light transmission efficiency vs brightness (1 Viewer)

[ksbird/foxranch]

A friend recently looked at some of the data developed to evaluate binoculars for US Gov't use. Allot of the data refers to immersibility, shock resistance etc. But of particular interest to me was the "light transmission efficiency" rating as a % of total wideband light projected through an aperture 2.5mm x 2.5mm. The ratings for the best binoculars were the Fujinon FMTR-SX and Nikon Prostar (I didn't have a Nikon model # so I don't know if this is the current ED version or previous model) at 97.4% and 97.5%. Then came many, many other binocular models like the Fujinon M-22 (96.2%). I guess the Sandia, Lawrence Livermore, Los Alamos, JPL mandate was to test just about every binocular they could find. But the highest rated roof prism model was the Swarovski SL 10x50 at 93.3%, with the Zeiss FL 10x56 T* right behind at 93.2%.

Sadly even the Steiner 7x50 Marine and Tasco Waterproof 322BW had better light transmission than the Swarovski and Zeiss (96.4% and 95%). There were many other Swaro and Zeiss models tested at magnifications ranging between 7x and 15x and none did as well as their 2 best rated models except the Zeiss 7x50 Marine BG*T at 96.6%. The Leica Ultravid 8x50BR was the best performing Leica at only 91.5% light transmission. There were dozens of binoculars that had greater light transmission efficiency than this and many of those retail at less than US$200.

Supposedly light transmission efficiency considers reflection, dispersion and diffusion in its measurement. To the extent that chromatic abberation causes light to be bent away from the direct light path it would also be a factor in light transmission efficiency, but the quality of the glass used, the fineness of the grit used in polishing, the clarity of the cement used to make cemented pairs or triplets, the number of lens surfaces and the quality and completeness of the multicoatings are the dominant factors.

I'm just wondering if this difference in light transmission would be noticible as greater brightness at dusk or dawn. There were dozens of porro prism models that rated better than the best roof prism models for light transmission. Is there a way for roof prism models to compensate for this with larger objectives? For example would the Zeiss 10x56 only be as bright as the Docter 10x50 T*? How much less useful are roof prism models on cloudy days when dawn dawns later and evening comes earlier? Obviously during the brightest sunlight of midday the lower light transmission might be unimportant, but why aren't porro prism makers mentioning this more often in their marketing, especially on models with smaller exit pupils? It would seem to be an important consideration. By the way, older model uncoated binoculars had been tested years ago and the best uncoated models had light transmission efficiency of less than 75%.

 

[kmiernik]

I'm just wondering if this difference in light transmission would be noticible as greater brightness at dusk or dawn.

You have to remember that at the end of the optical system of binoculars you put your own eye. While in typical light your eye pupil has around 5mm diameter when it's dark it could be as large as 8mm. To be more precise this number is correct if someone is below his 40's and he has healthy eyes. Older people can't get pupil larger than 5mm so for them there's no point in buying and using night binoculars with exit eye pupil larger then 5mm.

The transmission is varying in scale of one percent. Light gathering power depends on objective size in square. Difference between 40mm and 50mm is really huge: it's like 1.56 to 1. Difference between 42mm and 40mm is 10% so in poor light conditions - when your eye has a chance to get bigger then 5mm - 8x42 get 10% more light then 8x40 if both has the same light transmission factor.

Taking into account that roof prism system has to have worse light transmission (as there is at least one mirror instead of total internal reflection) I'm almost sure that is the reason of tradition of putting bigger objectives in roofs then in porros (32 mm vs 30 mm, 42 mm vs 40 mm and so on).

All in all if you take 8x40 porro with 97% transmission and 8x42 with 90% transmission you get almost the same numbers concerning total light you see at the end of the binocular (97% vs. 110%*90% = 99%). But this numbers works only in bad light conditions and good eyes with eyes pupils larger then 5mm.

