Thread: Vintage Binoculars

Originally Posted by BobH

I do not know where this square root formula came from but the light going to your eyes from the ocular lenses comes through an opening, exit pupil, measured in mm. The larger the opening the more light gets to your eyes all else being equal but they are not. Poorly made binoculars with sub standard optical coating will not transmit the same amount of light as a properly made binocular with superior optical coating regardless of both having the same exit pupil. Add to that your eyes may not, if you are older, dilate wider than 5mm to make use of the extra light supplied by having an exit pupil larger than 5mm.

7X50 binoculars have an exit pupil of 5mm arrived at by dividing the size of the objective lens by the magnification. The 25X100 binocular has an exit pupil of 4mm which is less light that the 7X50. A 10X100 binocular would have an exit pupil of 10mm which not even a young person's eyes could dilate wide enough to use all the light being delivered. That is also twice the amount of light an older person's eyes could accommodate.

Bob

That's all very interesting, and my apologies for taking the thread in another direction. I just thought that maybe there might be some known examples. Maybe it's a typo, but 7x50 binoculars would have an exit pupil of around 7mm in following your formula, no?

I can't remember the source, and I'm away from my books, but the square of the power ideally being less than the diameter of the lens in mm may have been mentioned in Patrick Moore's Exploring the Night Sky with Binoculars. And yes, as I recall, it has to do with allowing the maximum amount of light into the eye for viewing certain star clusters, for example, which would otherwise remain invisible at the increased magnification of the binoculars being used.

Having a large exit pupil is about eye relief. If you wear glasses you will realize that.

Most binos come in just a few ranges. 7x50 are considered marine glasses and always have been while 8x30 or 6x40 and similar are considered field glasses. They make others for birding and they make specialty models for the military used on ships which are similar to astronomical glasses and mounted on tripods.

Binos are not commonly used for deep space viewing that's what a telescope is for. Most use them to catch movement of near earth objects where you need the larger field of view.

Originally Posted by Brontosaurus

That's all very interesting, and my apologies for taking the thread in another direction. I just thought that maybe there might be some known examples. Maybe it's a typo, but 7x50 binoculars would have an exit pupil of around 7mm in following your formula, no?

I can't remember the source, and I'm away from my books, but the square of the power ideally being less than the diameter of the lens in mm may have been mentioned in Patrick Moore's Exploring the Night Sky with Binoculars. And yes, as I recall, it has to do with allowing the maximum amount of light into the eye for viewing certain star clusters, for example, which would otherwise remain invisible at the increased magnification of the binoculars being used.

Yes, you are absolutely correct the 7X50 binos would have approximately a 7mm exit pupil. Yes it was a typo, I had meant it to be 7X35.

The square root thing still has me puzzled though. Remember I am not into using binos for star gazing. From reading, the higher power binos will let you discern more individual objects that are farther away in a crowded sky. There can be a difference between the maximum amount of light a bino can provide and what your eyes can accommodate. If you are under 30 and your eyes can dilate to 7mm then a bino having a 7mm exit pupil is giving you the max amount of light you can practically use. If you are older your eyes may not be able to dilate to 7mm but only to 5mm, for example, so that extra 2mm of light provided by the 7mm exit pupil is wasted. It all depends on the individual user's eyes as to what the maximum amount they can use is.

It has been interesting reading up a bit on star gazing binos so it has been an educational direction for me.

Bob

Originally Posted by thebigspendur

Having a large exit pupil is about eye relief. If you wear glasses you will realize that.

From what I have read, eye relief is down to how the objective/eyepiece lens is constructed and not exit pupil size.

Bob

Originally Posted by BobH

Yes, you are absolutely correct the 7X50 binos would have approximately a 7mm exit pupil. Yes it was a typo, I had meant it to be 7X35.

The square root thing still has me puzzled though. Remember I am not into using binos for star gazing. From reading, the higher power binos will let you discern more individual objects that are farther away in a crowded sky. There can be a difference between the maximum amount of light a bino can provide and what your eyes can accommodate. If you are under 30 and your eyes can dilate to 7mm then a bino having a 7mm exit pupil is giving you the max amount of light you can practically use. If you are older your eyes may not be able to dilate to 7mm but only to 5mm, for example, so that extra 2mm of light provided by the 7mm exit pupil is wasted. It all depends on the individual user's eyes as to what the maximum amount they can use is.

