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x42 bins vs. x32 bins - optical differences aside from brightness? (1 Viewer)

dries1

Member
ZD hart, are you looking through (directly comparing) an 8X32 SF and 8X42 SF, or are you comparing the view through a 8X32 SF and 10X42 SF?.
 

ZDHart

Well-known member
Supporter
The focal length of the 42mm is longer than the 32mm, so this goes the other way and increases resolution if the eyes are say 2.5mm pupil size.



It is far too simplistic to just compare a low powered 32mm and 42mm binocular.

Regards,
B.
My premise in this thread is comparing two bins of nearly identical quality, Victory SF 8x42 and 8x32, differing in objective size.

So, I'm confused when you state that the focal length of the 42mm is longer than the 32mm - aren't the focal lengths (8x in both cases) the same?
 

ZDHart

Well-known member
Supporter
ZD hart, are you looking through (directly comparing) an 8X32 SF and 8X42 SF, or are you comparing the view through a 8X32 SF and 10X42 SF?.
dries1.... in the comparison, I'm trying to hold as much constant as I possibly can, aside from objective size. So, in my example above: Zeiss Victory SF 8x32 vs. Zeiss Victory SF 8x42.

I think I've learned that the reason the x42 may provide a sharper image to the viewer, than the x32, other factors held constant, is because diffraction decreases as the size of the objective increases. And if diffraction is reduced, then sharpness of the light image is likely to be increased.

I think I've got that right, but perhaps someone much more knowledgeable about optics than I can confirm or correct this impression.
 
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ZDHart

Well-known member
Supporter
Which view do you prefer if you are stationary and viewing out to your back yard, the 8X32 SF or the 8X42 SF?
My SFs are 8x32 and 10x42, so quite different experiences between the two (8x vs. 10x powers). The 42s seem perhaps a little sharper, but they are 10x compared to 8x, so not a very good comparison.

Comparing my UVHD+ 7x42 to my SF 8x32... the Leicas seem to have an edge, for clarity and sharpness. Almost too close to call, though. And, they are two different powers, very different designs, different coatings, different producers... so, there are all those confounding variables involved. :rolleyes:
 
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Maljunulo

Well-known member
My premise in this thread is comparing two bins of nearly identical quality, Victory SF 8x42 and 8x32, differing in objective size.

So, I'm confused when you state that the focal length of the 42mm is longer than the 32mm - aren't the focal lengths (8x in both cases) the same?
No, probably not. If the 42 is longer, it probably has longer focal length objectives.

Again, one of the fundamentals of optics is that Magnification = Focal length of objective / Focal length of ocular, so there is an infinite number of combinations which will result in 8X magnification.
 
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ZDHart

Well-known member
Supporter
Diffraction!

It should have been so obvious to me as a commercial photographer! I've always known that when you stop a lens down to small apertures, the image quality tends to worsen. Most lenses perform best (sharpest) closer to wide open, than closer to having the aperture stopped down all the way. The smaller the aperture that the light has to go through, the most those light rays are diffracted, causing less sharpness.

Take a 50mm f/2 lens. You can expect the sharpest results from that lens to typically be around f/2.8 to f/4. If you stop that lens down to f/16, you can expect a relatively poor image quality to result. You'll get much deeper depth of focus, but you'll lose sharpness. This is due to the effects of diffraction.

So, it only makes sense that diffraction may influence a binocular lens of 32mm objective to be slightly less sharp than one with a 42mm objective - all other variables held relatively constant - due to diffraction.
 

ZDHart

Well-known member
Supporter
No, probably not. If the 42 is longer, it probably has longer focal length objectives.

Again, one of the fundamentals of optics is that Magnification = Focal length of objective / Focal length of ocular.
I am not very knowledgeable, at all, on internal lens design. So, you're saying that the magnification of an SF 8x32 lens is different than the magnification of an SF 8x42 lens?
 

Maljunulo

Well-known member
Not necessarily, but probably, if you are talking about the objective.

The focal length of a lens is determined by its curvature, and the refractive index of the glass (or whatever it is made of) not its diameter.

Fundamentals.
 

ZDHart

Well-known member
Supporter
I'm sorry, but you really do need to do some reading.

No, that's not what I said at all.
Perhaps you could re-state/elaborate on your your statement? To impart a little more understanding to the reader. Now, I understand you better - you're not interested in doing so.

Through this thread you seem to act as though it is beneath you to do so. In one moment, you speak as such an authority, quoting an equation. And then, when asked for the reason behind the concept, you have no clue. And then mostly you just say - go elsewhere are learn more there.