 

[Pileatus]

A friend recently looked at some of the data developed to evaluate binoculars for US Gov't use. Allot of the data refers to immersibility, shock resistance etc. But of particular interest to me was the "light transmission efficiency" rating as a % of total wideband light projected through an aperture 2.5mm x 2.5mm. The ratings for the best binoculars were the Fujinon FMTR-SX and Nikon Prostar (I didn't have a Nikon model # so I don't know if this is the current ED version or previous model) at 97.4% and 97.5%. Then came many, many other binocular models like the Fujinon M-22 (96.2%). I guess the Sandia, Lawrence Livermore, Los Alamos, JPL mandate was to test just about every binocular they could find. But the highest rated roof prism model was the Swarovski SL 10x50 at 93.3%, with the Zeiss FL 10x56 T* right behind at 93.2%.

Sadly even the Steiner 7x50 Marine and Tasco Waterproof 322BW had better light transmission than the Swarovski and Zeiss (96.4% and 95%). There were many other Swaro and Zeiss models tested at magnifications ranging between 7x and 15x and none did as well as their 2 best rated models except the Zeiss 7x50 Marine BG*T at 96.6%. The Leica Ultravid 8x50BR was the best performing Leica at only 91.5% light transmission. There were dozens of binoculars that had greater light transmission efficiency than this and many of those retail at less than US$200.

Supposedly light transmission efficiency considers reflection, dispersion and diffusion in its measurement. To the extent that chromatic abberation causes light to be bent away from the direct light path it would also be a factor in light transmission efficiency, but the quality of the glass used, the fineness of the grit used in polishing, the clarity of the cement used to make cemented pairs or triplets, the number of lens surfaces and the quality and completeness of the multicoatings are the dominant factors.

I'm just wondering if this difference in light transmission would be noticible as greater brightness at dusk or dawn. There were dozens of porro prism models that rated better than the best roof prism models for light transmission. Is there a way for roof prism models to compensate for this with larger objectives? For example would the Zeiss 10x56 only be as bright as the Docter 10x50 T*? How much less useful are roof prism models on cloudy days when dawn dawns later and evening comes earlier? Obviously during the brightest sunlight of midday the lower light transmission might be unimportant, but why aren't porro prism makers mentioning this more often in their marketing, especially on models with smaller exit pupils? It would seem to be an important consideration. By the way, older model uncoated binoculars had been tested years ago and the best uncoated models had light transmission efficiency of less than 75%.

Let's see the data so we can evaluate it in toto.

 

[Pinewood]

Hello to KSBIRD/FOXRANCH,

I think that anyone would be hard pressed to notice the difference between 91% transmission and 97.6% transmission without the special instruments available at Lawrence Livermore Labs. Ceteris paribus, it might account for no more than two minutes of dusk viewing, and transmission is not the whole story. There was a recent thread by someone who thought that Porros provide a more stereoscopic, or plastic, image which helps in twilight which may be true but contrast is certainly important.
Lawrence Livermore is quite capable of measuring physical attributes like light transmission but, as Elkcub might write, we are dealing with perception which is a psychometric with a lot more variables going on. If you would increase the objective size of an 10x40 by one millimeter to 41 millimeters, which would provide about 5% more light, the twilight factor would be 20.249 only .249 more than the 10x40's twilight factor of 20. In any case, between 91.5% and 97.5%, most folks would not find transmission to be of overwhelming importance. Contrast, internal baffling, quality of the coatings in reducing reflection from behind the observer, even design of the eyepiece may be more important to the user.

In daytime and for astronomy, reflections from backlighting are very irritating to me. I have experienced binoculars from several of the manufacturers mentioned, and I have found that Leica and Zeiss do the best job of suppressing those reflections. In short, the users may have preferences beyond light transmission.

What is amazing is that the best roof binoculars now have light transmissions so close to that of Porros. The differences would have been more pronounced, only a decade, ago. It is just a simple fact that light transmission is poorer with the mirror in most roof glasses. I would have thought the Zeiss' AK prisms would outperformed the Swarovski's Schmidt-Pechan, so I am surprised at that.

As long as the bird watching market wants roof binoculars, do not be surprised that companies will market them, especially the as per unit profit is greater on the more expensive roof binoculars. Zeiss' 7x50 is a marine glass and independent focussing, otherwise Zeiss binoculars are being sold to hunters and bird watchers. With the ending of the EII line, Nikon is also catering to the lovers of roof binocualrs, who are willing to spend more for the perceived benefits of roof glasses: light in weight, slender, more shockproof, easily made waterproof and closer focussing, even at the expense of light transmission and FOV.

