Zeiss low light binoculars question (2 Viewers)

The question I'd like to ask could apply to any make of binoculars but as the low-light binos I have are both Zeiss it feels right to post in this forum.

I have 7x42 Dialyt BGAT*P* and 8x56 Victory FL. Let's compare these as both have Abbe-König prisms; we could include the 7x42 Victory FL also but I have never had or used that pair of binos. All I am trying to do with this choice is keep other parameters apart from magnification factor and objective lens size as similar as possible.

My question is: is the faster speed -- I don't know the correct terminology here as I am more used to working with camera lenses, so I mean 'fast' in the sense of letting in more light -- of an 8x56 compared to a 7x42 likely to be lost through ageing of the human eye? Is there just one or are there several factors to be considered here? I have had differing answers from casual enquiries in a few shops, so I appeal to this forum's experience.

If we now include some non-Zeiss binoculars in the discussion, for instance Leica 7x42 Ultravid HD plus (I haven't got or used these), can different design features widen or narrow the difference in visibility between 7x42 and 8x56? I'm thinking here that SP prisms might increase the difference, while coatings and HT glass might work in the other direction.

Theoretical and practical answers welcome!

Tom

I like to think about a telescope (or binocular) in very cartoonish terms as a light funnel. The wide end of the funnel is the objective (sometimes called the entrance pupil of the telescope) and its area determines how much light is collected. The narrow end of the funnel where the light comes out of the eyepiece into your eye is called the exit pupil. The diameter of the exit pupil is the diameter of the objective divided by the magnification. I believe that the exit pupil may be what you are looking for when you describe "speed."

In this cartoon we are ignoring all the real world problems of an optical system, but it points out a few interesting things:

• Bigger objective gathers more light.
• Higher magnification increases area of image of an extended object.
• Surface brightness of an extended object image is light collected divided by area of magnified image. So surface brightness will be the same for two instruments if the exit pupil is the same.

This last point is part of what makes exit pupil an interesting specification. However there are many caveats including:

• If the entrance pupil of you eye is smaller than the exit pupil of the telescope, then not all the light gets into your eye even if the "funnel" is optically perfect.
• If there are any blockages in the telescope between the objective and your eye, then the exit pupil will be smaller and/or not fully illuminated.
• If there is scattering or absorbtion of light in the instrument you will lose light and your image with be distorted and/or lose contrast.
• No instrument is perfect.

One last important point is that while a 6x30 and a 10x50 have the same 5mm exit pupil, the larger binocular is presenting any extended object in the field of view at larger image scale with the same surface brightness (more light) and our visual ability to distinguish detail and small contrast differences improves with increasing image scale.

Hope this helps,
Alan

You won't realize the full advantage of the 56mm aperture unless your eye opens to 7mm, but whatever your entrance pupil may be in any light the extra magnification of the 8x56 will always give it a larger effective aperture. For instance, if your pupil is open to 5mm the 7x42 will be stopped down to 35mm and the 8x56 will be stopped down to 40mm. In that situation points of light like stars will be brighter in the 8x56 while extended objects will have the same surface brightness in each, provided the prism type, number of elements and the transmission of coatings and glass are identical.

Henry

If your eye pupil is able to dilate to 7 mm the 8x56 might look brighter in low light than the 7x42. 7x7/6x6 gives a 36% larger aperture for the 8x56. The extra light will help with contrast detection in low light.

There are charts on maximum eye pupil size vs age as it tends to decrease with age but it's only statistics. You have measure it to know.
http://calgary.rasc.ca/telescopes/index.htm

Extra magnification might also be useful as the resolution of the eye decreases in low light. But I think it's not that significant in low light as the eye resolution can drop a lot more than the extra 14% magnification the 8x will give you. But it depends on light level.

As important are coatings, glass and prisms-types etc., they might increase transmission as follows*:

HT-glass: 2-5% (higher value in the blue/violet part of spectrum)
AK-prisms: 3-5% (vs Schmidt-Pechan)
Good coatings 5-10% (latest vs older coatings)
Porro-Prisms: 5% (vs Schmidt-Pechan)

High blue transmission (@500nm) seems to be extra important in twilight conditions.
https://en.wikipedia.org/wiki/Scotopic_vision
https://en.wikipedia.org/wiki/Purkinje_effect

*There might be other opinions on this. Feel free to correct me.

