Telescope Reviews: Advantg/Disadvantg of BV/Binoc

Hi Ed, thanks for the response.....

> I cannot tell how you made your assumptions or what they represent.

I would be happy to explain any parts you don't understand.... I think the information is very relevant to diffraction limits for close terrestrial viewing, BV vs. Binoc.

> Do you realize this 'most that can be resolved' can only be accomplished at very high powers.

Your confusing two issues here. When resolving distant planets, more magnification is the "order of the day", indeed. But this discussion was regarding terrestrial viewing, where any viewing instrument can see an animal at 100 yards (including the un aided eye). So what my posts are addressing:

1) how much magnification you can achieve with x size apt., before apt. diffraction begins degrading the view. Of course larger apertures always win, but at the expense of weight and size.

2) More relevant to this thread though, this exercise addressed the "potential" pupil diam. diffraction effects as a result of the false exit pupil sizes which you discovered. You mentioned in your original thread, a BV forces "larger than normal" exit pupils vs. a straight scope or binocs, which we all agree. You then went on to explain how having a larger exit pupil vs. eye pupil diam. will create additional diffraction, as when the exit pupil is larger than the eye pupil diam, the circumference of the eye pupil now becomes a contributing diffraction source. (as opposed to the scope apt being the only source of apt diffraction) Your original thread went on, and used a factor of eye pupil diam. / exit pupil... which you then multiplied this factor against the Scope/binoc apt. diam. (for light loss and diffraction) I am in agreement with all this Ed. What I was curious about - was "how and when" this false exit pupil can actually degrade the image (from diffraction) due to the lowering projected resolution on the retina... i.e. degrading the projected image, below a resolution value the retina is capable of resolving.

The small exit pupils of .5mm that you refer to are at very high magnification, ~ 200 x, and at this magnification, surely the exit pupil is smaller than the eye pupil diam. So none of this applies....but 200x is rarely the case for terrestrial viewing, as this thread is about binoculars vs. BV for terrestrial viewing. In astro, this discussion is irrelevant. But many people consider the differences between BV/Binocs for terrestrial use.

> As I pointed out above, magnifications that we would be using on the order of 25x or 40x to 50x might allow you to see potentially 10 lp/mm or 50 to 80 lp/mm, but never as high as 200. In fact, I don't own any instruments that could be used at any powers that would allow me to see as high as 200 lp/mm.

We are talking about two different resolutions here. You refer to the resolution the eye can resolve off a reflected target. I am referring to the aerial resolution, or the projected resolution of a "abberation free" optical system - on to the retina. Thanks to another poster on this forum, Jared, he offered cross references between the two, 100 lp/mm projected resolution on to the retina is equivalent to approx., the unaided human eye resolving a 3 lp/mm reflected target at 10" viewing distance. This cross reference made perfect sense to me, as the best human vision can only resolve up to 6 lp/mm on reflected charts, hence the max. 200 lp/mm projected resolution onto the retina. (which also perfectly coincides with the human retina containing 400 cones per mm, or 200 cones pairs per mm)

When lenses are tested with optical test equipment, their ability to resolve is expressed in aerial resolution (in scopes we use the term projected resolution as everything is being projected onto the retinal). The basis for this is, many lenses are used in an optical system, whereas the lenses begin to degrade the total resolution with more lens elements. Hence why they isolate the aerial resolution of each element and test with sophisticated lab equipment, as the human is not capable of discerning such resolutions. As an example, the best photographic lenses can produce aerial resolution in the 600 - 800 lp/mm range. Scope optics are also some of the best optics made (of course this varies based on the quality of the scope) as everyone is chasing the "best possible view" for the dollar.

If you take your scope and focus on a close target and eliminate other seeing obstacles, you can test this for yourself. I have done this many times and the results are very consistent amongst many optical systems I own. To simplify the approach and see the effects, simply use the well used magn/inch method below as a reference.... and be sure the exit pupil is smaller than the eye pupil diameter, so you know for sure where aperture diffraction is coming from.

magnification per apt. diam (in inches) - Example - 43x magnification / 4" apt = 11.

At, 11 magnification per inch, the optics can produce a max. 200 lp/mm projected on the retina.

Therefore,

11x magn/inch (and < 11x) will produce "diffraction free" views.

22x magn/inch will produce views which are degraded 50% by apt. diffraction.
(vs. what the human eye is capable of resolving)

44x magn/inch will produce views are are degraded 75% by apt. diffraction. (50 lp/mm projected to the retina)

Again, assuming you have good optics (I am sure a pair of $15 binoculars can defy this formula as lens abbearations will rule) and eliminate other seeing variables, which means focusing on a relatively close target with no, or very few seeing variables, such as a bright lit target indoors at 50ft, then hopefully you will see the resolution fall-off as described above. I often use a block wall 100 ft away outside, in the morning, to reduce thermals. For me, the results are near perfect to the benchmarks above. Others I have tested seem to have the same findings. I am curious how others on this forum would fare at this simple test. Anyway, if you have a high quality optics, this is an excellent way to isolate the variables and "see" the diffraction in varying degrees.