Happy bird watching,
Arthur Pinewood

 

[ksbird/foxranch]

I guess there are different ways to measure binocular light transmission amongst labs. This link for the Bushnell Elite claims 90% transmission for their 10x43 is the brightest in its class. This seems to go against the Zeiss test results.

http://www.opticsplanet.net/bushnell-elite-10x43-binoculars.html

Nikon and Zeiss don't seem to want to talk about their transmission specs but previous Nikon literature says the Prostar transmits 97+% of all the light entering. This would be 1/2 a db difference and should be visible. For some reason here in Kansas there can be periods of low light that can easily last for an hour after dawn and before sunset on highly overcast days.

The mention of the word "Marine" describing the Zeiss 7x50 BG*T always made me wonder. If anything, all the glare off the water makes boating or shore situations brighter than on land. If the only purpose of these binocs is to be held steady and have a wide field so they are usable on a boat, then why isn't the size more like 7x30, unless 7x50s should be used mostly at night?

We're not boaters here although we do take people to the Missouri River to see shore birds. Looking into the shadows created by overhanging trees makes a 7x50 useful but if we are in a more open area with 100 yards of open river and bright sunlight, the glare off the water can be too brutal to use 7x50s unless you have polarizing filters.

I agree about how irritating the "center halo" effect is when a bright sky near sunset or sunrise illuminates a large part of the field of view (especially when looking into strong shadows). The Hensoldt DF 8x30 has been rated as one of the best binoculars in the world in this respect, because there is a cone baffle against the prism that allows most incoming stray light to be "damped out". This stands to reason since this was a military binocular design and if you can't see an incoming hazard, it might kill you. Generally though we find that good light baffling makes a binocular "bigger" without adding much if any weight, and since most porros are already larger than most roofers there seems to be more ways to cope with extra light baffles in a porro design. Good light baffling would seem to run directly counter to making a small binocular with a roof prism design.

Hello to KSBIRD/FOXRANCH,

I think that anyone would be hard pressed to notice the difference between 91% transmission and 97.6% transmission without the special instruments available at Lawrence Livermore Labs. Ceteris paribus, it might account for no more than two minutes of dusk viewing, and transmission is not the whole story. There was a recent thread by someone who thought that Porros provide a more stereoscopic, or plastic, image which helps in twilight which may be true but contrast is certainly important.
Lawrence Livermore is quite capable of measuring physical attributes like light transmission but, as Elkcub might write, we are dealing with perception which is a psychometric with a lot more variables going on. If you would increase the objective size of an 10x40 by one millimeter to 41 millimeters, which would provide about 5% more light, the twilight factor would be 20.249 only .249 more than the 10x40's twilight factor of 20. In any case, between 91.5% and 97.5%, most folks would not find transmission to be of overwhelming importance. Contrast, internal baffling, quality of the coatings in reducing reflection from behind the observer, even design of the eyepiece may be more important to the user.

In daytime and for astronomy, reflections from backlighting are very irritating to me. I have experienced binoculars from several of the manufacturers mentioned, and I have found that Leica and Zeiss do the best job of suppressing those reflections. In short, the users may have preferences beyond light transmission.

What is amazing is that the best roof binoculars now have light transmissions so close to that of Porros. The differences would have been more pronounced, only a decade, ago. It is just a simple fact that light transmission is poorer with the mirror in most roof glasses. I would have thought the Zeiss' AK prisms would outperformed the Swarovski's Schmidt-Pechan, so I am surprised at that.

As long as the bird watching market wants roof binoculars, do not be surprised that companies will market them, especially the as per unit profit is greater on the more expensive roof binoculars. Zeiss' 7x50 is a marine glass and independent focussing, otherwise Zeiss binoculars are being sold to hunters and bird watchers. With the ending of the EII line, Nikon is also catering to the lovers of roof binocualrs, who are willing to spend more for the perceived benefits of roof glasses: light in weight, slender, more shockproof, easily made waterproof and closer focussing, even at the expense of light transmission and FOV.

Happy bird watching,
Arthur Pinewood

 

[FrankD]

Let's see the data so we can evaluate it in toto.

I would love to see it as well. Even recently I have often wondered if there was an independent lab that has reviewed light transmission numbers for the various binoculars on the market today. Though valid points have been mentioned previously I still think the comparative data would prove enlightening to some extent.