Sorry to have annoyed you.

My question was directed to Binastro. My thought was that the bins both being 8x, that they were the same focal length. That is not true, apparently.

You state that there are two different focal lengths within the binocular, one for the ocular and one for the objective. I didn't realize that distinction.

Obviously you are too busy to take any time to elaborate on that, or you just don't care to elaborate. I'm trying to understand the optic qualities better. My lack of knowledge about internal lens design seems to annoy you.

I'm sorry, but I'm sure that there are many readers, some here in this thread at present, who may want to learn more about this subject and, in your view... we just need to go somewhere else to gain some more understanding.
 
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Binastro

Well-known member
Dear ZDHart,

As you are a commercial photographer I am surprised at some of your questions.
Although I appreciate that professional photography is an art as well as science.

Normally a 50mm f/2 lens is sharpest at about f/4 to f/5.6.
I suppose a really expensive lens might be best between f/2.8 and f/4.

Although some very high quality lenses are used wide open, such as monochromatic lenses producing chips. These lenses cost $100,000 or more.

An 8x32 binocular with f/4 objectives has objectives of 128mm focal length.
Eyepieces 16mm focal length.

An 8x42 binocular with f/4 objectives has objectives of 168mm focal length.
Eyepieces 21mm focal length.

The fact that the eyepieces differ so much has implications regarding complexity, eye relief etc.

A person with exceptional eyesight, 20/8 or better, might notice the difference in long focus astro scopes of 32mm and 42mm aperture at say 12x, 10x or possibly 8x.
But binoculars are so poor optically. I doubt that anybody would notice a difference between 8x32mm and 8x42mm binoculars unless boosted magnification was used.

Regards,
B.
 

jring

Well-known member
My premise in this thread is comparing two bins of nearly identical quality, Victory SF 8x42 and 8x32, differing in objective size.

So, I'm confused when you state that the focal length of the 42mm is longer than the 32mm - aren't the focal lengths (8x in both cases) the same?

Hi,

the magnifications are the same... the (objective) focal lengths are different - as can be seen by the longer 42mm instrument.

Magnification = objective focal length / eyepiece focal length

Joachim
 

Maljunulo

Well-known member
Perhaps you could re-state/elaborate on your your statement? To impart a little more understanding to the reader. Now, I understand you better - you're not interested in doing so.
I thought better of that post and edited it, before I even saw your response.

Go read it again.
 

ZDHart

Well-known member
Supporter
Dear ZDHart,

An 8x32 binocular with f/4 objectives has objectives of 128mm focal length.
Eyepieces 16mm focal length.

An 8x42 binocular with f/4 objectives has objectives of 168mm focal length.
Eyepieces 21mm focal length.

The fact that the eyepieces differ so much has implications regarding complexity, eye relief etc.

A person with exceptional eyesight, 20/8 or better, might notice the difference in long focus astro scopes of 32mm and 42mm aperture at say 12x, 10x or possibly 8x.
But binoculars are so poor optically. I doubt that anybody would notice a difference between 8x32mm and 8x42mm binoculars unless boosted magnification was used.

Regards,
B.
Generally speaking, camera lenses tend to be sharpest when stopped down two to four stops or so. As you stop down more, diffraction begins to creep in - but there is a fair bit of variation between individual lenses, different brands, different imaging formats, and different qualities/price points of camera lenses.

Typically, when creating images with cameras, I am most concerned with apertures, due to that influence on the look of the resulting image. I select an aperture for the visual effect that aperture produces in the resulting image. Usually, I have less concern for selecting an aperture purely on sharpness. In the event where choosing a specific shutter speed for visual result is primary, then aperture and ISO become more variable. Back to the topic at hand...

Binastro... I have read comments by several members of this forum, stating that bins of 42mm objective will typically have greater resolution/sharpness than bins of 32mm objective - other parameters held relatively constant.

So that got me wondering why that might be the case. Why are members here stating that you will get higher resolution, in addition to more brightness, by choosing 42mm over 32mm? Especially if whatever difference is not visible by the human eyes, as you assert?

Getting to the answer to that was my ENTIRE purpose of this thread! Calculations are much less meaningful to me than understanding the causative factors of why that might be so. I learned, elsewhere, that the difference may well be largely due to diffraction.

Your comments would indicate that there is no applicable purpose to this thread. None of us should expect to see any difference in sharpness/resolution between two "sister" binoculars (like SF 8x32 and SF 8x42) - owing to the difference in objective size.

If you are correct (and I would not deny that you are), then statements regarding 42mm objective bins having greater resolution/sharpness than 32mm objective bins, of the same magnification, are moot.

