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ZEISS DTI thermal imaging cameras. For more discoveries at night, and during the day.

Consideration of twilight factor in Zeiss AK optics (1 Viewer)

S

SeldomPerched

Well-known member
This is a general question but I'm hoping it's OK to post it on the Zeiss forum as this particular brand caters particularly well for low light users with its A-K prism models, many of which have been discontinued but remain in wide circulation.

How much does the twilight factor hint at the comparative suitability of different specification binoculars for low light use? For instance a 7x42 Dialyt BG/AT*P(*) has a t/f of 17.15; so does of course a 7x42 Victory T*FL whereas the 8x56 T*FL sports a t/f of 21.17 and the 10x56 T*FL which has a smaller exit pupil than the 8x version just mentioned nevertheless has a higher t/f of 23.7 because of the higher magnification combined with the same 56mm objective diameter.

I can see the way the maths works but if we now factor in limitations on eye flexibility for the older user does the twilight factor still provide a clue to which binoculars will provide the most detail to that user in practice as the light is failing? And then again there are differences in transmission values and if those range between - say for sake of argument - 93% and 95% will that difference be enough to affect the order of the instruments' effectiveness in giving us the detail in the dull/twilight conditions?

(As an aside and for what it's worth - which is probably nothing - I sense more is apparent to me at dusk through an 8x42 HT than the 8x56 FL used in rapid succession. In the day time the HT gives me a washed, brighter view though the larger FL has the easier, more relaxed one.)

So after completing this post I'm wondering if there's a good reason why twilight factor is something that hardly gets mentioned in most posts I've seen regarding light gathering performance and the picking out of detail.

Tom
 
Last edited by a moderator:
Perhaps twilight factor was a better indicator back when lens coatings were not so good. And as you hint at, the calculation doesn't take into account AK prisms or HT glass. Some folks (including me) have also pointed out that a 42x7 binocular will give the same t/f result as a 7x42. And these days we know that people differ in how their eyes/brain combo reacts to light consisting of different balances of colour content. I think if folks think about t/f at all it is only as a rough and ready indicator and not as an infallible guide.

Lee
 
SeldomPerched said:
How much does the twilight factor hint at the comparative suitability of different specification binoculars for low light use?

I can see the way the maths works but if we now factor in limitations on eye flexibility for the older user does the twilight factor still provide a clue to which binoculars will provide the most detail to that user in practice as the light is failing?
Click to expand...

Here's a link to an article by Holger Merlitz that covers some of these aspects. I find figures 5 and 6 particularly relevant: http://www.holgermerlitz.de/bino_performance/bino_performance.pdf
 
Mark9473 said:
Here's a link to an article by Holger Merlitz that covers some of these aspects. I find figures 5 and 6 particularly relevant: http://www.holgermerlitz.de/bino_performance/bino_performance.pdf
Click to expand...

Thank you for this, Mark. When I started to look at the article I despaired of its complexity but the sections you mention (fig.5 & 6) made clear sense to me and were something I'd not seen or at least understood in the past.
Holger's examples of the large camouflaged boar vs. the small white mouse and also the visibility of a tiny candle flame helped a lot.

Tom
 
Troubador said:
Perhaps twilight factor was a better indicator back when lens coatings were not so good. And as you hint at, the calculation doesn't take into account AK prisms or HT glass. Some folks (including me) have also pointed out that a 42x7 binocular will give the same t/f result as a 7x42. And these days we know that people differ in how their eyes/brain combo reacts to light consisting of different balances of colour content. I think if folks think about t/f at all it is only as a rough and ready indicator and not as an infallible guide.

Lee
Click to expand...

Yes, that makes sense, Lee. And unless I'm imagining it my limited experience so far suggests that the AK systems really do give me better and extended viewing in dullest daytime conditions and at twilight. A 42x7 - I can't imagine what sort of shape that would have - like a long metal rod maybe?

Tom
 
Let's not get carried away here - an A-K prism can only be worth 1~1&1/2 odd % extra transmission or so over a dielectric mirror S-P system. Any claims beyond that are only so much blue pixie dust ! :cat:





Chosun :gh:
 
SeldomPerched said:
Yes, that makes sense, Lee. And unless I'm imagining it my limited experience so far suggests that the AK systems really do give me better and extended viewing in dullest daytime conditions and at twilight. A 42x7 - I can't imagine what sort of shape that would have - like a long metal rod maybe?

Tom
Click to expand...

