Low light performance (1 Viewer)

Hi All,
Another quick question.
All things being equal (i.e.lense and coating quality) which has the greater effect on low light performance.. A larger objective lense or a lower power magnificaton? Essentially I'm looking at 7x50 or 8x56 and wondering which will be better in the twilight hours. Thanks!

Afterthought: What exactly determines a set of binoculars depth of field. It seems porro's have the lead where this is concerned and as usual it got me to wondering. Off topic I realize but I didn't want to flood the place w/ questions.

Best calculation I've seen (other than square of exit pupil) is square root of multiplication times aperture. So holding either constant and one variable should have equivalent diffs.

7x50 = 18.7 "twilight factor". 8x56 = 21.

However, that 8x is going to be heavier and the difference in 19 and 21 in "stops of light" will be small. I'd go for lighter and more compact in this decision. Also, age factors in. If over 40 or especially 50, you may only be able to take advantage of a 5 or 6 mm exit pupil.

This is one thing that despite all the fun calculations, only you can answer by experience. Hopefully others have more experience. My experience says go light. What you can hold you will use

Matt

Lower power, regardless whatever else, almost never shows more. I think the "twilight factor" is pretty good. I like to watch owls as night falls. My favorite two binos for this are an 8x42 roof that only transmits 82%, and a huge 7x50 porro that transmits 95%. Sure, the 7x50 looks brighter, but the extra 1x brings the roof right neck and neck with the "night glass" until it is actually, bona fide, all the way "NIGHT". Then, the 7x50 is merely real bad, and the 8x42 is quite terrible, or, pretty much, who cares, time to go home.
Ron

Things get complicated in low to very low light. One's age and entrance pupil diameter (dilated pupil diameter) certainly factor in. My eyes max out at slightly under 6mm so 7x50s, for the 7.1 exit pupil size, don't give me an advantage. But Ron's right that more mag always shows more information and disproportionately raises twilight factor, which I think is a more useful number than simple "brightness factor". Of course high mag has it's price to be paid in the form of shake, field of view, and depth of field.
Unless you're using the bins in the very lowest of light, an 8x50 or if you can hold them, a 10x50, will give higher twilight performance.

Read Alexis' post here:
http://www.birdforum.net/showthread.php?t=111513

From ASTRONOMICS website:

Relative Brightness
A number used to compare the brightness of binoculars or spotting scopes of similar magnification. The relative brightness is determined by squaring the diameter of the exit pupil. The larger the relative brightness number, the brighter the image.

Twilight Factor
A number used to compare the effectiveness of binoculars or spotting scopes used in low light. The twilight factor is found by multiplying the size of the objective lens (in mm) by the magnification and then finding the square root of that result. The larger the twilight factor, the more detail you can see in low light. A twilight factor of 17 or better if usually required for reasonable low light use.

I'm thinking there has to be a point of diminishing returns for twilight factor. By this logic, a 15x42 (if such a thing exists) would have more detail in low light than a 10x42 or 8x42. I'm thinking that point of diminishing returns is at exit pupils smaller than one's pupil entrance.

I've used bins for astro and for very low light. Lower magnification is easier to focus and contend with. 7x50 is very popular for astro (since Astronomics was mentioned). Higher magnification concentrates more light, but at low light, it's still better to have more depth of field to facilitate focusing.

See the graph. I took some magnifications and did two main sets (plus a couple other common magnifications). The two main sets are "holding magnification constant" and "holding objective constant". I show both twilight performance and relative brightness.

Look at the 8x binoculars. 8x42 , 8x50 and 8x56 are all 18-21. Look at the 42mm binoculars. 7x42, 8x42 and 10x42 range from 17-20. It doesn't look to me like magnification has a disproportionate impact on low light performance. In the real world, I'll take easier to focus and exit pupil at or greater than my own pupil. With low light, the strategy is to be able to see shapes and motion. Easier with a 7x than 8x all things being equal. That still makes the 7x50 the winner for the OP.

Matt

Yup, it's definitely time to start writing this stuff down. Either that or hook up 2 or 3 more computer screens so as to access all this info. w/out having to flip back and forth. I plugged in the numbers for a 10x50 and they seemed to balance as well as the 10x42's. Just a little higher up. I like that.
It's funny you should mention the law of diminishing returns. I was perusing the ebay binocular offerings and found some numbers that confused me. Exit pupil=Objective/magnification right? Well I found some with their magnifications larger than their objectives...? Does that invert the view requiring another mirror/lense or will your head implode when viewing w/ the focus set to infinity?
I've gotta say, trying to figure out which bino's to get is almost as much fun as being out in the field.... Kind of.... well maybe not... it is fun though.
Still wondering where the depth of field gets defined. Or is that another thread?

