal length 750mm =f5). The lowest powered eyepiece I use with my f5 scope is a 30mm. That gives a magnification of 25x with an exit pupil of 6mm and a 2° field of view.

If I were to put my 40mm eyepiece in my f5 6” (D=150mm, F=750mm) reflector in an attempt to get a brighter image, I would get 18.75x magnification with a 2.6° fov, but exit pupil would be 8mm. Two things would happen. My secondary mirror might show up in the image (although I've never seen it) and (more important to this discussion) my maximum dilated eye pupil of 6.5mm would cut off the outer edges of the exit pupil. In effect, my eye would reduce the light delivered by my telescope to an effective aperture of 6.5 x 18.75 = 122mm.

Certainly this low magnification and oversized exit pupil is acceptable if my goal is to get maximum field of view, but if it is maximum bright image I desire, I should have paid closer attention to my own eye pupils and used an eyepiece that produces a maximum 6.5mm exit pupil. If you employ this tactic to gain field of view with your scope, you must accept that you are losing the use of the full aperture. The net effect is a reduction in the total light gathering. While some observers use this tactic to surround an object with more field, the reduction in effective aperture will have the affect of reducing contrast on low light objects.

If eye pupil remains larger than the exit pupil in use, then the entire exit pupil beam enters the eye and exit pupil controls the amount of light delivered to the eye. The condition will sometimes occur where eye pupil is smaller than exit pupil and it helps to have an understanding of the implications.

What you will find in the references provided is all say eye pupil is an extension of exit pupil. If at any point before the exit pupil reaches the eye a portion of the light beam is clipped, the effect in the exit pupil, or in this case the eye pupil, has a like affect on the entrance pupil. Ray trace diagrams may help to visualize the effect. It is the cumulative affect of all rays from every point on the objective that give the result in the image. If any of those rays are clipped and do not make it through the entire system, which in this case includes the eye, then the net result is a reduced effective aperture. While a reduced effective aperture will have no affect on your field of view, it will result in lower resolution and a less bright image.

When using a binocular, if eye pupil is smaller than exit pupil while the fixed binocular magnification does not change, does resolution still remain constant? Does image brightness remain constant? What else, if anything, in the optical system might change?

The key concept here is aperture, magnification and exit pupil are inextricably bound together. If exit pupil varies and magnification doesn’t change, something else must change.

In the binocular, (with the exception of those with interchangeable eyepieces) magnification is constant. There is no disputing the fact that the resolution and light gathering is delivered to the focal plane by the full aperture. But next, the eyepiece selected by the manufacturer must deliver the image to the eye. The eyepiece determines the magnification and the size of the exit pupil. The size of the exit pupil determines the image brightness.

What happens to resolution and image brightness when it is delivered to an eye pupil smaller than the exit pupil?

As I vary the eyepieces and magnification in my telescopes, the aperture remains constant, so exit pupil changes as magnification changes and resolution remains constant. The ability to see that resolution increases as we approach optimum magnification. Image brightness varies with the size of the exit pupil.

That’s not what happens in binoculars and in some cases telescopes at very low powers. In binoculars, many times, entrance pupil can be made smaller by the eye but magnification cannot be changed in a fixed power binocular. Because of this and the laws of optics, something else is forced to change and that is referred to as “effective aperture.”

If eye pupil is smaller than exit pupil in binoculars, or any optics, given all other parameters in the system remain constant, one parameter has to vary for the laws of optics to still hold true. We could force aperture to remain constant, but then the only way you could get a smaller exit pupil with a fixed set of eyepieces, or a constant magnification, is for focal length to change. However, focal length is not changing. With magnification still constant, something else is changing. Magnification and focal length in a binocular are fixed, so the only other parameter left to change is the “effective aperture.”

A proper application of the laws of optics can give only one result. If eye pupil is smaller than exit pupil while magnification remains constant, only one other parameter of the system can change and that is referred to as “effective aperture.” The net affect reduced “effective aperture” has on resolution and brightness can then be explained by the laws of optics. Effective aperture is considered that which would provide the equivalent exit pupil that matches the smaller eye pupil. Resolution and brightness would be based on that effective aperture.