Probably of more importance in really low light is the person's eyes, particularly the chemical effects as one adapts to the low light.
This takes longer as one ages, and is less effective.

Ones eyes also become less efficient with age, but would be same with different binoculars.

All the posts are very helpful. I am going to need to re-read them, some of them more than once or twice, but they are excellent reference texts. The 'cartoonish' funnel description was the best possible way to get started too. Yes, by 'speed' I think 'exit pupil' was what I meant.

Good to know that the 8x56 will have its advantages; I like it very much but obviously there can be times when the weight will have me wondering.

Tom

Tom, you may find this thread relevant:
https://www.birdforum.net/showthread.php?t=358393

According to Holger Merlitz´ book (p.79), your pupil will on average only open to the 7mm your 8x56 offers if you are below age 20. For age 65, average open pupil is 4.75mm.

Secondly, in twilight there is the Stiles-Crawford effect diminishing the effective exit pupil size.

Comparing an AK Zeiss glass (in my case the 7x42 Victory FL) to the SP glasses of other manufacturers - yes the difference in brightness is notable even and especially in bright daylight. The Ultravid + is way darker, and the Nikon EDG even more so. I compared them all at the same time.

Moreover, AK glasses tend to be much sharper.

The effect of HT glass is IMO not very important but I never directly compared the FL to the HT.

If you have a good sample of an AK Victory FL, that should be - except viewing comfort and edge performance - a state-of-the-art glass.

SeldomPerched said:

The question I'd like to ask could apply to any make of binoculars but as the low-light binos I have are both Zeiss it feels right to post in this forum.

I have 7x42 Dialyt BGAT*P* and 8x56 Victory FL. Let's compare these as both have Abbe-König prisms; we could include the 7x42 Victory FL also but I have never had or used that pair of binos. All I am trying to do with this choice is keep other parameters apart from magnification factor and objective lens size as similar as possible.

My question is: is the faster speed -- I don't know the correct terminology here as I am more used to working with camera lenses, so I mean 'fast' in the sense of letting in more light -- of an 8x56 compared to a 7x42 likely to be lost through ageing of the human eye? Is there just one or are there several factors to be considered here? I have had differing answers from casual enquiries in a few shops, so I appeal to this forum's experience.

If we now include some non-Zeiss binoculars in the discussion, for instance Leica 7x42 Ultravid HD plus (I haven't got or used these), can different design features widen or narrow the difference in visibility between 7x42 and 8x56? I'm thinking here that SP prisms might increase the difference, while coatings and HT glass might work in the other direction.

Theoretical and practical answers welcome!

Tom

Click to expand...

Tobias, post 8,
In a paper published in Night Vision 1987 the result of the investigation of many persons was published in the age range 5-90 years old. In the age range 15-20 years of age pupil size varies between 6-8,5 mm, for 65-70 year old the size varies between 4-6 mm, so averages are useful but if you want to know how your eyes perform in low light you must investigate it.
Gijs van Ginkel

Tobias Mennle said:

Comparing an AK Zeiss glass (in my case the 7x42 Victory FL) to the SP glasses of other manufacturers - yes the difference in brightness is notable even and especially in bright daylight. The Ultravid + is way darker, and the Nikon EDG even more so. I compared them all at the same time.

Click to expand...

Tobias,

In bright daylight your pupils are probably 2-2,5 mm diameter. Wouldn't they dilate a little to compensate for the "way darker" UV HD+ and EDG?

Considering that the human eye has a dynamic range of up to one hundred trillion to one, or 46,5 f-stops, that differentiation is somewhat ambitious!

John

Where does the one hundred trillion come from?
Presumably 10 to the 14th power.

The brightest the eyes can tolerate is the level of a zenithal full moon or magnitude minus 12.5. The faintest stars seen may be magnitude 7.5. So 20 magnitudes or 10 to the 8th.

The Sun is magnitude minus 26.7 so with magnitude 7.5 stars this gives a range of 34.2 magnitudes or less than 10 the 14th, but with a stretch 10 to the 14th.
However, looking directly at a full zenithal Sun will destroy central vision in less than a minute, and seriously damage it in 10 seconds.
But one will retain, hopefully, side vision so indeed a full range will still be available, but with permanently damaged central vision.