A 42x7 would be ludicrous and offering it as part of the discussion is a round-about way of saying taking the twilight factor as an infallible guide is ludicrous too.

Lee
 
You do not have to dive deep in mathematic formula to get a little insight in the behaviour of light in prisms.
The difference between AK-prisms and porro prisms?
The amount of light passing through those prisms is dependent on the amount of optical glass light waves have to travel; that means how many cm of optical cm and how much light is absorbed during these cms. Optical glass is highly transparant so that will not be a lot. Reflections on air-glass surfaces are more serious causes of light losses, but in AK prisms to my knowledge only 4 perfect reflections occur just as in porrosystems. Schmidt-Pechans need more reflections and moreover light losses will take place if no dielectric mirror is present on one light leaking prism surface.
Gijs van Ginkel
 
Seldom Perched,

While the twilight factor has passed into legend it seems strange that it is so widely misunderstood.

Two Zeiss scientists, Köhler and Leinhos published their study on the use of binoculars in 'twiilight' in 1955, the year after a similar study by Berek from Leitz that Holger Merlits explores further in his publication. The two studies looked at different aspects of binocular performance. Zeiss, the effective acuity and Leitz the contrast threshold for detection. Obviously the data they produced only applied to the set of binoculars used for the study. In the Zeiss study it was 12 binoculars ranging from 6x to 15x and objective diameters from 24mm to 50mm. Both studies found that magnification alone was the principle determinant for their own definition of performance in the daylight or photopic range. Below this level the exit pupil becomes an increasingly important component factor. Köhler and Leinhos found that the results between 10^ -1 and 10^-2.5 apostilb approximated to the formula we know as the twilight Factor. Unfortunately, I suspect few, even in those days, understood what an apostilb was, but the authors helpfully point out this is actually below the level of typical moonlight. What we call the twilight factor might more helpfully termed the moonlight factor. .

I suspect for the large majority, birdwatching is essentially a daylight activity. Most head for home as soon as it becomes impossible to determine the colour of plumage clearly. The so called twilight factor is irrelevant. Unfortunately none of those major studies by Köhler and Leinhos, Berek, and Blackwell in the US considered the ability to distinguish colour important at the time.

David
 
Chosun Juan said:
Let's not get carried away here - an A-K prism can only be worth 1~1&1/2 odd % extra transmission or so over a dielectric mirror S-P system. Any claims beyond that are only so much blue pixie dust ! :cat:
Chosun :gh:
Click to expand...

Chosun,

Whatever the figures, I don't think it's my wishful thinking that in very dull/cloudy/rainy conditions towards sunset and beyond - with the sun often therefore invisible - I have noticed the binoculars that pick out the shadowed detail best, for instance looking past tree trunks into a wood, are all AK-prism kit of various magnifications (two different 7x42, an 8x42, and an 8x56). So even if the transmission difference figures max out at 1.5% Zeiss are giving me something perceptible that I'd be as happy to see from the S-P brigade.

In brighter conditions I can't say I see any differences for certain in either direction between any of the glass.

Tom
 
SP,

The available light can be heavily blue shifted when it's "very dull/cloudy/rainy conditions towards sunset and beyond" and the binocular's transmission at shorter wavelengths becomes increasingly important.

David
 
typo said:
Seldom Perched,

While the twilight factor has passed into legend it seems strange that it is so widely misunderstood.

Two Zeiss scientists, Köhler and Leinhos published their study on the use of binoculars in 'twiilight' in 1955, the year after a similar study by Berek from Leitz that Holger Merlits explores further in his publication. The two studies looked at different aspects of binocular performance. Zeiss, the effective acuity and Leitz the contrast threshold for detection. Obviously the data they produced only applied to the set of binoculars used for the study. In the Zeiss study it was 12 binoculars ranging from 6x to 15x and objective diameters from 24mm to 50mm. Both studies found that magnification alone was the principle determinant for their own definition of performance in the daylight or photopic range. Below this level the exit pupil becomes an increasingly important component factor. Köhler and Leinhos found that the results between 10^ -1 and 10^-2.5 apostilb approximated to the formula we know as the twilight Factor. Unfortunately, I suspect few, even in those days, understood what an apostilb was, but the authors helpfully point out this is actually below the level of typical moonlight. What we call the twilight factor might more helpfully termed the moonlight factor. .

I suspect for the large majority, birdwatching is essentially a daylight activity. Most head for home as soon as it becomes impossible to determine the colour of plumage clearly. The so called twilight factor is irrelevant. Unfortunately none of those major studies by Köhler and Leinhos, Berek, and Blackwell in the US considered the ability to distinguish colour important at the time.