Does the change in "effective aperture" result in an effective f ratio? As effective aperture is forced to decrease, it would seem effective focal ratio increases. One formula we have at our disposal is exit pupil = focal length of eye piece / f#. When the eye pupil effectively reduces the size of the exit pupil, and hence the effective aperture, we can divide the focal length of the system by the effective aperture to get a new Effective Focal Ratio. We can see that same change in the above exit pupil formula, as the only way we could get a smaller exit pupil would be to divide the eyepiece focal length by a higher f#. It can be seen Focal Ratio, or f#, while it is a number which gives us a quick indication of the performance of a system, is a resultant quotient, not an affect, therefore it is of much greater relevance to discuss the changes in aperture than to direct this discussion towards f ratio.

If resolution does change, why is it we may not be able to see this?

The magnifications used in binoculars are so far below the optimum magnification to see full resolution provided by the objective lens, you will never see maximum potential resolution of the objective in a binocular. Binoculars are designed for maximum brightness of image, not maximum resolution of detail. While a 70mm lens has the potential to provide a maximum resolution about 2 arcseconds, a premium 16x70 binocular is capable of resolving at best only 8” to 10” arc, depending on the acuity of the observer. Why? Because magnification is too low. The 70mm objective would need a magnification about 87x to see maximum resolution.

Why then are we not able to see the difference in brightness?

Consider this. If you have been using binoculars with an exit pupil larger than your eye pupil, all along your eye pupil has been creating an effective aperture. You cannot get more light (a wider beam of light) into your eye than can be allowed thru your pupils. If you’ve been using a 10x70 binocular with a 7mm exit pupil and your eyes dilate to only 5mm, you have never received a larger beam than 5mm into your eyes. Therefore, all along, the binocular has been performing as if it were a 10x50.

So you might ask then, in bright daylight viewing, if eye pupils dilate to only about 2.5mm, would it not be the same to use a 10x25 as opposed to a 10x50?

Let's assume a terrestrial viewer is using a 50mm binocular with a 5mm exit pupil (10x50). Now assume the observer’s eyes in daylight can dilate to only a maximum of 2.5mm. The difference between the full aperture exit pupil of 5mm and the effective aperture exit pupil as a result of the 2.5mm eye pupil, for a 50mm binocular, observing an object at a distance of 100 feet is 5.5 arcseconds of angular resolution, a linear dimension on the object of interest of only 0.03 inches, approximately the width of a thin vein on a leaf. While this is a very real number, it is not likely something most people are able to notice.

What happens in low light terrestrial viewing where many seem to prefer a larger exit pupil because it seems brighter?

Now we need to go back to the cardinal rule that states exit pupil controls effective aperture. There would be no more light delivered by either binocular if eye pupil is 2.5mm, BUT, as seen through a 2.5mm eye pupil, there would seem to be a difference in the two binoculars, and you might be able to see this difference. The 10x25 binocular is using the entire objective lens and we have seen from vignette studies, many (if not all) binocular systems block (vignette) portions of the light from the periphery of the objective lens. The 10x25 is using the entire diameter of the lens to light the exit pupil and the light from the outer portions of the objective is reduced in the exit pupil due to vignette. Not all of the light gathered by the 10x25 reaches the eye.

Most vignette studies show that approximately the central 50% of an objective lens provides 100% illumination of the exit pupil. The light delivered from the area outside the central 50% of the objective does not all reach the exit pupil. In the 10x50 binocular, stopped down by the daylight contracted eye pupil to 10x25, only the central 50% of the objective is putting light into the exit pupil. The key is this 2.5mm exit pupil is 100% illuminated from the cenrtal portion of a stopped down 50mm lens. A stopped down 10x50 is providing a lot more illumination to the same size 2.5mm exit pupil than a full aperture 10x25. That would account for a significant difference in apparent brightness between the two, even though both situations have a 2.5mm exit pupil. As a bonus, that same 10x50 binocular will serve the user better under lower light conditions when eye pupil is enlarged to possible 3mm or 4mm.

What are the implications of choosing a binocular with too large an exit pupil vs. one with the same aperture and a higher magnification that would result in a smaller exit pupil, one my eyes can take it all in?