As I mentioned earlier, this projected resolution method of un abberated optics is a simpler method to determine the projected resolution on the retina. When aperture diffraction (from scope apt. or eye pupil diam.) begins to degrade the image below 200 lp/mm, we can begin to attribute the degraded view to "aperture diffraction." As mentioned prior, the 200 lp/mm comes from the fact the retina contains 400 cones per mm, or 200 "cone pairs" per mm, which I have expressed "cones pairs" as line pairs, as its simply an expression of density. It's no different then expressing digital sensors as pixel pairs per mm, which often we see mentioned as lp/mm.

Of course it's always possible there can be other contributing factors to degradation, but this discussion revolves around aperture diffraction only. As mentioned early, there is also the human element here, (age, quality of retina, brains interpretation, retina density, etc.) so this exercise will only get us "in the ballpark", which in optics, is never easy :-) It's not uncommon to miss the ballpark and be in the wrong country :-)

> In fact, I don't own any instruments that could be used at any powers that would allow me to see as high as 200 lp/mm.

I am sure you do Ed, a decent pair of $100 binocs can accomplish this with ease.... hence this discussion.

> Where are you getting your 1500 lp/mm from?

The formula is, 1500/f stop = aperture diffraction lp/mm. This is resolution limit imposed by diffraction, expressed in lp/mm. As you know, there is other ways to express diffraction, but coming from a photographic background, this is the preferred method when dealing with reflected targets and human vision. This formula has been around since the early 1800's and has been used in photography since then. Myself and many photo colleagues use this formula daily, for most of our lives. It is an amazingly accurate value, and prvoven by countless tests, by the best researchers out there, including Zeiss. Since there is no human element, aperture diffraction its VERY predictable, hence this thread. I have included at the bottom of this post, a reference from the LF photography home page that explains this in more detail.....

> How is it used in reference to a 5" aperture?

If the scope is 800mm fl, then 800/125mm apt = f 6.4. 1500/6.4 = 234 lp/mm, the max. aerial lp/mm the lens can project. But when you introduce it in the form of a scope, you have to alter the effective fl to account for magnification of the entire system, including the fl of the human eyes, as the eye becomes a lens in the optical train before the light rays hit the retina. Hence why my calculations showed the effective fl of the entire optical system, effective fl of the combined "objective fl, EP fl, human eye fl", then applied the aperture diffraction to this combined "effective fl".

Additional diffraction information.......

Diffraction
A beam of light passing through a circular aperture spreads out a little, a phenomenon known as diffraction. Diffraction is a physical phenomena which is unescapable. The smaller the aperture, the more the spreading. For photographic lenses, diffraction depends only on the f-number N.

Strictly speaking, diffraction is a function of aperture size or the physical size of the hole and that is how it would be defined in a physics textbook. Which means that the larger area aperture in a 300mm lens at f/16 (as compared to a 50mm lens at f/16) should provide lower diffraction. However, diffraction patterns are angular patterns and as such are dependent on how far from the aperture you place the screen used to view it also. In photography, the aperture is at the optical center of the lens and the screen is (for infinity focus) one focal length away. The physical size of the diffraction blur is then the focal length divided by the apparent size of the aperture i.e., the definition of the f stop. Thus, in photography, diffraction is only a function of f stop and not a function of the focal length. In simpler terms, the larger aperture of the 300mm lens does offer lesser diffraction at the diaphragm (i.e., less bending around the diaphragm) but since the light now has a longer distance to travel (as compared to the 50mm lens), the smaller bending still results in a fair bit of blur at the viewing screen. N Dhananjay

The image of a point light source (such as a star) after diffraction appears not as a point, but as a central circular spot surrounded by a series of dark and bright rings. See references for details and formulas. Because a point source is imaged as a disk rather than a point, it will appear blured, and this limits the maximum resolution of the lens. The formula R = 1500/N explaned below, gives the maximum resolution of the lens as a function of the f-number N.

