does size matter? (of exit pupils)

[Takis]

Hello from a new member,

I just bought a Zeiss 8x30 B/GA (on sale) and am very happy with it (though the pouch is silly). Now, I read that its exit pupil (30/8=3.75) is relatively small and that a larger exit pupil, e.g. on an 8 x 42 (=5.25) would offer better performance in low light conditions, but not otherwise. This is supposed to be related to my eyes' pupils, if these have a maximum opening of 5mm (allegedly related to age), then larger than 5mm is of no use. Two things worry me about this:

a) Looking through my small binos the image always appears just as bright as with the naked eye, be it in broad Greek sunlight or at night. (This was not the case with my previous cheap binos.)

b) Why should the pupils of my binos and of my eyes 'match'? After all, I don't put the one on the other (the pupils of the binos are optically produced inside and those of my eyes are at least 1 cm back).

One explanation: The larger binos (e.g. 8 x 42 vs. 8 x 30) are always brighter. However, in daylight conditions, my eyes' pupils close as much as necessary to show an average brightness. At dawn, they open to max and the difference in brightness shows. (This would be like wearing very light sunglasses; you don't notice it during the day, but it gets dark at night.) - If this is true, the maximum opening of my pupils is irrelevant.

However, in low light, my eyes could operate with less opening of the pupils on a larger bino, which would result in a sharper image.

Hoping for enlightenment,

Takis

P.S.: Just saw that http://www.birdforum.net/showthread.php?t=13550 does have something on this, esp. Jonathan on no. 12., who seems to come to the same conclusion. Is this a settled matter and do we agree that the story "large binos for old age are useless" is a myth?

[Tero]

Well, for 10x, I am definitely going to go with 10x42 over 10x30. With 8x, the smaller ones may be OK.

[Takis]

Well, for 10x, I am definitely going to go with 10x42 over 10x30. With 8x, the smaller ones may be OK.

So, you think there is a lower limit to exit pupil size in a bino (around 4mm), below which it becomes a problem?

And you agree that maximum pupil size of the viewer is irrelevant?

[Tero]

Not sure of any of that. Just empirically tested a few models which told me to stay away from 10x30, besides complaints of smaller depth of field etc. The Exit pupil is just one of many factors. But we all love bright binoculars with wide fields and so steady anyone can hold them without shaking.

I was just trying to get some more responses from our resident experts since nobody replied.

[Jane Turner]

I find that the size of the exit pupil is an issue only in "difficult" conditions i.e. Low light/high shake.

Personally I am a fan of a big exit pupil - I use 8x42,s but I also have a pair of 8x20s for travelling. These perform just as well in good conditions.

[henry link]

There is one misconception in your original post which may be confusing you. The exit pupil of a binocular does not form "inside" as in your statement labeled "b)". It forms behind the eyepiece at a distance that corresponds to the binocular's eye relief. This is where you place the pupil of your eye. You can see the bright sharp circle of the exit pupil if you place a white card behind the eyepiece. Move it back and forth until you see the smallest sharpest circle of light, that is what falls on your pupil. It's actually a tiny focused image of the objective lens. If this circle is larger than the pupil of your eye then some of it will fall on the iris and never enter the eye. That reduces the useable exit pupil and effectively reduces the aperture of the binocular. So if you eye is closed to 3mm in daylight an 8x56 binocular effectively becomes an 8x24. If your eye only opens to 5mm in the lowest light that same 8x56 will effectively become an 8x40.

[FrankD]

And you agree that maximum pupil size of the viewer is irrelevant?

In terms of brightness maybe but I would think that one also has to consider the "comfort level" (for lack of a better word) that a larger exit pupil provides. I guess what I am trying to relate is that a larger exit pupil makes it easier for you to move your eye around to examine the image in front of you. In other words if I want to look at something in the upper right hand corner of the image then with an exit pupil of 5 mm or larger I can just move my eye to focus on that point. With a 4 mm or smaller exit pupil I actually have to swing the binocular so that the center of the image is focused on whatever object I am trying to pay attention to.

[Tero]

Frank, I never move my eye that much, hardly at all. Always the binocular moves. It is good to have a wide field to note activity in the periphery, but I catch it always immediately to the center.

[Martin_O]

When talking about brightness it's primarly the exit pupil and aperture we diskuss.
I'm also use bins for astromomical use.
The most common size is 15x70. It will give you an EP about 4,6mm. Another common size is 11x70. That size will give you an EP about 6.4mm. This EP is not useful if you don't use illegal stuff |8.|
When I use my 25x100 I will get an EP about 4mm, but with its big aperture it collect so much light that it's hard to use in daylight.

/Martin

[postcardcv]

In terms of brightness maybe but I would think that one also has to consider the "comfort level" (for lack of a better word) that a larger exit pupil provides.

I totally agree. I use a 7x42 and an 8x32 and see very little difference in brightness except for in very extreme conditions (most of the time my 8x20 are fine too). However the 7x42s are the most comfortable to use, I figured that the larger exit pupil made eye position less critical, and therefore making them easier to use.

[Tero]

I think it has more to do with the details of the eye piece construction. In a small binocular, 8x25, the eye piece is small too. A few small ones have comfortable eye pieces and cups, ProMaster for instance.

[Takis]

Thanks for the replies. I tried the white card thing Henry suggested and, yes, the "exit pupil" is really behind the lens, about where my own pupils should be when looking through. (So, this is really a very different thing from my pupils or the iris diaphragm of a camera.) Understood this way, it's really a miracle that my pupils should match these tiny spots of light.

So, I gather the two 'pupils' really do match. And the old notion that an exit pupil larger than that of my eye is useless is correct. (An exit pupil that is larger than the pupil of my eye does not make the image brighter, but it makes the binoculars easier to use, easier to avoid these black rings intruding from the edges.)

I did not really understand Martin's comment. Are you suggesting that aperture determines brightness, irrespective of magnification and thus exit pupil? (Keeping glass & construction constant.)

[Swissboy]

(An exit pupil that is larger than the pupil of my eye does not make the image brighter, .....)

And from what I understand, as we get older, we can't open our pupils that much any more. That is at least one reason why we need more light. I, for one, am always amazed how my son and his family can live without keeping the lights turned on in their darkish (to me) hallway. But to get back to the binoculars, it means that the only benefit I gather these days from my old 7x50 "nightglass" is that comfort of not having to match the two pupils (eye and optics) very precisely. When I was a teenager, I had bought those binoculars precisely for making optimal use of low light conditions when observing foxes and the like. I guess, when I find the time to do this again (I'll retire a year from now), I'll have to get myself a fine night-vision instrument now. I just hope they come down in price a fair bit in the meantime.