Assume for example the user has a maximum dilated 5mm eye pupil. His choice in binocular is narrowed down to a 10x70, a 7x50 or a 10x50. For this observer with 5mm eye pupils, the 7x50 will be performing like a 7x35. The 10x70 will be performing like a 10x50. And finally the 10x50 will perform as designed, like a 10x50. It will see a lot more than the 7x50 and it will likely weigh (and cost) a lot less than the 10x70.

What consideration should be given to exit pupil if the binocular is to be used in brightly lit light polluted skies?

In this case you might consider even smaller exit pupil than your maximum dilated eye pupil. You can vary the values in the relationship of aperture/magnification (which gives exit pupil) by either decreasing aperture or increasing magnification. Either choice will decrease exit pupil and therefore increase the apparent contrast in the image by decreasing the brightness of the background sky. This should help bring out the images of target objects.

For some users, another good reason to keep binocular exit pupil slightly smaller than eye pupil would be to mask deficiencies in the eyes. It is a well known fact that the affects of astigmatism (in the eye, not in the instrument) become far less obvious in the image as the exit pupil in the optical system is reduced. Personally, I can still see the astigmatism inherent in my less than perfect eyes when viewing thru exit pupils of 2mm or larger. So I keep my glasses on for all binocular viewing and allow myself the luxury of utilizing much larger binocular exit pupils.

So what’s the right choice for the brightest astro images?

If you are looking for the brightest image, consider the maximum size of your own dilated eye pupils before you determine the size exit pupil you want in a binocular. It might be OK to go slightly larger with the exit pupil, assuming you would have ample opportunity to get out under the darkest of skies and use it to it’s real performance potential. Keep in mind the sweet spot affect outlined in terrestrial use above. There is some advantage to not using the entire objective lens, but potentially at a significant price/gain ratio. Then, buy a magnification that suites your needs. Since the light gathering of different apertures varies so little, aperture might be the single aspect you should allow to vary the most. But don’t buy a 7mm exit pupil because that classy 10x70 binocular is so hyped up by everyone. If your pupils dilate to only 5mm, a 10x70 binocular is not the right choice for you. Stick with a 5mm exit pupil and vary magnification and aperture to suit your needs.

Reference Sources:
Rutten, Harrie G., & Martin van Venrooij, "Telescope Optics", Willmann Bell, 1988-2002
Sidgwick, J.B., “Amateur Astronomer’s Handbook”, Dover Publications, 1971
Suiter, H.R., “Star Testing Astronomical Telescopes”, Willmann-Bell, Inc. 1994-2001

Edz
Aug. 2004

#2 lighttrap

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Posted 11 August 2004 - 04:17 PM

Ed,
There are two concepts I'd like to see introduced into that article.

1) Low light terrestrial observing requires maximum light gathering, and is a totally different concept than binocular astronomy. In fact, a lot of folks who's eye pupils may not dilate to 7mm still prefer a 7mm exit pupil optic for low-light terrestrial use, even if they prefer a smaller exit pupil optic for astronomy.

2)The effect of choosing a matching or even smaller exit pupil optic for astronomy, is to lessen the effect of extraneous light scatter, such as from light pollution, and therefore increase contrast between stellar objects and background sky.

Mike Swaim

#3 EdZ

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Posted 11 August 2004 - 04:23 PM

Hi Mike,

I tried to address #1 in the article. The low light gains from increased aperture are no where near the brightness losses from decreased exit pupil. But they are there and I mentioned it.

There can be an advantage to using even a smaller exit pupil than eye pupil if observing under light poluted skies. Although that was not the crux of the article it would do well to include it. that is worth adding.

edz

both items edited, edz



#4 KennyJ

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Posted 11 August 2004 - 05:07 PM

Ed,

You deserve applause and appreciation for this comprehensive effort to bring up to date ,conclusions to a subject matter "mind -boggler" I know you have spent well over a year studying and researching.

And I agree with almost everything stated.

One aspect I feel would serve to increase reader -awareness of the REAL DEAL is that what we refer to as "brightness" in a binocular image ( a very desireable quality for most terrestrial applications -- but often less so for celestial applications ) is , from my observations , AT LEAST as variable as a result of QUALITY of optics as it is purely on a basis of exit -pupil.