 

[Pinewood]

The mention of the word "Marine" describing the Zeiss 7x50 BG*T always made me wonder. If anything, all the glare off the water makes boating or shore situations brighter than on land. If the only purpose of these binocs is to be held steady and have a wide field so they are usable on a boat, then why isn't the size more like 7x30, unless 7x50s should be used mostly at night?

Dear KS,

The 7x50 configuration is most certainly useful at night, when the large objectives are considered quite an asset. In this day and age, radar is probably more useful than visual aids but the binocular is still considered a necessity at sea and even on the Big Muddy and on the Father of Waters.

Happy bird watching,
Arthur Pinewood :scribe:

 

[Klos]

Good info but it might as well be a question of quantity vs quality. Say, pc coating would surely bring light transmission efficiency down, but I doubt I'll ever go with ones without it.

 

[Sout Fork]

Heck when it comes right down to it at that difference in efficency a good neck strap compared to a poor one makes more real-world practical difference than any difference in brightness.

 

[henry link]

I think I probably place a higher value on high light transmission than many people do. For me high transmission is what makes the image vivid and life like. I've noticed some people actually prefer lower transmission images as more relaxing and comfortable, but those images just look dull to me. I spend a lot of time subjectively evaluating the light transmission of every optical device I own from binoculars to scopes to eyepieces and I'm always looking for hard data, so I hope ksbird will provide us with more details from the lab tests they have seen.

In the meanwhile here is a link to a test done for the Finnish military: http://www.birdforum.net/showthread.php?t=9306&page=2

Transmission figures in it are much lower than the few examples given by ksbird/foxranch for both porros and roofs. A member here was kind enough to send me data from two German tests. One from 2000 was particularly interesting in that the tests were conducted by Leica and Swarovski on the very same binocular specimens. Sometimes the measurements were in pretty good agreement, but often there were 3-5% differences in transmission for the very same binocular as measured by the two labs. There doesn't appear to be any standard method for making these measurements. The Finnish tests, for instance, used a 1mm diameter light beam rather than a 2.5mm. I've seen eyepiece tests that used a laser as a light source. Measuring equipment and protocals are probably different in every case.

Measurements of over 97% for the Nikon Prostar and Fujinon FMT-SX are unusually high and difficult to reconcile with the complexity of the designs. This would have to be the measurement of a peak in transmission and even then it's higher than the highest point on a graph of the light transmission curve of the Fujinon 7x50 FMT-SX supplied by Fuji (about 96%, supposedly as measured by JPL and even that figure seems suspiciously high to me). 97% is closer to what I've seen for eyepiece transmission alone in relatively simple eyepieces. Transmission tests sometimes use a graph representation with the light transmission plotted from perhaps 700nm to 400nm (the best way to present the data IMO), or sometimes wave lengths at which the measurements were made are specified. In the German tests two wavelengths were used, one called "Day" and the other called "Night" which I assume means 555nm and 510nm, the wavelengths to which the eye is most sensitive in daylight and in darkness. The Finnish test probably used 510nm which partly accounts for the low numbers. Every binocular suffers a loss of transmission at shorter wave lengths, sometimes dropping to below 50% at 400nm.

Here are a few more comments on the content of ksbird's original post in no particular order:

In tests using only the center 2.5mm of the binocular objective "real world" factors like optical aberrations, internal reflections and vignetting would play no significant roll in the measurements, but of course these things do effect contrast and apparent brightness for an actual observer.

I don't think transmission differences of less than about 3% are detectable by eye. At least that's my threshold in a very careful direct comparison between two optical devices. 5% is relatively easy to detect in a direct comparison and 10% is very obvious. I'm reasonably certain that fractions of 1% are invisible. There is probably a 1-3% sample variation in different specimens of the same model binocular. Since light transmission curves in optics are never straight lines we often detect differences in the transmission curves of two devices as color differences rather than brightness differences.

I think uncoated binoculars have transmission closer to 50% (a 4-5% loss at each glass to air surface). 75% is more like the transmission of binoculars with single layer coatings.