So, I guess that your comments have pretty much negated the usefulness of this thread!
 
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dries1

Member
ZDhart,

As I see it you seem to appreciate the larger aperture, however it would help that you looked a 8X42 with the 8X32 SF together, and your eyes will tell you which one you prefer. As long as I am able, I will always reach for a 8X42 over an 8X32, however others will prefer an 8X32, and folks prefer these formats for different reasons larger EP on the 8X42, light weight of an 8X32, etc. The best way is to determine your self with your own eyes.
 

ZDHart

Well-known member
Supporter
ZDhart,

As I see it you seem to appreciate the larger aperture, however it would help that you looked a 8X42 with the 8X32 SF together, and your eyes will tell you which one you prefer. As long as I am able, I will always reach for a 8X42 over an 8X32, however others will prefer an 8X32, and folks prefer these formats for different reasons larger EP on the 8X42, light weight of an 8X32, etc. The best way is to determine your self with your own eyes.
Hi dries1. Thank you. I absolutely agree that one needs to judge for oneself. If I had an 8x42SF to compare with my 10x42SF, I would certainly be doing that - perhaps to an extreme degree, with a tripod, just to see for myself.

Aside for situations where I would clearly prefer the very light weight and ocular balance of the SF8x32, which is a very enjoyable pair of bins to see with! I have no doubt that I too would typically reach for an SF8x42 over the SF8x32. My UVHD+ 7x42 and SF 10x42 have taught me how nice x42 can be.

I enjoy my 7x42 and 10x42s for what they offer. They are stunning. And I enjoy my 8x32s for their particular attributes. The surprisingly feather weight SF8x32 presents an especially stellar image, as well.

Lacking the SF 8x42 to compare with my SF 8x32, I just thought I'd ask the question of the forum. I didn't expect the responses, though. It's been like pulling teeth to try to find an explanation as to why the 42 may offer greater resolution. Didn't need to be that way, but one never knows how such queries will play out.

And still, much to my surprise, no one aside from myself has put forward any explanation as to why the x42 in this particular comparison might offer a sharper image/more resolution, than the x32.

As I mentioned previously, I think diffraction is the primary explanatory factor.


I'm done with the inquiry now, as I'm sure most visitors to this thread are, as well. ;)
 
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Hermann

Well-known member
And still, much to my surprise, no one aside from myself has put forward any explanation as to why the x42 in this particular comparison might offer a sharper image/more resolution, than the x32.
What you perceive as a "sharper image/more resolution" in a binocular is a combination of many different factors. It doesn't just depend on diffraction:

  • It's also factors like the presence or absence of stray light or glare, especially veiling glare. These affect contrast in a big way. Stray light isn't necessarily related to the size of the objective lenses, there are e.g. 8x32s that suppress stray light very well, others don't.
  • Then there's the question of ease of view - and ease of view is to some considerable extent related partly to the size of the exit pupil. The larger the exit pupil the "sharper" a binocular may seem, even in bright light. Ease of view is also related to the construction and the focal length of the eyepiece.
  • Then there are ergonomics and weight. They also affect your ability to actually "see" a sharp image. Ergonomics and weight are or course highly subjective, they depend on the shape of your hands, your strength and so on. As an aside: Note lighter isn't always better, I personally find for a steady and "really sharp" image (of an unstabilized binocular) a weight between 700 and 800 gr. ideal. And of course the steadier the image is the sharper it looks. YMMV.
This list doesn't cover all the factors.

My own take of the situation is that size matters if you want the very best ("sharpest") image. This holds true as long as the optical quality of the binoculars is exactly the same. (This isn't necessarily the case, not even in binoculars from the same series. The old Leica 8x32BA for instance was IMO "better", i.e. "sharper" than the 8x42BA.) So if you compare, say, an 8x20, 8x32, 8x42 and 8x50/8x56 the 8x50/8x56 wins even in bright light and/or if your eye pupils don't open to 7mm anymore. In fact, if the exit pupil is below ~4mm the binocular isn't even in the running, at least not in handheld views. On a tripod it may be alright, handheld it isn't.

However, using binoculars with large objectives comes at a price. So even though it may have the "best/sharpest image" with the highest resolution it may not be what you want in a given situation. I did a pretty thorough comparison years ago between an 8x32, 8x42 and an 8x56. The 8x56 had the "best", sharpest image in any light even when I had the binoculars on a heavy tripod. BUT in some circumstances even a puny 8x20 may be a better choice than an 8x32.

I sure wouldn't want to carry an 8x50/8x56 monster around for hours on end.