Anyone of a neutral disposition, who has compared the BRIGHTNESS of images through , for example , a Zeiss 7 x 42 and any "typical" , medium priced 7 x 50 binocular ,and for example , those through Nikon 8 x 32 Superior E and any "typical" medium priced 8 x 40 binocular will probably know what I mean.

Sadly , not everyone who has ever contributed to this great scientific Grail we call "optical theory" has had the pleasure of looking through such great binoculars.

Kind Regards , Kenny.

#5 EdZ

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Posted 11 August 2004 - 06:07 PM

Duely Noted Kenny,

and edited to reflect

thanks

edz

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Posted 11 August 2004 - 08:28 PM

Thanks for the article EdZ. Many newbies, including yours truly back in the day, did not reflect upon exit pupil size vs my dilated eye size when purchasing my vistas. As luck had it, they match, but I wish I had known about this importance prior so that I could have made a more informed purchasing decision.

Mike, your 2 points are quite good. Being that I am 90% astronomy, I would have never considered the terrestrial needs.

#7 sftonkin

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Posted 12 August 2004 - 01:43 AM

One other important aspect, IMO:

Those of us whose eyes are less than perfect usually find that the aberrations are worse at the periphery of our lenses. For example, my eyes are astigmatic. With large exit pupils, the astigmatism is apparent; with small ones, it isn't.

#8 Rammysherriff

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Posted 12 August 2004 - 03:57 AM

EdZ,

I don't know how long it took to research and write this colossus, but thankyou - it is an excellent article, and should be pinned to the top. It is probably worth a link to the eyepieces forum; Larry Sayre kindly posted matters relating to exit pupil size and real pupil size when I was researching eyepiece choices.

This is science we should not ignore!

#9 werewolf6977

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Posted 12 August 2004 - 06:13 AM

Very informative. Explains why I do better with 12X50's, 15X70's, etc. And I am going to look through an excellent pair of bins yet! Pete :jump:

#10 EdZ

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Posted 12 August 2004 - 06:51 AM

Mike,

you had me up laying awake last night. I knew I had violated the basic tenet of this study, but couldn't figure out why. As relates to low light terra. At first I answered that even though the eye at 2.5mm stops down the light entering, the larger binoc would provide a brighter image to the terra user because it gathers more light in the first place. That's wrong. That violates the optical rule that exit pupil (in this case eye pupil) controls and the "effective aperture" takes over.

The answer lies in what portion of the objective lens is illuminating the exit pupil. I remember a discussion with Kenny a while back during the vignetting studies that this new found information would answer a lot of questions about terra use. That was it.

A 10x25 binocular uses the entire objective lens to illuminate the 2.5mm exit pupil. The light from the objective past 50% out from center is more and more vignetted from illuminating the exit pupil. By the time you trace the light rays from 70% out to 100% out on the objective, only 50% to 25% or less may be illuminating the exit pupil. Not all the light gathered by a 10x25 binocular passes thru the 2.5mm exit pupil.

In the 10x50 binocular, the eye dilation of 2.5mm restricts the exit pupil to using an effective aperture of 10x25. The central 50% of the objective lens is what gets used. Vignetting studies show that approximately the central 50% of objective lenses in good binoculars pass nearly 100% of the gathered light. The vignetting is predominantly outside that area. The 10x50, stopped down by the eye, is using the sweetest spot on the lens to illuminate the exit pupil. it will be a lot brighter than the exit pupil formed by the 10x25.

I have edited the article to reflect my lost sleep.

edz

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Posted 12 August 2004 - 08:04 AM

Quite true EdZ. I am glad that someone recalled the discussions about vignetting.

#12 lighttrap

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Posted 12 August 2004 - 08:31 AM

Sorry for your lost sleep, Ed. I probably could've saved you some time, had I thought about it before posting. Your vignetting idea definitely comes into play, but I think there's a couple more practical reasons for the superiority of very large exit pupil binoculars for low light terrestrial use.

1) A larger exit pupil can mask a bit of hand tremor.
2) It's possible that well dark adapted terrestrial users are actually dilating their pupils wider than when astronomizing. The reason would be the effects of starlight and/or sky glow. (ie. The forest is darker than the meadow.)
3) The rods are specialized to pick up and center on motion. This argues for a wide FOV, and to my way of thinking, a restrictive exit pupil could be thought of in terms of limiting potential FOV. (Yeah, I know that one's kind of grasping.)