The central spot size due to diffraction, called the Airy disk, has a diameter of d = 2.44 x lambda x N. Note that this formula depends on the f-stop and not on the physical aperture of the lens. For a wavelength near the middle of the visible, lambda = 550 x 10 exp(-9) meters , d = 1.34 x 10 exp(-6) x N meters, or d = N / 750 if d is in mm. No lens can make a spot smaller than that value. 86% of the power is in the Airy disk, so it can be viewed as the image of the point. However unlike in the case of defocus, the brightness within the central diffraction spot is not constant, but it decreases from the center to its border, where it reaches 0, resulting in a dark ring. If we use the Rayleigh's criterion that two points are resolved when the dark ring of one coincides with the center of the Airy disk of the other, the corresponding resolution is R = 2/d.

fstop resolution limit
N R (lp/mm) d (mm)
11 136 .014
16 93 .021
22 68 .029
32 46 .042
45 33 .059
64 23 .085

Ed...

> Your missing a portion of the light. Not all rays are visible at other than the exit pupil distaance. that's where all rays come together to form a completely illuminated image.

I understand the explanation, but I NEVER once experienced this... with my eye almost slammed on the glass, I see every ounce of image that I see at the ER position. Do you not experience this also? I always notice people slamming their eyes into a Bino viewer for this same reason....

> this statement of yours pretty much lets me know that you agree or at least seem to think the BV provides stereo images. But then this opens to question, how much?

Well, this is getting into semantics. True stereo vision as it relates to depth perception requires two images of the subject, both taken from a different prospective, such as our Inter Pupil spacing in natural viewing or in binocs, .... but NOT in BV. In a BV, depth perception produced by two distinct images does NOT exist. So any depth we feel is "perceived" or "simulated" by the brain. But the BV capitalizes on the fact the brain uses many depth cues to interpret depth, not just two distinct images....this is evidenced by many of the 2d images you look at to trick your vision into seeing depth, a popular one was a series of books called Magic eyes. So it's possible to perceive depth, (sometimes amazing depth) where NO depth exist from two distinct images produces from two sources, such as our two eyes, or two objectives in a binoc.

What was stunning to me, after years of stereo research... my TV BV creates a wonderful and quite natural depth feel, even though it's ALL "simulated depth". IMO, I believe the BV is creating perceived depth by capitalizing on EVERY OTHER depth cue the brain uses to create the depth illusion to us, - except true depth :-) ! Keep in mind, this discussion so far only relates to relatively close terrestrial viewing.

The other depth cues the brain uses are resolution, sharpness, contrast, AFOV (big factor), using both eyes (even bigger factor). IMO, a BV in a sharp scope with very sharp EP's used at low magnification literally overwhelms all the other depth cues of the brain, enabling the brain to offer us the feeling of depth. One of the biggest factors of perceived depth in 2d imagery is "immersion". Immersion is felt by your angle of view. This is evidenced by the semi spherical movies you see in big Amusement parks or National parks visitor centers, museums, etc. When you stand, and are surrounded by the projected movie image, the immersion is so great, you brain begins to simulate that depth which actually existed when the scene was photographed. But yet, it was recorded with one lens, one film.... and projected with one lens, one film and is viewed with the un unaided eye. The depth feeling is so powerful, you are instructed to hold onto the handrails in front you, as the effect is so powerful, your brain actually places you in the scene, and the movement can alter your senses to the point of swaying or even falling. Anyway these movies, are a perfect demonstration how the brain can reproduce near perfect depth from 2d imagery. When these movies are done well, with super high rez images, I often get a much better depth feeling vs. an Imax 3d movie that was recorded with dual images and projected with dual images, and appropriate eye wear must be worn to achieve the two different images into each eye. (there is many different methods of getting two different images into each eye, but that is off topic here) Again the point being, when you overwhelm the brain with enough depth cues, it has the ability to reproduce depth.

I also have experienced the exact same when testing stereo viewers. Often I can use 2 identical images in a stereo viewer, but the views are so sharp, and the AFOV so big, the brain sees depth where no depth exist in the images. Now when I introduce images with depth (two distinct images shot with two side by side cameras) even more depth is introduced and appreciated.

Trust me, the TV BV I have been using has caught me by surprise and is opening up new viewing possibilities for me, hence my interest in fine tuning the oddity that a BV represents, i.e. One image, split into two views. Other than stereo microscopes, I have never seen this type of optical trickery in terrestrial or amateur astronomy gear, till recently. So I am willing to bet, the final chapter has not been written on this... your threads really stimulated many of us to think about this, and maybe advance what you have started....

> Is the huge reduction enough to be a non-issue? Or is the affect still there and somee people have a problem with it? This would be helpful to fully understand. Either a huge reduction would allow being able to say to people who claim 3D, there is no stereo affect with a binoviewer, or the acceptance that there is a stereo affect and acknowledgement that people can see 3D with a BV means that it is similar to a binocular. My only question is, which is it?

For starters, a BV produces a single, flat, 2d image. So does a scope with single eye viewing. However, even with a high quality scope with single eye viewing, at very low magnification, the resolution is so sharp (always when seeing conditions are right), contrast is so high, edge sharpness is superb, it will overwhelm some of the brains depth cues discussed above and you can sense a 3d feel.

