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Incorrect apertures and magnifications on binos?!?

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#1 jgroub

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Posted 02 September 2014 - 06:21 PM

I was reading this review of my 12x60 Oberwerks:  

 

http://www.cloudynig...ar-review-r2854

 

and the reviewer discussed not only how he measured that these particular binoculars only had 51mm of effective aperture (as opposed to 60mm, natch), and how the magnification was actually only 11.4x, but also, that all binos in this class would have similar specs.  (He also found the field of view to be narrower than advertised as well.)  

 

Are the specs on binos accurate? Is this an industry-wide "problem"?  Is the reviewer's statement true about other binos?  Is there any way we can know this before purchasing, assuming there aren't any reliable reviews like this one?  I was thinking about some 15x70s as an upgrade to my, ahem 11.4x51s, but now I'm not so sure.  The magnification isn't quite as important to me as the aperture.  How can I make sure that I'm getting the aperture that I think I'm getting?  



#2 Rich V.

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Posted 02 September 2014 - 07:22 PM

Jon, many binoculars fall short on effective aperture.  While the claimed objective aperture may be correct (measuring the glass), internal prism apertures and other restrictions to the light path may make the effective aperture smaller than the claimed aperture.  For instance, most lower priced 70mm binos actually measure 63mm and 80mms measure 72-73mm.  If the magnification is less than claimed also, you may be fooled into thinking everything works out OK.  For example, you may have a "20x80" and you measure the exit pupil to be 4mm; exactly as it should be.  It turns out that the bino is actually an 18x73, though, which also has a 4mm exit pupil.  Fooled you!   :blush:

 

When you step up to the "premium" binos like the KUO BA8 porros and the Fujinon FMTs, you can expect the effective aperture to be within a mm of the claimed aperture.

 

If you look at EdZ's binocular measurements in the "Links to 433+ Binocular Mini Reviews in this Forum", you'll see the pattern...

 

We can use a "flashlight test" to measure the true effective aperture of binos; a flashlight beam is aimed through the eyepiece at a distance of a foot or more behind the eyepiece and the circle of light projected onto a wall through the objective lens will be the effective aperture.  Now you can measure the exit pupil with a caliper and divide it's size in mm into the effective aperture in mm.  This will give you a more accurate magnification.

 

Rich


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#3 DarkDisplay

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Posted 02 September 2014 - 07:22 PM

This is also a source of aggravation to me. Much care should be taken by binocular companies to provide exact information to their customers. If it's not actually 10x, 15x, 50mm or 70mm, just say so. Better yet, do it right or not at all. Perhaps I'm being a bit unreasonable. But...that little bit you don't have might mean a lot. I've become very careful about researching a bino before I get serious about it.

Best wishes,
Frank
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#4 jgroub

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Posted 02 September 2014 - 07:30 PM

We can use a "flashlight test" to measure the true effective aperture of binos; a flashlight beam is aimed through the eyepiece at a distance of a foot or more behind the eyepiece and the circle of light projected onto a wall through the objective lens will be the effective aperture.  Now you can measure the exit pupil with a caliper and divide it's size in mm into the effective aperture in mm.  This will give you a more accurate magnification.

 

Rich

Thanks, Rich.  But if you're not at the store, and don't have a review handy, how can you know what the real specs are?  Or is it just that, with the exception of a couple of specific brands like Fujinon and KUO (?), they're all doing it to roughly the same degree, so that your 15x70s are the same as everybody else's, so I really shouldn't worry about it?  



#5 Rich V.

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Posted 02 September 2014 - 07:34 PM

I should also point out that even well regarded binocular test sites like Allbinos.com don't report the effective aperture of the binos they "test".  They merely measure the objective lens diameter and call it "real front lens diameter", FWIW, totally missing the point.   :p

 

Likewise, "tests" done by the astro magazines gloss over most every measurable parameter of a bino.  Instead they give you subjective impressions woven into a lot of glossy BS instead of measurable data.  You can only count on reports like EdZ's where the time was taken to objectively review the binocular including pertinent measurements of magnification and true aperture.