Abbe-Koenig roof prisms have light transmission similar to Porro. Both have 4 reflections and no mirror coating is required. Zeiss claims a 7% increase in transmission for AK over "comparable" Schmidt-Pechan roof prisms with silver coating. S-P's have 6 reflections with one prism face requiring mirror coating. The loss at that face may be only 0.5% with dielectric mirror coating, 3-6% with silver coating or 10-15% with aluminum. The reflection losses particularly with aluminum may be higher at short wavelengths.

If you wanted to design a binocular for maximum light transmission you would keep the design very simple and use cemented groups wherever possible. The Swarovski SL porros were a design like that (ksbird, did you mean Swaro 10x50 SL or SLC?). The 7x42, 8x56 and 7x50 SL's used a cemented objective, a cemented porroprism cluster and a simple 3 element, 2 group Kellner eyepiece. There were only 8 glass to air surfaces in the entire optical train. Compare that to 14 surfaces in the Fujinon 7x50 FMT-SX, (8 in the eyepiece alone) and 16 in the Nikon 7x50 Prostar (10 in the eyepiece). Premium roof prism binoculars are all quite complex, usually using a triplet objective (one cemented doublet, one singlet), a focusing element and an eyepiece with 3 or 4 groups for a total of 16-18 surfaces. Inexpensive porros usually have only 10-12 surfaces and the inherently high transmission of the porroprism, so if good coatings are used they can be quite bright, as bright or brighter than high end roof prisms.

 

[ksbird/foxranch]

I think I probably place a higher value on high light transmission than many people do. For me high transmission is what makes the image vivid and life like. I've noticed some people actually prefer lower transmission images as more relaxing and comfortable, but those images just look dull to me. I spend a lot of time subjectively evaluating the light transmission of every optical device I own from binoculars to scopes to eyepieces and I'm always looking for hard data, so I hope ksbird will provide us with more details from the lab tests they have seen.

In the meanwhile here is a link to a test done for the Finnish military: http://www.birdforum.net/showthread.php?t=9306&page=2

Transmission figures in it are much lower than the few examples given by ksbird/foxranch for both porros and roofs. A member here was kind enough to send me data from two German tests. One from 2000 was particularly interesting in that the tests were conducted by Leica and Swarovski on the very same binocular specimens. Sometimes the measurements were in pretty good agreement, but often there were 3-5% differences in transmission for the very same binocular as measured by the two labs. There doesn't appear to be any standard method for making these measurements. The Finnish tests, for instance, used a 1mm diameter light beam rather than a 2.5mm. I've seen eyepiece tests that used a laser as a light source. Measuring equipment and protocals are probably different in every case.

Measurements of over 97% for the Nikon Prostar and Fujinon FMT-SX are unusually high and difficult to reconcile with the complexity of the designs. This would have to be the measurement of a peak in transmission and even then it's higher than the highest point on a graph of the light transmission curve of the Fujinon 7x50 FMT-SX supplied by Fuji (about 96%, supposedly as measured by JPL and even that figure seems suspiciously high to me). 97% is closer to what I've seen for eyepiece transmission alone in relatively simple eyepieces. Transmission tests sometimes use a graph representation with the light transmission plotted from perhaps 700nm to 400nm (the best way to present the data IMO), or sometimes wave lengths at which the measurements were made are specified. In the German tests two wavelengths were used, one called "Day" and the other called "Night" which I assume means 555nm and 510nm, the wavelengths to which the eye is most sensitive in daylight and in darkness. The Finnish test probably used 510nm which partly accounts for the low numbers. Every binocular suffers a loss of transmission at shorter wave lengths, sometimes dropping to below 50% at 400nm.

Here are a few more comments on the content of ksbird's original post in no particular order:

In tests using only the center 2.5mm of the binocular objective "real world" factors like optical aberrations, internal reflections and vignetting would play no significant roll in the measurements, but of course these things do effect contrast and apparent brightness for an actual observer.

I don't think transmission differences of less than about 3% are detectable by eye. At least that's my threshold in a very careful direct comparison between two optical devices. 5% is relatively easy to detect in a direct comparison and 10% is very obvious. I'm reasonably certain that fractions of 1% are invisible. There is probably a 1-3% sample variation in different specimens of the same model binocular. Since light transmission curves in optics are never straight lines we often detect differences in the transmission curves of two devices as color differences rather than brightness differences.