Now, I believe it's this slight depth felt in a single eye view, is the starting point of understanding how the brain can simulate or perceive 3d effects where there is NONE. Single eye viewing in a scope at very high rez, specially on bright subjects, demonstrates a slight depth effect. Look at local trees with your scope, can you tell which tree is in front of the others? It's quite easy... try to stay at super low magnification so "Depth of Field" does not cloud the issue too much. Anyway, this tiny bit of depth in a single eye view I believe is created by the ultra sharpness of the optics. With the optics adding contrast to the image and edge sharpness, with a low enough pupil diameter, say 2mm, (which is the perfect balance between the eyes abberations vs. pupil diffraction), you can actually see a view NOT possible with the unaided eye when viewed at a close distance to represent the same magnifications. IMO, it's this high resolution / high contrast, trickery, that induces the brain into simulating depth.

Now, project that same image into both eyes.... now the brain goes into depth mode - as with 2 eye viewing, our brain is in depth mode 99% of the time, except when looking at a 2d image. So our brains are trained, when each eye sees an image (either distinct or the same image), then depth should exists.

Your brain as no training or previous knowledge on using both eyes, seeing an unusually sharp image (not like a 2d reflective print which has much limited contrast, brightness and resolution vs. low power magnification scope viewing) and not interpreting this as true dual stereo vision (i.e. two distinct images, as in true stereo vision) The only exception to this, maybe people who use stereo microscopes all day, as their brain receives very unusual training, i.e. seeing 2 sharp distinct views of the identical image.

To further this concept, in photographic printing, I have noticed for many years, when a 2d print is made on translucent back lit medium, at extraordinary high levels of resolution, contrast and edge sharpness (now often attributed by digital manipulation), people viewing such, all comment..... Ahhhh, it seems 3d, almost like there is depth, like looking through a window. This furthers my position on how we can trick the brain....

Another huge variable in tricking the brain is the AFOV. When those same back-lit images above which were printed at 50" and viewed relatively close, are now reduced to 14", and viewed relatively close, we stop hearing the, " I see depth" comments. This I relate to sheer AFOV, clearly one of the biggest depth cues.... with little or nothing in the periphery, our brain realizes this is not a real view, as the eye never sees tiny circles with no periphery.

So a Bino viewer, at low magnifications, with very sharp optics, well lit subject, large AFOV EP's preferably 80 degrees, comfortable viewing (not allowing black out in the eyes, problems with ER, glasses, etc.) then this BV has just provided every single depth cue to the brain, except ONE - distinct images. And that's the cue most would think contributes the most. So this again demonstrates how creative our brains are, as it seems our brain fills in the only missing piece - true depth! So the more depth cues (and the quality of the depth cues) you give the brain, the better your brain will fill in the missing ones.

Now for astro viewing, it's a different ball game, as our eye spacing and brain is not capable of perceiving depth past relatively short distances. With Astro BV'ing, the magnifications are often very high (except maybe the moon), resolution is often very diffraction limited. However, wit the right EP's, we can still offer a large AFOV and good contrast can still go a long way to trick us us into slight depth simulation. But with Astro / BV, I think a safer definition is, we feel like we are in a spaceship, looking out a huge window. In which case, even at this vantage point, these distant objects provide no depth from our natural stereo vision. Our brain is required to use its other depth cues to determine spatial relations.... So with astro, its a mix bag, much of it probably depending on the quality of the view, AFOV, the contrast of the scene, the size relations of the subjects in view, and even the colors. But clearly the BV even in astro provides tremendous immersion and slightly better sharpness than one eyed viewing, all very favorable IMO, vs. one eye viewing. I would be very curious to hear from BV users with lots of experience viewing deep space.... would they describe what they see as immersive (spaceship view) or depth. Anything is possible, and the manual on this topic has not been written. We are all writing it as we go along.

But I think the most dramatic sweet spot of a BV falls into terrestrial viewing..... as in most cases we are comparing the BV/Scope with binocs, in which case, the simulated depth allows us the use of a single bigger apt. (vs. two smaller ones in a binoc) and a smaller package, with the potential to go to any magnification with a change of EP's (which is possible in some binocs) Also, per dollar spent, I believe the BV/Scope, you get you much better views after you pass the normal binoc range of 15 - 20x magnification. Below 15x magnfication, a pair of Canon IS binocs just can't be beat for the view vs. dollars spent. From my analysis though, BV /Scope fills that 15 - 60x range more cost effectively, better optically (higher magnfication views at the same diffraction thanks to larger aperture) and produces a smaller, lower weight system vs. the old standard - Binocs.

I would be interested in hearing other opinions....