 

Rich


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#6 GlennLeDrew

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Posted 02 September 2014 - 11:57 PM

The stressing of light weight and compactness is the real culprit, with what are really objectives of too short focal length being employed. A faster (shorter focal length or given aperture) objective produces a more steeply converging light cone. The optical path length 'consumed' by the prism system demands that this light cone not be too steep, else the front prism aperture clips the outer cone and thereby reduce the effective aperture. Bino objectives should be no faster than about f/4, but many are more like f/3.7-3.8.

 

Many will instantly arrive at 'the fix' for this; bigger prisms. Nope. It's all prportional. A bigger prism set consumes a commensurately longer optical path, and so there is no gain to be had here. The only benefit of a bigger prism system is that it permits to use larger field stops on eyepieces, affording a larger FOV.

 

What some--too few--makers do is employ a 'tapered' front prism, which on the 'input' side is oversized and so can accommodate the light cone where it's widest. This is costlier. Interestingly, my early Celestron 25X100 uses such tapered prisms, providing full 100mm performance in spite of the f/3.9 aperture ratio.


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#7 Jon Isaacs

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Posted 03 September 2014 - 06:53 AM

 

We can use a "flashlight test" to measure the true effective aperture of binos; a flashlight beam is aimed through the eyepiece at a distance of a foot or more behind the eyepiece and the circle of light projected onto a wall through the objective lens will be the effective aperture.  Now you can measure the exit pupil with a caliper and divide it's size in mm into the effective aperture in mm.  This will give you a more accurate magnification.

 

Rich

Thanks, Rich.  But if you're not at the store, and don't have a review handy, how can you know what the real specs are?  Or is it just that, with the exception of a couple of specific brands like Fujinon and KUO (?), they're all doing it to roughly the same degree, so that your 15x70s are the same as everybody else's, so I really shouldn't worry about it?  

 

 

Jon:

 

In part, you can guess based on price. Full aperture binoculars are not cheap..   And too.. the difference between a 63mm objective and a 70 mm objective is 19% in light gathering.. not a lot, about 0.2 magnitudes. Binoculars are generally under magnified with large exit pupils so the difference in light gathering is not critical.. More important is the quality of the construction... Are the darn things going to stay collimated.. 

 

Best to do your research before you go shopping or take your smart phone with you and read the reviews in the store.  Of course there are few stores that carry higher quality large aperture binoculars, where are you shopping?

 

Jon



#8 jgroub

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Posted 03 September 2014 - 04:15 PM

Of course there are few stores that carry higher quality large aperture binoculars, where are you shopping?

 

Jon

 

Just poking around on-line.  Thinking about the next size up at Oberwerk:  

 

http://www.bigbinoculars.com/1570.htm

 

but if I'm only getting 60mm or so, then maybe not.  



#9 hallelujah

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Posted 03 September 2014 - 08:28 PM

 Thinking about the next size up at Oberwerk:.........but if I'm only getting 60mm or so, then maybe not.  

 

You would have to go to the Oberwerk Ultra series to get full aperture.

 

http://www.bigbinocu...com/ultra70.htm

 

Stan


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#10 jgroub

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Posted 04 September 2014 - 10:32 AM

 

 Thinking about the next size up at Oberwerk:.........but if I'm only getting 60mm or so, then maybe not.  

 

You would have to go to the Oberwerk Ultra series to get full aperture.

 

http://www.bigbinocu...com/ultra70.htm

 

Stan

 

Very good to know.  Thanks, Stan.  



#11 Binojunky

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Posted 04 September 2014 - 10:44 AM

 

Of course there are few stores that carry higher quality large aperture binoculars, where are you shopping?

 

Jon

 

Just poking around on-line.  Thinking about the next size up at Oberwerk:  

 

http://www.bigbinoculars.com/1570.htm

 

but if I'm only getting 60mm or so, then maybe not.  

 

 These come from the same factory in China as the Celestron Skymasters, but with better coatings, better quality prisms and lenses, I have these and found them to be excellent for the price, DA.


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#12 152ED

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Posted 08 September 2014 - 12:08 AM

Speaking only for my 15 year old "bear" brand 15x70 chinese binos (which look exactly like the latest ones on the auction site only without the textured prism housings, same $70 price range), they are definitely more apertre and mag than a similar pair of 9x60s I had at the time, which supposedly had better prisms and coatings.  I sold the 9x60s but still have the bear binos, though they were miscollimated for years.  They are just on the limit of what is hand holdable, for weight and mag.  It's a nice size between regular binos and giants.