As to pupil size, 9mm is not unknown in youngsters.
An astronomer reported 3mm maximum pupil size at age 70.
An optician 7mm at age 65.

Binastro said:

Where does the one hundred trillion come from?

Click to expand...

Here:- https://en.wikipedia.org/wiki/Human_eye#Dynamic_range

John

Thanks John.

I think that some of that wikipedia article is misleading.
Indeed, one can detect extremely bight light levels for a fraction of a second, but it is harmful to the eyes for longer periods.

I will have to calculate how bright their candelas per square metre is.
But I think it could harm the retina if concentrated, and not only central vision.

Also it is incorrect to say that exposure to light completely negates dark adaptation.
If the light exposure is very brief, even white light, it has little effect.

P.S.
I looked further at the 10 to the 8th candelas per square metre, and it states 'Possible retinal damage'
In fact, I think that prolonged exposure to that bright a scene would cause actual retinal damage.
This would also be cumulative damage.

I would not look at anything that bright for more than 1 second.
One needs a scene to be much less bright for eye safety.

Although the discussion is of low light level binocular performance, the post is also concerned with visual performance.

The Wikipedia article kindly mentioned in post 12 seems to be missing some important points.
The absolute low level detection of illumination is over a wide area, not just point source magnitude 7.5 stars.
So similarly, one would expect for consistency, a wide area of illumination at 10 to the 8th candelas per square metre.
What is missing from the article is the size of the illuminated scene.

This reminds me of an Arthur C. Clarke story about a south American football match where one side did not like the referee. Presumably a zenithal Sun. Each member of that side had a full size mirror, and at an appointed signal they all reflected the Sun onto the referee.
There was a bang and a cloud of smoke and no referee.

I'm not sure as to how long this would actually take, but at 1/10th the Sun's surface illumination, a person subjected to this amount of radiation would quickly become a gone person.

A while ago a forum member was kind enough to give me his old Sekonic light meter so I thought I'd check out how my visual impression compared to meter readings.

Early this morning the sun was shining directly on the rear of my white painted garage. The masonry paint had obviously yellowed a bit but the woodwork paint still looked reasonably fresh even though it was at least ten years old. I then placed a newly washed cotton sheet alongside for comparison. To my eyes the cotton sheet was clearly the brightest white. If that was 100% I would have said the woodwork was 90% and the masonry somewhere in the low 80s. My wife ranked them the same but thought the differences were larger. The meter gave quite a different answer. From the same range and angle the cotton sheet was 18500cd/m2, the wood paint 21500 and the masonry 17000.


If the woodwork had 22% higher luminance than the cotton sheet why did I think the sheet was 10% brighter? When the two were separated by a few feet the difference seemed mostly brightness, but when placed side by side the sheet was an obvious colour difference. The sheet was relatively blue and the wood paint yellow, but not nearly so yellow as the masonry paint.

When the different surfaces were viewed in isolation the pupil of the eye adjust the retinal light level independently as well so making luminance estimation between different views very difficult. There is still information on the depth of contrast and colour within the view which give clues to the strength and spectral distribution of the both the illuminating light, reflectance, and colour of the target. The net result was the cotton sheet looked brightest. The interesting thing was how pronounced the difference in the wood paint and the cotton sheet was side by side. The sheet was obviously a blue white and the wood paint yellow white. This was much more pronounced when viewed separately. The sheet still had 22% lower luminance but still appeared brighter.

Of course there is nothing new about bluer looking brighter. They were adding blue enhancing additives to the weekly wash in my Grandmother's day and I imagine a long time before that. Colour evidently trumps luminance when it comes to brightness perception. The eye is quite incapable in detecting small differences in luminance between different views as it would when comparing binoculars. There are several scientific studies that show the eye can spot a 2% difference in luminance within the same field of view, when the colour is unaltered, but would be impossible when viewed separately. The only explanation is that a HT appears brighter because it's bluer and a UV+ might seem darker than a FL due to the higher red content.