Your exit pupil calc is correct. I think there are situations where the manufacturer has different exit pupil than objective/mag when either the mag is not quite even or maybe the bins are internally stopped down as with some cheap models. I think with any decent models, you'll find it's objective/mag.

BTW, I went through an exercise recently trying to observe depth of field to apply written theory to my uses. You can read the escapades here. My observations were consistent with the consensus that magnification alone has the biggest impact on depth of field.

Bear in mind a lot of people are using the "3D" effect interchangeably with depth of field. Depth of field is about what's in focus within a given range. 3D effect you get from porro prism binoculars with the objectives further apart to give the sense of 3D. I tested a good range of porro and roof prisms and I didn't find depth of focus to vary due to that prism type.

It's bewildering at first to look at binoculars and the specs alone don't guarantee performance.

Matt_RTH said:

I'm thinking there has to be a point of diminishing returns for twilight factor. By this logic, a 15x42 (if such a thing exists) would have more detail in low light than a 10x42 or 8x42.

Click to expand...

Matt,

It does. I've tested this with a pair Nikon 8-16x40 zoom binoculars. In low light there is more detail visible at 16x than at 8x or 10x. The image is much dimmer but smaller details can be discerned in spite of that. Twilight Factor is supposed to account for that effect. If a bright looking image is what you want then go for an exit pupil that matches the dilated eye, but you will see more detail with higher magnification even if the image appears much darker .

Henry

Always with the compromise.

Henry, I wish I could test that but really, to the OP's point, what is "low light performance"? Dim/detailed or brighter? Shake, depth of field, field of view and contrast would all suffer at higher mags, wouldn't they? I'm not being stubborn, I think my original statements still stand in terms of practical use.

Yippeekiay - All things being equal both objective diameter and magnification in low light performance count. This evening was one of those exceptionally clear days in central Montana. No wind or clouds. As the sun had already set, I took my 8x32 Fls and Cascade porro 8x42 for a comparison. I was looking directly to the west with the mountain ridge line still distinguishable, but the small town below me was nothing but shades of gray with an occasional street light or window contrasting the darkness. Now to me anyway, this is the most severe test to subject a binocular to for seeing into the shadows with no reflection other than ambient light. How did the binoculars compare?
Well, the Cascades clearly outperformed the Fls. I could see more detail. Evidently that extra 10mm of objective lens creating a a 5.25 pupil as opposed to the 4mm pupil of the 8x32 does make a difference. What this suggests to me is that a inexpensive porro (but with quality optics) can compete with the best. And the price differential is like 5:l. with the Zeiss costing over $1000. Leupold will probably discontinue the Cascade porro sometime. Too good for the money. If a user can deal with a smaller FOV, the Cascade
has to be among the best buy out there. I don't have a Zeiss FL 8x42 to compare against, and would like to hear from someone having both binoculars tested under the same difficult situation. John

Matt_RTH said:

I'm thinking there has to be a point of diminishing returns for twilight factor. By this logic, a 15x42 (if such a thing exists) would have more detail in low light than a 10x42 or 8x42. I'm thinking that point of diminishing returns is at exit pupils smaller than one's pupil entrance.

Click to expand...

I think the key is to maintain a certain amount of exit pupil though it doesn't have to be equal to one's dilated pupil. Twilight Factor seems to be relevant as it pertains to commonly used sizes of binoculars. When one uses the formula with theoretical extremes I think the numbers lose meaning. A 20x30 will yield a TF of 24.5 but with an with an exit pupil of 1.5mm it won't be much of a low light bin.

LOTS of great stuff going on here! I'm only 1/4 of the way through the "escapades" and my computer time is already up. Thought I'd write up another quick but hearty Thanks for the information. And hopefully, though I'm not a physicist, or even very scientific, I'll have some solid observations to contribute rather sooner than later. Perhaps what I appreciate the most (and this is by no means an easy thing for me) is a renewed detrmination to learn this math stuff. Long story...

BTW am really leaning towards the Vortex razor 8.5x50 now. Maybe a 10x later on but for now it seems a good balance.