David
Click to expand...

David,

Interesting history. I might be a daylight birder/nature observer turning into someone curious to see how long I can keep making out details after the colours have started to fade! What is the meaning of the ^ symbol in your apostilb range?

Tom
 
typo said:
SP,

The available light can be heavily blue shifted when it's "very dull/cloudy/rainy conditions towards sunset and beyond" and the binocular's transmission at shorter wavelengths becomes increasingly important.

David
Click to expand...

David,

Does this suggest that Zeiss as well as having AK prisms with higher transmission also quite independently of that are weighted towards shorter wavelengths? Or that AK prisms are in themselves going to bring about that weighting anyway?

To be honest, I am happy just to use what works for me but wanted to make the point that the AK-equipped binos are the ones that do the best job for me in low light.
 
SeldomPerched said:
David,

What is the meaning of the ^ symbol in your apostilb range?

Tom
Click to expand...

Tom,

Unfortunately I don't have the ability to do superscript on this keyboard, but I meant, 'to the power of', and probably should have used E instead of ^. In normal decimals it was between 1 and 0.0032 apostilb, or roughly 0.3 to 0.001 cd/m2 in current SI units if that helps? Possibly useful for badger or owl watchers?

David
 
Tom,

The primary advantage of AK prisms, HT glass and the latest improvements in lens and prism coatings in poor light conditions is to improve transmission at shorter wavelengths.

David
 
SeldomPerched said:
David,

Does this suggest that Zeiss as well as having AK prisms with higher transmission also quite independently of that are weighted towards shorter wavelengths? Or that AK prisms are in themselves going to bring about that weighting anyway?

To be honest, I am happy just to use what works for me but wanted to make the point that the AK-equipped binos are the ones that do the best job for me in low light.
Click to expand...
Well I ain't gonna tell ya what you see ! :brains: but all of the reasons myself, Gijs, and David have given you are the reasons you find (particularly the HT's) working for you.

So that's a little bit down to the 100% internally reflecting prisms, a little bit down to the HT glass, and a littler bit down to the coatings on these models which Zeiss have tuned to perform in low light conditions :cat:

If you look at the transmission chart for the Swarovski 10x42 SV on Allbinos you will see that it performs in the purple/blue even better than the Zeiss 10x42 HT. I suspect the Swaro is using a little bit of HT glass or equivalent too. Either of those may be your best bet and a lot lighter than lugging the 10x56 FL around ...... :h?:

When your eye's ability to discern colour is gone though, it really is picking nits (or stacking BB's as Bill is want to say ! :) ...... only of any practical use if you are viewing Unicorns (or Pixies ! :) after dusk - when I suspect control of the heart rate will be a much bigger factor anyway ;)





Chosun :gh:
 
Chosun Juan said:
Well I ain't gonna tell ya what you see ! :brains: but all of the reasons myself, Gijs, and David have given you are the reasons you find (particularly the HT's) working for you.

So that's a little bit down to the 100% internally reflecting prisms, a little bit down to the HT glass, and a littler bit down to the coatings on these models which Zeiss have tuned to perform in low light conditions :cat:

If you look at the transmission chart for the Swarovski 10x42 SV on Allbinos you will see that it performs in the purple/blue even better than the Zeiss 10x42 HT. I suspect the Swaro is using a little bit of HT glass or equivalent too. Either of those may be your best bet and a lot lighter than lugging the 10x56 FL around ...... :h?:

When your eye's ability to discern colour is gone though, it really is picking nits (or stacking BB's as Bill is want to say ! :) ...... only of any practical use if you are viewing Unicorns (or Pixies ! :) after dusk - when I suspect control of the heart rate will be a much bigger factor anyway ;)





Chosun :gh:
Click to expand...

Haha, Chosun.. all good stuff! The 10x42 EL SV you mention is one I tried out but ended up getting the 10x50 instead; do you know if the blue/purple performance you mention applies equally - maybe more - to that?

One thing I've found to do with a point you make, and it's something Chuck told me before I realized it for myself:

It's actually the 8x56 FL I have (not the 10x56) and the interesting point is I have a far steadier image from that than from my more recently acquired 10x50 EL FP. The thing is that though heavier I find the 8x56 giving me a FAR less shaky view. The lower magnification trumps the heavier weight in other words. It's not a glass I'd lug round for a hike but well within manageability for more static work especially with a tree or window etc to lean against. I'm not going to give up on the 10 x 50 though, at least not without a lot more practice first; it is a fantastic device and produces great sharpness from near to far.