It has been interesting reading up a bit on star gazing binos so it has been an educational direction for me.

Bob

I have since looked up the pupil on Wikipedia, which claims a maximum range of 9mm in exceptional cases. So if 7mm is the maximum in normal cases (to say nothing of aging or using belladonna), then that would sort of explain why 7x50 would appear to be the default--and I've always been curious as to why. Handheld, I can stabilize it, but it's much easier to use it with a tripod. From the thread, I've learned that 6x40 and 5x30 binoculars exist, so these might be more applicable for more discreet hand-held night-sky gazing.

Originally Posted by Brontosaurus

I have since looked up the pupil on Wikipedia, which claims a maximum range of 9mm in exceptional cases. So if 7mm is the maximum in normal cases (to say nothing of aging or using belladonna), then that would sort of explain why 7x50 would appear to be the default--and I've always been curious as to why. Handheld, I can stabilize it, but it's much easier to use it with a tripod. From the thread, I've learned that 6x40 and 5x30 binoculars exist, so these might be more applicable for more discreet hand-held night-sky gazing.

I finally found a reference to square root here https://www.astronomics.com/binocula...cations_t.aspx . Here also http://www.backcountrychronicles.com...light-factors/ . A little too technical for me though.

I'm sure my eyes at 64 will dilate a maximum of 5mm at night so anything bino that gives between a 4mm and 5mm exit pupil will work for me at night. During daylight they are more than good enough as your pupils will not be dilated. The other two things that may be as important as how bright a bino is would be a flat field of view out to the corners of the lens and how good the lens coatings are. For those to be really good you usually have to pay some money to get.

I have a pair of Nikon 8X32 SE bino that are my all arounders and with an exit pupil of 4mm they do pretty well at night for my aged eyes. Also a larger Fujinon 8X40 BFL binos that will give me all the light I can use at night having a 5mm exit pupil.

Bob

I used a pair of Zeiss porro-prism 10x50s for a long time. Fantastic glass but heavy.

I got a good deal on a pair of Swaro SLC 10x42s a few years ago and eBayed the old Zeiss' off. Wow, people pay A LOT of money for old Zeiss glass.

The Swaros are very nice along with being a little lighter and a little more compact. Big deal when packing through the hills and scanning for elk.

I could almost see the astronauts when the shuttle was going by the last time

Originally Posted by BobH

From what I have read, eye relief is down to how the objective/eyepiece lens is constructed and not exit pupil size.

Bob

"Eye relief is the distance from the outer surface of the eyepiece lens to the position where the exit pupil is formed (eyepoint). Looking through binoculars from the eyepoint, you can obtain the whole field of view without vignetting."

So a larger final lens will allow you to look at the image and with eyeglasses it makes viewing very easy.

Back in my college days I used to build newtonian reflector Telescopes.

Originally Posted by kelbro

I used a pair of Zeiss pro-prism 10x50s for a long time. Fantastic glass but heavy.

I got a good deal on a pair of Swaro SLC 10x42s a few years ago and eBayed the old Zeiss' off. Wow, people pay A LOT of money for old Zeiss glass.

The Swaros are very nice along with being a little lighter and a little more compact. Big deal when packing through the hills and scanning for elk.

I could almost see the astronauts when the shuttle was going by the last time

Zeiss is THE preeminent name in optics of all kinds and folks will pay a kings ransom for especially the older stuff from WWll era.

Originally Posted by thebigspendur

"Eye relief is the distance from the outer surface of the eyepiece lens to the position where the exit pupil is formed (eyepoint). Looking through binoculars from the eyepoint, you can obtain the whole field of view without vignetting."

So a larger final lens will allow you to look at the image and with eyeglasses it makes viewing very easy.

Back in my college days I used to build newtonian reflector Telescopes.

Argue with these guys then

Binoculars: Eye Relief or Birding Binoculars Q&A: Is exit pupil related to eye relief?

Yes, that is the definition of eye relief but the amount of eye relief depends on what design the eye piece is. If you are an eye glass wearer you need a minimum eye relief distance to be able to use the binos wearing glasses. Some binos just do not meet that requirement. Fortunately most do these days. I don't wear eye glasses so it is not that critical for me.

Bob