I've done some low light testing of 7x35, 7x50, 8x40, 10x50 and 12x50s. There's a pretty sharp cutoff as twilight encroaches and settles into darkness where the 7x35s and 8x40s simply loose ability to pick out details still obvious in the 7x50s, 10x50s and 12x50s. As the darkness settles in, the distance at which fine details can be seen closes rapidly. Because of their higher magnification, the 10x50s and 12x50s can still show finer detail at distance, up until the time that exit pupil becomes the limiting factor. Then, as the eyes start to dark adapt, and really strain to see fine detail, the 10x50s and 12x50s begin to loose detail when compared to the 7x50s. Astronomers misused this info long known to hunters, military and surveillance types to assume that 7mm exit pupil binos were best for astronomy. What they failed to take into account was the enhanced contrast that a smaller exit pupil bino can show for astronomy, and the effects of even small amounts of light pollution, either from artifical or natural sources. So far, the finest contrast, and best astronomical detail resolution thet I've ever seen came from a binocular telescope with eyepieces giving a 1.775mm exit pupil. That's not too surprising, since a lot of telescope users report their most used eyepieces give something around a 2mm exit pupil.

But there's an interesting joker in the pack, which you very wisely mention in your article. That is the issue of quality vs quanity. For instance, Nikon 8x32 SEs will actually perform as if they're 8x40s. Likewise Swarovski Habicht 10x40s actually perform as if they are 10x50s. That's also why a really top quality 7x50 will outperform a mediocre 8x56 or mediocre 9x63.

Mike Swaim

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Posted 12 August 2004 - 09:35 AM

"Then, as the eyes start to dark adapt, and really strain to see fine detail, the 10x50s and 12x50s begin to loose detail when compared to the 7x50s."

Mike, this statement does not quite make sense to me. Could you please explain more? I don't quite follow how 7x50 would show greater detail than 10x50 under dark conditions (or any conditions for that matter). Assuming 4-5mm pupil size under darkness, wouldn't the 10x50 exit pupil be a better match for this reason and for the reasons EdZ mentioned above? Perhaps, I need a text book definition of contrast.

Well actually, if it's totally dark, neither binos would provide any enhanced terrestrial detail because..well...it's totally dark.

#14 lighttrap

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Posted 12 August 2004 - 10:38 AM

"Then, as the eyes start to dark adapt, and really strain to see fine detail, the 10x50s and 12x50s begin to loose detail when compared to the 7x50s."

Mike, this statement does not quite make sense to me. Could you please explain more? I don't quite follow how 7x50 would show greater detail than 10x50 under dark conditions (or any conditions for that matter). Assuming 4-5mm pupil size under darkness, wouldn't the 10x50 exit pupil be a better match for this reason and for the reasons EdZ mentioned above?


Why assume only a 4-5mm pupil dilation? Like Ed, mine have been measured at 6.5mm. I don't claim to know what the experience would be like if one's pupils only dilated to 4mm. However, I'm still quite leary of theory that proposes that everything hinges on exit pupil. It's kind of interesting that when the RAF was interested in producing binoculars for low light plane spotting, that they chose at least one, if not more, models with 8mm exit pupils. I wonder if they'd make the same choice today, given more modern high light throughput coatings and optics?

Well actually, if it's totally dark, neither binos would provide any enhanced terrestrial detail because..well...it's totally dark.


That's true. In the case of a completely dark room. But, outside, there's almost always some ambient light.

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Posted 12 August 2004 - 11:37 AM

My dark pupil size is 7mm, but that's not near the average size for adults my age (33), so I just threw the value of 4-5mm out there for the sake of arguement. I though it was close to the average...that's all. So perhaps it's larger...say 5-6mm...still, the question about 7x50 vs 10x50 regarding constrast remains.

"That's true. In the case of a completely dark room. But, outside, there's almost always some ambient light."

I wish the ONLY ambient light was the moon or zodiacal light. But yes, you're right, there is always "some" degree of ambient light.