#13 Herr_Alien

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Posted 08 September 2014 - 02:05 AM

Funny, what I liked about that review was the conclusion: the whole "smaller-prism-bigger-objective" strategy seems to be a way to deliver good performance 10x50's using the cheaper production process of a "smaller-prism-bigger-objective" 12x60.

 

I use a 10x50 for almost 4 years now. So ok, it's a 10x43 actually, but it did allow me to see the Nova Del 2013. And countless of other objects.

 

Maybe I shouldn't say this in a forum dedicated to astronomical equipment, but, with an optical instrument that provides a decent image, it's the actual observing that matters most.  :getem:



#14 Mike B

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Posted 11 October 2014 - 12:20 AM

Hi Jon-

Glad to hear someone asking the good questions!

 

Thanks, Rich.  But if you're not at the store, and don't have a review handy, how can you know what the real specs are?  Or is it just that, with the exception of a couple of specific brands like Fujinon and KUO (?), they're all doing it to roughly the same degree, so that your 15x70s are the same as everybody else's, so I really shouldn't worry about it?  

 

Almost ALL of my astro gear gets researched out the wazoo before i ever make a purchase; to me it's just part of the enjoyment! So maybe someone would pop for a $20 Plossl on a whim (not me, but...), but they sure as tootin' ain't gonna do so for hundreds of dollars of gear. Purchases like that are likely gonna be well-considered & thoughtfully planned!

 

Fortunately, with the advent of the internet, EdZ & his marvelous research (plus the larger pool of user info on CN) was loosed on the world of information- so as far as 'nocs are concerned, read up before you buy- it's mostly "out there" for the diggin'!

 

 

Binoculars are generally under magnified with large exit pupils so the difference in light gathering is not critical.. More important is the quality of the construction... Are the darn things going to stay collimated..

 

Very true. I'd take a quality build set of 'nocs with excellent optics, even at a true 50mm, over a 70mm pair of 'nocs with mediocre optics of a true 60mm aperture.

 

Quality of optics will point-up stars far better, and over more of the field, yielding fainter stars & deeper grasp- and more satisfying views overall, than will mediocre optics of larger aperture. Magnification will also trump aperture- as quoted above. Add to this comfort of grip, action of focuser, solid collimation & comfort of view, and you have a total winner for an enjoyable & productive experience all around!

 

A perfect example of this is the pair of Nikon Axtion-Extremes i purchased a while ago. At 12x50, and a true 50mm, they were twice the cost of an inexpensive pair of 15x70's i'd had layin' around. Haven't really done a head-to-head... the 15x is nice, fersure, but the *quality* of the view in the Nikons just shames the bigger 15x70's!

 

Unfortunately, the Nikon level of quality 'nocs is prob'ly not gonna beat the rest of the market into submission re: fudging specs & advertising... we're still gonna have to "dig" to find the truth; but that's true in purdy much all of the retail realm- cars, vitamins, clothing, stereos, medicine... you-name-it.


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#15 JKoelman

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Posted 11 October 2014 - 10:13 AM

The stressing of light weight and compactness is the real culprit, with what are really objectives of too short focal length being employed.


The stressing of light weight and compactness can hardly be seen as culprit, given that the obvious fix is to reduce the objectives to a size commensurate with the dimensions of the prisms (whilst keeping their focal lengths unchanged). This won't change the light gathering capacity* nor the magnification and will improve contrast, while making the bino more compact and lighter. But it would force the seller to state the real aperture...

Note
* Perhaps it could reduce the far off-axis illumination? Not sure about this.

Edited by JKoelman, 11 October 2014 - 11:18 AM.


#16 GlennLeDrew

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Posted 12 October 2014 - 04:16 PM

Johanns,

You raise an interesting question. The too-fast objective, for which the full aperture is not contributing to any one image piont, does indeed reduce vignetting somewhat. At increasing off-axis angles, the entrance pupil becomes clipped by the prism apertures.This results in the image-forming light bundle translating across the objective. If the objective is 'over-sized', the light bundles for increasing off-axis angles do not become truncated as quickly as they slide toward the objective edge.