I think uncoated binoculars have transmission closer to 50% (a 4-5% loss at each glass to air surface). 75% is more like the transmission of binoculars with single layer coatings.

Abbe-Koenig roof prisms have light transmission similar to Porro. Both have 4 reflections and no mirror coating is required. Zeiss claims a 7% increase in transmission for AK over "comparable" Schmidt-Pechan roof prisms with silver coating. S-P's have 6 reflections with one prism face requiring mirror coating. The loss at that face may be only 0.5% with dielectric mirror coating, 3-6% with silver coating or 10-15% with aluminum. The reflection losses particularly with aluminum may be higher at short wavelengths.

If you wanted to design a binocular for maximum light transmission you would keep the design very simple and use cemented groups wherever possible. The Swarovski SL porros were a design like that (ksbird, did you mean Swaro 10x50 SL or SLC?). The 7x42, 8x56 and 7x50 SL's used a cemented objective, a cemented porroprism cluster and a simple 3 element, 2 group Kellner eyepiece. There were only 8 glass to air surfaces in the entire optical train. Compare that to 14 surfaces in the Fujinon 7x50 FMT-SX, (8 in the eyepiece alone) and 16 in the Nikon 7x50 Prostar (10 in the eyepiece). Premium roof prism binoculars are all quite complex, usually using a triplet objective (one cemented doublet, one singlet), a focusing element and an eyepiece with 3 or 4 groups for a total of 16-18 surfaces. Inexpensive porros usually have only 10-12 surfaces and the inherently high transmission of the porroprism, so if good coatings are used they can be quite bright, as bright or brighter than high end roof prisms.

Hi all. The numerical results I listed came during a telephone call with a person who does purchasing for the old INS now called Homeland Security Immigration. The information should be available thru the US govt "freedom of information". Various companies seem to run "light transmission tests" for their own purposes fairly often

http://www.opticsplanet.net/binoculars-light-transmission-lab-test.html

The US gov't tested Nikon Prostar info was for "some" model of this binocular but I didn't ask which one. There have been about 8 different versions in the past 15 years. Nikon used to list the light transmission spec for this model and they claimed it was about 98%, but with no definition of what that means. (I did Nikon seminar scheduling for a few years.) On the other hand this seems to account for statements in places like CloudyNights.com where they claim that a multicoated 2 surface lens (cemented or not) will transmit 99+% of the light coming to it, so even with 10 surfaces there may only be 1% light loss total for the binocular system.

http://www.cloudynights.com/item.php?item_id=179&pr=2x48x62

Steiner Binoculars does not list light transmission numbers in their website but they provide retailer salespeople with sales crib sheets that say that their 8x56 Nighthunter XP and 7x50 Nighthunter XP binoculars have peak transmission of over 96% in "important visual bandpasses".

http://www.muzzleloadingskills.com/site/1003902/product/595

I did not think to ask the Purchasing agent who I had on the telephone whether there were Steiner binoculars on the US govt test list, although I should have because Steiner makes some very nice binoculars and we use some of them here on the ranch. Steiner is odd because they make a number of porro prism and roof prism basic formats and then they claim that they use different coatings on each of the final model binoculars they market based on the niche the binocular will be used in. Their highest transmission bins seem to be for night/dusk/dawn use and marine use (perhaps because binoculars are used in the middle of the night on boats?). In any case 96% light throughput for a $500 binocular seems to be very high.

Steiner uses this quote in their website

http://www.steiner-binoculars.com/news/news_commander_1.html

"The binoculars that Houghtaling uses, the German-made Steiner 7x50 Commander III, boast the highest light-gathering ability of any compass binocular, with a light transmission of over 95% and more than 99.8% per lens element, they’re built to a tough shock test (20g) and U.S. Navy waterproof specifications. The Commander III also offers a 30-year warranty against leakage or defects."

Nikon will try to send me edited copies of their own tests disputing Steiner's claims. Holger Merlitz discusses the tests for light transmission done by German and Chinese labs and Nikon and Fujinon may be two of the best makers for light transmission.