#16 KennyJ

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Posted 12 August 2004 - 11:38 AM

I believe that the very latest amended version by Ed , coupled with the "still in debate" issues surrounding Mike's comments come closer than any text I have ever seen written about this fascinating , so crucial , yet so commonly misunderstood and understated topic.

The "accepted formula" for calculating "twighlight factor" of multiplying magnification by objective size and then obtaining the square root of the sum , theoretically goes against what Mike has observed, which is probably why it doesn't quite make sense to Nightwatch.

It ought not to make sense to many people really , but , and it is a BIG BUT , I must admit that I have similar experiences to Mike in this respect , and in my post regarding Twighlight Factor back in September 2003 , I put the anomoly down to differences in QUALITY and DESIGN.

Whether or not this is the REAL reason I do not know for sure , but there is no doubt that comes a point during the natural darkening process ( i.e gradual loss of light around and just after sunset ) when I can actually see things ( terrestrial objects at close and medium distance) MORE CLEARLY through Zeiss 7 x 42 than I can through Swift 10 x 50s.

Of course one way to eliminate the "quality and design" factor might be to compare images through 7 x 42s and 10 x 50s of EQUAL quality and design.

Now is someone has, say Swarovski SLCs in both these sizes, perhaps that would be a more fair comparison.

Regards , Kenny.

#17 Henry Link

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Posted 12 August 2004 - 12:25 PM

Another consequence of the eye's pupil size reducing "effective aperture" is that "effective focal ratio" is increased at the same time. In the example of a 10X50 and a 10X70 both becoming effectively 10X50's when matched to a 5mm eye pupil, for instance, if both have F:4 objectives the 10X50 remains F:4 while the 10X70 becomes "effectively" F:5.6 with a reduction in visible aberrations. The increase in "effective focal ratio" in daylight is even greater. The optics of a 7X50 can bcome "effectively" F:10 or more.

#18 lighttrap

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Posted 12 August 2004 - 01:01 PM

Another consequence of the eye's pupil size reducing "effective aperture" is that "effective focal ratio" is increased at the same time. In the example of a 10X50 and a 10X70 both becoming effectively 10X50's when matched to a 5mm eye pupil, for instance, if both have F:4 objectives the 10X50 remains F:4 while the 10X70 becomes "effectively" F:5.6 with a reduction in visible aberrations. The increase in "effective focal ratio" in daylight is even greater. The optics of a 7X50 can bcome "effectively" F:10 or more.


Henry, that's a really interesting point. I've never really thought about it in those terms. But it's pretty widely known that as binocular magnification goes up, so does CA. And as magnification goes down, depth of field increases. Your explanation makes sense of that.

#19 EdZ

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Posted 12 August 2004 - 01:12 PM

You are correct, the f ratio changes.

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#20 KennyJ

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Posted 12 August 2004 - 01:28 PM

Yes Henry, a very relevant point , which I'm sure Ed will add to his epic summary ( I know he KNOWS about this point anyway , so can only assume it got lost in the effort )

I've just been re -checking what happens when :

1. Masking down the objective

2. Placing a round central obstruction over objective.

Masking also affects effective focal ratio , but what I can't quite get my head round is that IF , as Ed says , SO much of the effective brightness is coming from the CENTRAL section of the objective , why is it that even a central obstruction of say 15mm diameter in a 42mm objective lens has so LITTLE an effect on the end product ?

At around 20mm , things start getting noticeably darker and more opaque.

Am I correct in putting this down to comparitive AREA rather than diameter ?

After all , although 21mm represents 50% of diameter , the area thus blocked only represents around 30% of total.

Nonetheless, from the vignetting data presented , one would expect a more dramatic effect from such a central obstruction.

Thinking out loud ,

Kenny.

#21 EdZ

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Posted 12 August 2004 - 01:50 PM

Kenny,

The differences would be judged on area, such as central obstruction in an SCT. I would guess definitive testing would probably show greater losses than the eye is capable of easily seeing.

The vignetting is very much dependant on the quality of the binocular.

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#22 EdZ

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Posted 12 August 2004 - 03:34 PM

In the example of a 10X50 and a 10X70 both becoming effectively 10X50's when matched to a 5mm eye pupil, for instance, if both have F:4 objectives the 10X50 remains F:4 while the 10X70 becomes "effectively" F:5.6 with a reduction in visible aberrations. The increase in "effective focal ratio" in daylight is even greater. The optics of a 7X50 can bcome "effectively" F:10 or more.