 

 

 

This is much like requirement for 'oversize' front lenses on wide angle lenses. The iris/entrance pupil is well behind the front lens, and so for far-off-axis angles that lens must be made large enough to not clip the light bundle on its way to the iris/entranc pupil. A bino's front prism aperture is much like a camera lens's iris, even though it's not *supposed* to be the entrance pupil.



#17 MartinPond

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Posted 12 October 2014 - 08:26 PM

Sometimes the measurement is not taken right.

I saw someone claim some Leupolds had a lower aperture,

and they did look like that with the rough method of looking at the front from a few feet away.

 

However.....I looked into the face from 20 ft away with a spotter and a beam LED,

and the inner iris did not cut in on the apparent diameter.  Measuring from a few feet

away looking at the face has far to much divergence to find the true aperture.

 

That said, methods that involve passing all the way through can still be the best,

since they find any restriction in the chain.   Those also must be done with very

collimated/parallel light.  They can fail as well from a few feet away. 

 

I see in the review that the laser was 'panned' across the ocular.

However, I don't see if was kept perfectly parallel and moved on an x-y stage, or

pivoted from a few feet away.   If it was panned by hand, it would have to be from

many feet away. We know nothing about the divergence (the pivoting) or errors in the panning.

I have done it across the room, and I needed a long dowel and an assistant to do it right.

 

BTW, flashlights a foot away have wildly differing divergences and even more variable source width.

Most flashlights are best approximated by a scattering of different point sources inches apart

because of the large imperfect reflectors.

Just sayin'...those are the ray-facts.  You can buy a flashlight that will focus a spot beam that

spreads very little over many feet. That makes a sensible source, and/or going many feet away.


Edited by MartinPond, 12 October 2014 - 08:53 PM.


#18 MartinPond

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Posted 13 October 2014 - 06:30 AM

I'm noticing Glenn's 2nd post (referred to in a nearby thread) is pretty good. 

A point source is at least more controlled than a big mottly reflector.

I'd take that technique, maybe with a bit more distance to the eyepiece.


Edited by MartinPond, 13 October 2014 - 08:22 AM.


#19 Swedpat

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Posted 13 October 2014 - 06:44 AM

I understand it's annoying if you don't get what you supposed. I would not find this acceptable for an expensive high-grade glass. But even with consideration that these binoculars actually are 11,4x51 you still get an impressive optical quality and performance fo the price! 


Edited by Swedpat, 13 October 2014 - 06:45 AM.


#20 MartinPond

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Posted 13 October 2014 - 08:16 AM

I'm wondering if the optical engineering definition of the aperture  (using parallel rays from infinity making a light cone through the instrument) and the perceived definition

that's being tested differ a bit. 

 

As a thought experiment, imagine the 'idela' test for what you see out of the eyepiece:

----At some great distance (distance is dozens or hundreds of objective widths away) ,

           there is an arc a laser can travel on. The arc subtends the angle claimed as the field of view.

----At various points along that arc, the laser is played across the face of the objective.

----The width of the view [for that source point] can be seen coming out of the eyepiece.

 

The "aperture" used for optical engineering is  technically just the one obtained at the center of the arc.

If rays are clipped in various ways along the filed-of-view arc, they cause vignetting, a dimming

near the edges of the FOV.

 

The curvature of the light cone on either side becaomes critical to testing.

A 6-degree FOV in the real world, for example, creates a 60-degree apparent fov at the eyepiece at 10 power.

This means that a 6-degree 'light cone' becomes a 60-degree light cone. 

What the heck does that mean?

Assuming reciprocity,

it means a true test throught the eyepiece would have to start with a

light cone CONVERGING at an angle of 60 degrees (bent in from a collimated beam).

However, the flashlight is DIVERGING, which alters the light cone coming out of the objective.

 

I keep hoping actual engineers for the bino companies will decloak   at these forums, but they don't.

I just have na optics course and support work for optical engineers in my past, but rays and cones

seem to explain a lot.   The few times I have gone looking for trouble and used the techniques of

my old optics lab, the trouble has evaporated and the irises are exactly where the engineering

would have them be (to avoid restriction and usually the fov).

 

Getting proper control of light convergence/divergence and understanding definitions could be at the heart of this.


Edited by MartinPond, 13 October 2014 - 08:21 AM.


