A tale of two lenses

[chris malcolm]

A Tale of Two Lenses by Chris Malcolm, on Flickr

In fact two binoculars, but the general principles are the same. The smaller is an 8x21, the larger an 8x56. They both give the same field of view, the same magnification, and the same image quality. The difference is in the exit pupil, what corresponds in binoculars to camera lens aperture.

Binoculars use the pupil of your eye to change aperture according to lighting conditions. In dim light the pupil of your eye gets bigger, up to a maximum of around 7mm. That's the exit pupil of the big binocular. That means you can use the big binocular in the dimmest light, and whatever your eye can see, the binocular can magnify. Whereas the exit pupil of the little binocular is 2.6mm, suitable only for bright light.

In dim light the small one is useless, but in bright light the performance of these two binoculars is the same. The small ones are very much smaller, lighter, and cheaper, and can easily be carried around all day in a pocket. So even if you wanted the big one for occasional dim light use, it could make sense to have the small one as well just because it's so much easier to carry around. And if you have the small one, do you really want to pay all that extra money just for the occasionally useful extra low light performance the big one provides?

These considerations also apply to camera lenses

 

[PINNACLE]

I sort of see your point but with a camera lens you also get shallower depth of field so better separation from background, faster shutter speed if needed, usually nicer bokeh, they are usually better made so image quality improves and they also offer extras like weather proofing.

I do not think people buy a 50mmf1.2 over a 50mmf1.8 just to use it at night or the same for a 400mmf2.8 v 400f5.6. The big brother usually offer quite a bit more including weight.

 

[sk66]

The objective lens is the entrance pupil and it's diameter generally sets the limit for the max aperture and what is transmitted through the focusing lens/eyepiece ("exit pupil")... In other words, the entrance pupil determines the exit pupil, and the max amount of light transmission.

 

[sk66]

I sort of see your point but with a camera lens you also get shallower depth of field so better separation from background, faster shutter speed if needed, usually nicer bokeh, they are usually better made so image quality improves and they also offer extras like weather proofing.

I do not think people buy a 50mmf1.2 over a 50mmf1.8 just to use it at night or the same for a 400mmf2.8 v 400f5.6. The big brother usually offer quite a bit more including weight.

A lot of variables/generalizations... IME, it is just as likely that *additional* compromises have been made in order to achieve even larger max apertures. I.e. the faster lens will have more CA/vignetting, "cat eye" bokeh, and softer edges (if not overall) at the wider apertures. And they will achieve similar IQ (sharpness/contrast/etc) at similar aperture settings. Although this may not always be the case if comparing w/ a VERY expensive lens, the differences then are usually rather subtle or even subjective.

I think the point is, if the lenses are going to be used at the (nearly) same aperture setting (for desired DOF/sharpness) then there is no real benefit to the faster lens in terms of image quality... And how many really ever make use of weather sealing? It's not going to do you much good if your body isn't equally sealed... even then a solid drenching is a risky proposition and a weather shield is advisable.

 

[HoppyUK]

I have two similar pairs. In good light, they're pretty much the same, but in low light the smaller ones are next to useless, not helped by the fact that I wear glasses. Also, the bigger ones are much easier to align with your eyes if you pick them up quickly, preferably with a longer 'eye-relief' distance. Highly recommended IMHO - if you want something smaller, I'd get a monocular with a large exit pupil.

Exit pupil is simply size of the objective lens divided by the magnification, eg 7x42 = 6mm

 

[Nod]

Like Richard and the OP, I have 2 similar pairs (well, several pairs but 2 that correspond closely to the pair under discussion!). The big ones are too cumbersome to shove in a pocket for a walk, especially if I'm already carrying a camera so I use the little ones during daylight (don't often go walking in the dark...) However, if I want to look into the night sky, I'll take the big ones out to use the extra light gathering properties.

 

[sk66]

if you want something smaller, I'd get a monocular with a large exit pupil.

I tried that...
I found the monocular much harder to place on target regardless of which eye I tried. And even though the magnification specification was the same, the image *seemed* smaller/lower resolution... IDK why either was the case; I suspect it must be down to something with binocular vs monocular vision, even w/ a 2d image (it was a Vortex model).
My small binoculars are now 10x25 Bushnell Legend Ultra HDs (highly recommended for the price, ~ $170), and my large ones are 10x42 Vortex Razor HDs (fairly expensive, ~ $1200).

Similarly (and I think more to the point of the OP), I have a 400/2.8, a 120-300/2.8, a 70-200/2.8 and an 85/1.4... OR I have a 28-300/3.5-5.6 and an 80-400/4.5-5.6. And given good conditions I can take similarly good images with any of them... that's why I own the super zooms.

 

[HoppyUK]

I tried that...
I found the monocular much harder to place on target regardless of which eye I tried. And even though the magnification specification was the same, the image *seemed* smaller/lower resolution... IDK why either was the case; I suspect it must be down to something with binocular vs monocular vision, even w/ a 2d image (it was a Vortex model).
My small binoculars are now 10x25 Bushnell Legend Ultra HDs (highly recommended for the price, ~ $170), and my large ones are 10x42 Vortex Razor HDs (fairly expensive, ~ $1200).

Similarly (and I think more to the point of the OP), I have a 400/2.8, a 120-300/2.8, a 70-200/2.8 and an 85/1.4... OR I have a 28-300/3.5-5.6 and an 80-400/4.5-5.6. And given good conditions I can take similarly good images with any of them... that's why I own the super zooms.

I'll admit that I've never owned a monocular, though I tried couple very recently and they were excellent - but everything looks good out of a shop doorway on a sunny day There must be a reason why they're not very popular and I'd guess you're on the right path with the monocular/binocular vision theory. I'll still get one sometime, as it can't be worse than a pair of those tiddlers in less good conditions.

 

[sk66]

I'll still get one sometime, as it can't be worse than a pair of those tiddlers in less good conditions.

I was hoping so... but I wound up gifting mine away.

 

[Snapsh0t]

A Tale of Two Lenses by Chris Malcolm, on Flickr

In fact two binoculars, but the general principles are the same. The smaller is an 8x21, the larger an 8x56. They both give the same field of view, the same magnification, and the same image quality. The difference is in the exit pupil, what corresponds in binoculars to camera lens aperture.

Binoculars use the pupil of your eye to change aperture according to lighting conditions. In dim light the pupil of your eye gets bigger, up to a maximum of around 7mm. That's the exit pupil of the big binocular. That means you can use the big binocular in the dimmest light, and whatever your eye can see, the binocular can magnify. Whereas the exit pupil of the little binocular is 2.6mm, suitable only for bright light.

In dim light the small one is useless, but in bright light the performance of these two binoculars is the same. The small ones are very much smaller, lighter, and cheaper, and can easily be carried around all day in a pocket. So even if you wanted the big one for occasional dim light use, it could make sense to have the small one as well just because it's so much easier to carry around. And if you have the small one, do you really want to pay all that extra money just for the occasionally useful extra low light performance the big one provides?

These considerations also apply to camera lenses

I went for the compromise 8x42.....

 

[GeeJay57]

I have a pair of Swarovski EL 8x42 SV. Not too big and as sharp and bright as I'll ever need. Bl**dy expensive too, though they amaze me every time I use them.

 

[Jelster]

I have a pair of Swarovski EL 8x42 SV. Not too big and as sharp and bright as I'll ever need. Bl**dy expensive too, though they amaze me every time I use them.

Having just invested in a Swarovski scope, I can fully appreciate the light capturing abilities from their glass. The detail I can pick up of wildlife at a distance sometimes just amazes me. Expensive? Yes, Worth it? Damned right....