I've yet to find readily understood scientific explanation of how the physiology and psychology works, but there is a lot of interest in the role of melanopsin photoception. It is believed to have a mechanistic role in pupil dilation and brightness perception. I've not found anything that it is the principal agent involved in these kind of binocular comparisons, but it does have a peak sensitivity in the blue.

David

Many thanks for that enlightening post, David.
If enough people read it, it might put an end to some of the esoteric discussions on BF, but I'm not optimistic!

John

David,
How do you guess that trio will read on the meter, if it still works,
and will appear to you, at dusk and at night?
Better of course, would you be able and like to try that out?
[Don't worry about neighbors, they probably already have interesting
theories about you ]
Thanks! Adhoc

Had a look at 5:30 this morning. The sun was up but the garage was still in the shadow of a line of trees. The sky was a cloudless blue and consequently all three targets appeared blue as well as you might expect. The cotton appeared relatively brighter than the comparison in full sun. The meter readings, were about a 30th of the full sun levels but interestingly the luminance ratios were almost identical.

David

typo said:

A while ago a forum member was kind enough to give me his old Sekonic light meter so I thought I'd check out how my visual impression compared to meter readings.

Early this morning the sun was shining directly on the rear of my white painted garage. The masonry paint had obviously yellowed a bit but the woodwork paint still looked reasonably fresh even though it was at least ten years old. I then placed a newly washed cotton sheet alongside for comparison. To my eyes the cotton sheet was clearly the brightest white. If that was 100% I would have said the woodwork was 90% and the masonry somewhere in the low 80s. My wife ranked them the same but thought the differences were larger. The meter gave quite a different answer. From the same range and angle the cotton sheet was 18500cd/m2, the wood paint 21500 and the masonry 17000.


If the woodwork had 22% higher luminance than the cotton sheet why did I think the sheet was 10% brighter? When the two were separated by a few feet the difference seemed mostly brightness, but when placed side by side the sheet was an obvious colour difference. The sheet was relatively blue and the wood paint yellow, but not nearly so yellow as the masonry paint.

When the different surfaces were viewed in isolation the pupil of the eye adjust the retinal light level independently as well so making luminance estimation between different views very difficult. There is still information on the depth of contrast and colour within the view which give clues to the strength and spectral distribution of the both the illuminating light, reflectance, and colour of the target. The net result was the cotton sheet looked brightest. The interesting thing was how pronounced the difference in the wood paint and the cotton sheet was side by side. The sheet was obviously a blue white and the wood paint yellow white. This was much more pronounced when viewed separately. The sheet still had 22% lower luminance but still appeared brighter.

Of course there is nothing new about bluer looking brighter. They were adding blue enhancing additives to the weekly wash in my Grandmother's day and I imagine a long time before that. Colour evidently trumps luminance when it comes to brightness perception. The eye is quite incapable in detecting small differences in luminance between different views as it would when comparing binoculars. There are several scientific studies that show the eye can spot a 2% difference in luminance within the same field of view, when the colour is unaltered, but would be impossible when viewed separately. The only explanation is that a HT appears brighter because it's bluer and a UV+ might seem darker than a FL due to the higher red content.

I've yet to find readily understood scientific explanation of how the physiology and psychology works, but there is a lot of interest in the role of melanopsin photoception. It is believed to have a mechanistic role in pupil dilation and brightness perception. I've not found anything that it is the principal agent involved in these kind of binocular comparisons, but it does have a peak sensitivity in the blue.

David

Click to expand...

Hi David,

Not following BF much, and just very briefly skimming along, but I've always been curious as to what was behind the amendments to the CIE Standard with regard to increased blue weighting.
In the Wiki page on Luminosity Intensity https://en.m.wikipedia.org/wiki/Luminous_intensity you might try digging through the source research for the ammendments:
Judd–Vos 1978 modified data[c 3]*(dashed),
and the Sharpe, Stockman, Jagla & Jägle 2005 data[c 4]*(dotted).
Looks like book references, though there is some minor information in the abstracts. Over to you Sherlock


Chosun :gh:

Tobias Mennle said:

Moreover, AK glasses tend to be much sharper.

Click to expand...

I'm not sure whether that's your observation or Holger's, but in any case I'd be curious why this would be so. (Assuming comparable phase coatings etc)