#21 GlennLeDrew

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Posted 13 October 2014 - 11:02 PM

Martin,

Considering the off-axis light is an additional complication, for one is then usually encountering additional obstructors which modify the vignetting. For our purposes, we merely accept that in virtually every bino vignetting to some extent is present, and restrict to assessing the effective aperture on-axis only.

 

As to the means of utlizing light to test aperture clipping...

 

The first and most important condition is that for an afocal instrument we have parallel light entering the objective and parallel light emerging out the eyepiece. (We can neglect the non-parallelism for incoming light from nearby subjects and the outgoing light for non-corrected myopes and presbyopes.) It is hence quite unnecessary to factor into it the apparent field angle of the eyepiece when considering the light bundle for any one image point.

 

For example (using the 10X60, 6º bino, and assuming zero vignetting). The light bundle for a point source at the field edge is defined by the objective diameter. This 60mm diameter cylinder is brought to a focus at the field stop edge. It then continues on, to be collimated by the eyepiece into a 6mm diameter cylinder inclined 30º to the optical axis and filling the exit pupil. That perfect collimation is not required results from the short focal length of tge eyepiece, which assures a suitably small 'point' source formed at the focus and which will not induce too-large shadow blurring on the emerging bundle out the objective (which a short projection distance further helps with.)

 

Indeed, the exit pupil is really the superposition of a multitude of such emerging cylinders of light for innumerable image points. And so to check aperture/pupil clipping, one just has to direct a collimated beam of light at least as large in diameter as the exit pupil into the exit pupil.


Edited by GlennLeDrew, 13 October 2014 - 11:10 PM.


#22 MartinPond

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Posted 14 October 2014 - 06:30 AM

Maybe this is the crux:

You can ignore non-parallelism when it involves nearby objects or badly corrected eyes

when you're actually using the binoculars, but neither close distance nor bad eyes

should be involved in the measurement, and both do effect the light path through the widget.

Uncorrected eyes 'want' a non-parallel flow coing into them and that effects the obstacles they see.

Curvature coming in unavoidably effects the obstacles hit and the curvatures up front.

 

I shined the flashlight in the 'unfocused' mode last night, and

it made different sized discs on the other side when it was moved from a foot to 4 ft away.

I switched to the 'collimated light' focused setting, and the disc stayed the same size. 

This is an empirical observation.

 

But there are other examples.

The most straighforward wouls be...

--I look into an objective from 2 feet away. I see an edge of a ring that is well inside the objective glass diameter.

--Then I look at the same objective from 20 feet away, with another pair of binoculars (or a spotter from further away).

    result: the edge is not in the way anymore.

 

There is a curvature in rays of the the observation cone coming from my eye.

It is very significant compared to the focal length of the objective. It gives a false image of the iris.

This is an even easier empirical observation.  If you don't have a clear edge,

you can look in and see the 'ribbed tunnel easily from a foot or two away, but you cannot

see it from 20 feet away.   Same effect. This is like watching a stick bend going into water.

 

The fact that binoculars (well, some of them) work up close or work for uncorrected eyes

does not erase the fact that both of those conditions affect the aperture experienced.

Compare the distance of a few feet with the focal length of the objective.  It is very significant.

Being afocal doesn't alter that, except to perhaps aggrevate it, not erase it.

 

Afocal means you will focus sharply on you retina through a range of curvatures.

It doesn't mean that you never focus sharply on your retina at all.

For engineering, it involves tow systems with two values for the eye's lens. That's it,

no evasion....focus must be attained. Focused for infinity is preferred, of course.


Edited by MartinPond, 14 October 2014 - 06:32 AM.


#23 Jon Isaacs

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Posted 14 October 2014 - 09:22 AM

 

There is a curvature in rays of the the observation cone coming from my eye.

 

What do you mean when you say there is a curvature in the rays?    

 

In my mind, the question of the effective aperture is quite straightforward.  For example, the 70mm Skymasters have objectives that are 70mm in diameter.  The question is, how much of that 70mm is contributing light to the image?  With a telescope, this can be easily determined because the eyepiece can be removed, a pinhole placed at the focal plane and a measurement made.  The simplest test is to simply see if the edge of the objective can be seen. If so, then there is nothing cutting into to the light cone and the scope is operating at full aperture.  If one wants to look at the off-axis illumination, then the pinhole can be moved off-axis and the appropriate observation or measurement can be made.