Henry, that's a really interesting point. I've never really thought about it in those terms. But it's pretty widely known that as binocular magnification goes up, so does CA. And as magnification goes down, depth of field increases. Your explanation makes sense of that.


Mike,

Your assumptions and Henry statements seem to be unrelated. Nearly all binoculars, regardless of magnification, are within a very narrow range close to f4.2 to f4.3. So, as binocular magnification goes up, it's not commonly because they are changing focal length in the binocular, it's just because they are using a shorter focal length eyepiece.

The reason CA might increase as magnification increases is the image scale size now makes the difference in the focal point color blur easier to see. It's like magnifying the apparent separation between a close double, you can't see it until it is magnified large enough. In this case, it may be unrelated to focal ratio and more dependant on magnification. By the way in the BT100, as magnification increases, CA is nearly absent.

In Henry's statement, focal ratio changes, but magnification does not. If there were any changes noticed in CA, it would be as a result of using a smaller area of the lens, which by the way still has the same focal length. Under any circumstances, magnification does not change.

edz

#23 lighttrap

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Posted 12 August 2004 - 05:23 PM

Ed,
Henry's post reminded me of some lakeside tests I've done using a whole range of binoculars from 7x50 to 16x70. Keep in mind that we're talking about daylight observing in bright sunlight. (ie. Think pupil constriction, not dilation.) For sake of argument, let's assume all binoculars are roughly f/4. (I'll get to changeable eyepiece models in a minute. They're often quite different.)

If it's true that as the pupil contracts, that the focal ratio of the binocular effectively lengthens, then the 7x50s actually wind up with the longest focal ratio because they throughput more light, thus causing the pupil to contract the most. While your point about increased magnification changing image scale and allowing color focal blur to be noticed is a good one, is it the only issue? Henry's post made me theorize that perhaps it's not the only issue. What if you're really, then comparing, say a 16x f/4.3 binocular to a 7x binocular that's operating at f/11? (I'll let you do the actual numbers, since you're lots better at that kind of thing than I am.) Do you see what I'm getting at? The sheer difference in light throughput could be causing radical shifting in pupil dilation. And if that's what's the effective focal length producer in the system at that point, then it's something to think about.

As for the interchangeable eyepiece binos that I have experience with, one is a legitimate f/8 to start with; (the Miyauchi Saturns), and the other (BT80) uses builtin focal length correctors to make it nominally f/11 or so. You're right, those didn't suffer from CA to any noticeable extent, but look at how radically they differ from the usual f/4-f/4.5 binos. I don't know what focal length your BT100s are, but I bet their not f/4.

Can we agree that when it comes to achromats, that focal ratio determines amount of CA? Can we further agree that when it comes to low magnification achromats that even relatively small variances in focal ratio might become highly relevant? If so, and if constricting pupils do change focal ratio, then perhaps you can see the tie in that I alluded to.

I urge you to go out some bright sunny day to a nearby lake and try out your new collection of Nikon Extremes along with your Fuji 10x70s and Fuji 16x70s. Let the binos be your sunglasses, no matter how bright. Hopefully, you can find some nice white sailboats to look at. Unfortunately, it's the wrong time of year for snow geese, but sailboats will do nicely. I'll bet that you'll find that amongst similar models, magnification does indeed serve as a pretty good predictor of CA observed. Furthermore, I think it's more complicated than simply attributing it to more mag delivering larger image scale. I could be wrong on that point, but that's certainly the idea that Henry's post set in motion in my head.

Anyway, whether I'm technically right or not, that's the idea I had in mind.

Mike Swaim

#24 KennyJ

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Posted 12 August 2004 - 05:34 PM

Mike,

As I mentioned in an earlier post , I think you are correct in what you are saying about observational differences.

I suspect though ,that you MIGHT be mixing focal LENGTH with focal RATIO.

Think about it.

Regards, Kenny.

#25 lighttrap

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Posted 12 August 2004 - 05:39 PM

Valid enough, Kenny. See if you like the edited version better. (just changed the word length to ratio in a few sentences)


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