AK or otherwise I have equal time for all the makes in my small arsenal of glass: L and N for their look and easy view; S for incredible sharpness; and Z for brightness and wide FOVs on the 7s and the 8x32. Each has a different colour slant which I particularly notice on things like red berries and autumn / fall leaves; each appeals to me, just as different films or digital film emulations have their appeal from Fuji, Kodak, (once upon a time) Agfa, etc.

This makes me think - I know we are getting away from brightness here but one thing leads to another! - that one day it might be that digitalized binoculars would include, as does a very highly rated professional camera series by Fujifilm, different colour, and contrast etc emulations that could be switched at will to suit a viewers colour preferences and the prevaling conditions. Now I'm not saying that would be a good or desirable thing but with some thought a mixture of highlight, shadow, sharpness, colour tone, contrast, and other controls could be applied. Not saying I want that but it's a thought! As with digital cameras there would be features that would largely go unused by each user but looking at the whole ownership worldwide I bet someone somewhere would find a handy use for at least one feature.

Thanks for listening if you got this far... ;-)

Tom
 
SeldomPerched said:
Haha, Chosun.. all good stuff! The 10x42 EL SV you mention is one I tried out but ended up getting the 10x50 instead; do you know if the blue/purple performance you mention applies equally - maybe more - to that?

One thing I've found to do with a point you make, and it's something Chuck told me before I realized it for myself:

It's actually the 8x56 FL I have (not the 10x56) and the interesting point is I have a far steadier image from that than from my more recently acquired 10x50 EL FP. The thing is that though heavier I find the 8x56 giving me a FAR less shaky view. The lower magnification trumps the heavier weight in other words. It's not a glass I'd lug round for a hike but well within manageability for more static work especially with a tree or window etc to lean against. I'm not going to give up on the 10 x 50 though, at least not without a lot more practice first; it is a fantastic device and produces great sharpness from near to far.

AK or otherwise I have equal time for all the makes in my small arsenal of glass: L and N for their look and easy view; S for incredible sharpness; and Z for brightness and wide FOVs on the 7s and the 8x32. Each has a different colour slant which I particularly notice on things like red berries and autumn / fall leaves; each appeals to me, just as different films or digital film emulations have their appeal from Fuji, Kodak, (once upon a time) Agfa, etc.

This makes me think - I know we are getting away from brightness here but one thing leads to another! - that one day it might be that digitalized binoculars would include, as does a very highly rated professional camera series by Fujifilm, different colour, and contrast etc emulations that could be switched at will to suit a viewers colour preferences and the prevaling conditions. Now I'm not saying that would be a good or desirable thing but with some thought a mixture of highlight, shadow, sharpness, colour tone, contrast, and other controls could be applied. Not saying I want that but it's a thought! As with digital cameras there would be features that would largely go unused by each user but looking at the whole ownership worldwide I bet someone somewhere would find a handy use for at least one feature.

Thanks for listening if you got this far... ;-)

Tom
Click to expand...
👍
Sometimes the Allbinos transmission charts can be a bit strange :cat: with a few anomalous results in there. The 10x50 SV doesn't show the same purple/blue transmission results as the 10x42 SV. Part of that would be the amount of glass in there - not sure if test vagaries also are a factor. Regardless our own Gijs Van Ginkel also has a swag of transmission test results available which show more consistency :t:

Agree that the Swaro 10x50 SV is a super nice bin (it just happens to hold very steady for me).

A few more things I should mention while I am here:-
* Glare control is also an important factor in preserving contrast, and I think the HT's do a little better than the SV's here (as do larger Exit Pupils in general)
* Higher Total Transmission also plays a part. The higher the transmission the less losses in the system (reflection, absorption, transmutation) and so the less stray light bouncing around needing controlling.
* As you have said steadiness of the view is also a factor. For the most part magnification rules significantly, but it must be usable (dof also plays a factor here - degree of difficulty rising as magnification does). This leads me to mention the Canon 10x42 IS.

I have been a vagrant visitor only to the bino forums the last couple of years so I'm not sure if there has been a specific Canon 10x42 IS vs 10x42 HT low light shootout. The Canon has Porro II prisms so will also be 100% internally reflecting. If there was such an animal as this comparo then perhaps Chuck was the one to wrangle it ? :cat:




Chosun :gh:
 
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