 

With binoculars the eyepiece complicates the issue but I think Glenn has explained it better than I ever could.  In the past, I have measured the effective aperture of a number of refractors using a laser focused with sufficient beam diameter focused into a widefield eyepiece to diverge the beam and gotten agreement to within a millimeter..  

 

Jon



#24 MartinPond

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Posted 14 October 2014 - 09:51 AM

Using reciprocal models can lead to confusion...sorry.

Curvature is based on....the radius of...the light cone you mention. That's all.

Divergence of convergence is the measure of the rays passing apart or together.

One should not measure with outside rays that are spreading apart.

 

To relate it to your method:

You mentioned moving the pin-hole off-axis.  When you do that, it sees a different ray passing through from a different place

 in the lens, and next lens, etc.  As the volume the different pinhole-positions increses, it subtends a conical area, or actually

cones passing through the instrument. you are sampling bits of the light cone, and when you see black it was blocked..

 

I agree the question is straightforward, or at least that the test methind should make it straightforward. 

When you talk about the light-cone in straightforward use, aren't you talking about a solid flat beam

coming from a star in parallel, which is then formed into a light cone that might be blocked somewhere along the line?

 

I'm just saying......for the answer to be straighforward, you really need parallel light passing into the objective

or the eyepiece.   If a laser is moved to spot A and the beam is still parallel to the beam form spot B, this is good.

Having flashlight shining from a foot away (assuming a point source) is equivalent to  pivoting a laser to make non-parallel rays...

...there is a 'curvature'.    The small focal length of the eyepiece does make it less sensitive, but the deviation of these

rays from parallel is problematic.  It violates your wish for something 'straighforward' (literally).  It gets more

confused with a large reflector on the flashlight, of uneven finish...just shine it at the wall and it's obvious.

 

If you move a laser and keep it parallel .... perfect.

If you have a distant source and move a pinhole....perfect (if it's focused for infinity).

 

If you shine a flash in either end from a foot away, it's not like either of those....it's a set of dispersing beams a foot away.

Unless....you just use a collimating (distance-beam-focus) flashlight.

Simply pivoting a laser pointer a foot away without shifting it to make a parallel beam would be the equivalent problem.

 

My 'observation cone' looking into an objective from one foot away has me looking into the lens at an angle,

and that makes the iris appear to be blocking the objective. That is just an illusion.  From 20 feet away, the iris would not

be in the way at all (in binoculars I have looked at).  I am looking at the objective at an angle close-up, and that

distorts my view of what's behind it.  Another term for this is:  a magnifying glass.

Is the difference insignificant at 1 foot?  Then magnifiers would not exists.

 encourage you to take a look...it's right there.   The baffle tunnel also usually appears to be straight

when it's actually conical. Another magnification illusion, and a nifty one, because the magnification

increases as you go down the tunnel. But.....from 20 ft away, you don't see it at all.


Edited by MartinPond, 14 October 2014 - 10:05 AM.


#25 Jon Isaacs

Jon Isaacs

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Posted 14 October 2014 - 11:24 AM

 

To relate it to your method:
You mentioned moving the pin-hole off-axis.  When you do that, it sees a different ray passing through from a different place
in the lens, and next lens, etc.

 

When I do this (use a pinhole at the focal plane to determine the effective aperture), I am looking at the light cone at a point on the focal plane. I see all the rays that are pass through the entire area of objective that are not blocked that reach that point on the focal plane. On-axis or off-axis, what I see is the portion of the objective that is contributing to the image at that point on the focal plane.  If the scope is operating at full aperture, then I will see the entire objective. As I move off-axis, as long as I am seeing the entire objective, I know that at that location on the focal plane, the image is fully illuminated, the scope is operating at full aperture.

 

At some point, I no longer see the entire objective, a baffle or some edge begins to block light coming from the objective, then I see the objective that is partially blocked by a larger circle at the edge.  That area I see is equal to the illumination level at that particular location on the focal plane.  

 

Important here is understanding the difference between a point on the focal plane and a point in the exit pupil. A point on the focal plane sees the entire objective (or portion that is not blocked) but only shows a single point of the object.  

 

A point or pencil at the exit pupil shows the entire field of view but only corresponds to a tiny aperture.  

 

Jon

 

 




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