Hermann
 
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ZDHart

Well-known member
Supporter
What you perceive as a "sharper image/more resolution" in a binocular is a combination of many different factors. It doesn't just depend on diffraction:

  • It's also factors like the presence or absence of stray light or glare, especially veiling glare. These affect contrast in a big way. Stray light isn't necessarily related to the size of the objective lenses, there are e.g. 8x32s that suppress stray light very well, others don't.
  • Then there's the question of ease of view - and ease of view is to some considerable extent related partly to the size of the exit pupil. The larger the exit pupil the "sharper" a binocular may seem, even in bright light. Ease of view is also related to the construction and the focal length of the eyepiece.
  • Then there are ergonomics and weight. They also affect your ability to actually "see" a sharp image. Ergonomics and weight are or course highly subjective, they depend on the shape of your hands, your strength and so on. As an aside: Note lighter isn't always better, I personally find for a steady and "really sharp" image (of an unstabilized binocular) a weight between 700 and 800 gr. ideal. And of course the steadier the image is the sharper it looks. YMMV.
This list doesn't cover all the factors.

My own take of the situation is that size matters if you want the very best ("sharpest") image. This holds true as long as the optical quality of the binoculars is exactly the same. (This isn't necessarily the case, not even in binoculars from the same series. The old Leica 8x32BA for instance was IMO "better", i.e. "sharper" than the 8x42BA.) So if you compare, say, an 8x20, 8x32, 8x42 and 8x50/8x56 the 8x50/8x56 wins even in bright light and/or if your eye pupils don't open to 7mm anymore. In fact, if the exit pupil is below ~4mm the binocular isn't even in the running, at least not in handheld views. On a tripod it may be alright, handheld it isn't.

However, using binoculars with large objectives comes at a price. So even though it may have the "best/sharpest image" with the highest resolution it may not be want you want in a given situation. I did a pretty thorough comparison years ago between an 8x32, 8x42 and an 8x56. The 8x56 had the "best", sharpest image in any light even when I had the binoculars on a heavy tripod. BUT in some circumstances even a puny 8x20 may be a better choice than an 8x32.

I sure wouldn't want to carry an 8x50/8x56 monster around for hours on end.

Hermann
Good post Hermann! Thank you for that. All salient points in the discussion.

I quite agree that having some weight and size works to advantage in stabilizing the view with bins. Much as I enjoy the SF8x32, they feel so feather-weight, as compared to my 42s, that they are a bit less stable in my hands. Which, as you point out, can lead to softening the "apparent" sharpness.

Speaking of "horses for courses" (which is always applicable when comparing bins)-
Back in the late 1970s I bought a pair of Trinovid 8x20BCA bins for use when attending the San Francisco Symphony, which I enjoyed quite regularly. They were quite suitable for that application, where I would not have wanted anything any larger than those little Leicas. Great sharpness was not among the significant qualities of those bins, but they served the purpose quite well! I could carry them in a fine suit pocket with little distorting to the shape of my suit jacket. By today's standards, those Trinovids don't measure up optically... as binocular coatings and glass have come quite a long way in the last 40-45 years. I'm sure that current production Trinovid 8x20s are noticeably nicer.
 
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MCLV

Member
As I mentioned previously, I think diffraction is the primary explanatory factor.

This is definitely not the case. Diffraction has no effect in the case that is discussed due to two reasons:
1) Telescopes with exit pupils sizes of 3 mm or larger are definitely not diffraction limited. Decrease of sharpness gets observable when one gets to exit pupil size of 1-1.5 mm or lower.
2) If we assume daylight observation with pupil size of 3 mm, then both 8x32 and 8x42 binoculars would effectively work as 8x24 instruments since it is the diameter of eye pupil and not diameter of objective lens that is the limiting factor. Both binoculars would have the same theoretical resolution limit due to diffraction in this case.

Hence, there is no physical limitation that would prevent the smaller binocular to achieve the same sharpness as a larger one in principle. However, then the real world happens. Real optics always exhibits some aberrations. And these aberrations gets smaller as you stop down the objective/the angle of rays gets smaller/F number of increases. And basically, you stop down the larger binoculars with your eye pupil more than the smaller ones.

Since both 8x32 and 8x42 binoculars operate at the same effective aperture if the eye pupil is smaller than exit pupils of both instruments, the objective lens of 8x42 binoculars is working effectively at higher F number than the objective lens of 8x32 ones (assuming both were designed with same F number when using full aperture).

This means that the 8x42 binoculars perform better in the real world not because they have larger diameter but because they are longer.
 

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