Others get more bizarre when discussing binoculars made for other governments. I don't even know who Weems & Plath are except that they make navigation equipment but they seem to supply military binoculars for naval use. They have 3 7x50 models, one with 95% light efficiency, one with 80% and one with a compass with 85% light effificency.

http://www.weems-plath.com/uploaded_files/binoculars.pdf#search='light%20transmission%20binoculars%20test'

Finally based on Steiner and Nikon's 99.8% light transmission certitude for either a 2 sided super-multicoated single lens like we would find as a separate air spaced eyepiece lens element or for one surface of a cemented lens doublet or triplet, even a lens with 14 air to glass surfaces would have 98% light transmission efficiency all other things being part of a perfect world. Assuming Nikon's losses in their best ED/crown/flint(dense) glass is similar, the 97+% figure doesn't seem to be impossible. The Schott website is replete with glass types that WITHOUT Coatings are 99% non-reflective, transmitting almost all the light as "efficient light throughput".

http://www.us.schott.com/special_applications/english/products/non_reflective_glass/index.html

there are supposed to be companies in Japan that make specialty glass for binocular makers on this page but I couldn't find them. Maybe you'll have better luck.

http://glassproperties.com/links/

My main question was whether the 5-8% difference in light transmission was obvious and possibly important. Holger Merlitz seems to incorporate one test of this genre in his comparative testing, and he often comments about how much better one 7x50 is compared to another 7x50 (you supply your own magXobj size) in terms of light transmission and says this seems very important on overcast days especially in winter when the amount of light hitting the "object viewed" is lower anyway. My friends and I seem to see a great different trying to read test paper charts (see the Jupiter Test thread for a description of this test), because we always keep the sun to our backs and at dusk, the brighter binoculars in the same category of size (eg. 10x42) ALWAYS win, even if they didn't win during the brightest times of day at 10am and 2pm (noon is considered a bad time for the letter resolution test because small curls in the paper cast shadows on various rows of letters). So now the feedback I have here in this thread says that for some people light transmission efficiency and total light transmission are important and for others it isn't. And so we seem to have made no progress.

 

[elkcub]

I think I probably place a higher value on high light transmission than many people do. For me high transmission is what makes the image vivid and life like. I've noticed some people actually prefer lower transmission images as more relaxing and comfortable, but those images just look dull to me. I spend a lot of time subjectively evaluating the light transmission of every optical device I own from binoculars to scopes to eyepieces and I'm always looking for hard data, so I hope ksbird will provide us with more details from the lab tests they have seen...

Henry,

I knew you'd pick up on this thread at some point. You're quite correct, the JND (just noticeable difference) for brightness is about 3% for the average trained observer. It would probably take a 10% difference (3+ JNDs) to make an obvious and reliable brightness distinction, ceteris paribus, as Arthur likes to say.

Personally, however, I'm not convinced that modest transmission differences are the primary determining factor for life like images. After using my Swift 804ED Audubon for some time, and comparing it with the standard 804R, several other Swifts, Swaro SLCs, and Nikons Es, my impression is that its crystal clear, life-like images result from superb control of chromatic aberration. That's the one feature that distinguishes it from all the others. My ED may or may not transmit better than the other models, I have no way to measure that, but it always seems to be brighter even when I'm not engaged in side-by-side comparisions. In other words, apparent brightness, clarity, and realism all impress me as being part of an integrated perceptual response. From what you've mentioned on other threads, I gather that your most "life like" binocular is the 8x42 FL, which may share this CA control property, and therefore produces similar perceptions.

Anyway, I'm just throwing in my two-cents again.

Regards,
Ed

 

[Leif]

I wouldn't argue with much of what has been said, except that I look for high contrast, rather than the highest transmission, though the two are coupled. High contrast helps give a natural image, though edge to edge sharpness, low CA, and low distortion all help too.

And anyway what do the labs measure? Are they peak values for each binocular, or the value at a given wavelength?

 

[Pileatus]

Henry,

I knew you'd pick up on this thread at some point. You're quite correct, the JND (just noticeable difference) for brightness is about 3% for the average trained observer. It would probably take a 10% difference (3+ JNDs) to make an obvious and reliable brightness distinction, ceteris paribus, as Arthur likes to say.

Personally, however, I'm not convinced that modest transmission differences are the primary determining factor for life like images. After using my Swift 804ED Audubon for some time, and comparing it with the standard 804R, several other Swifts, Swaro SLCs, and Nikons Es, my impression is that its crystal clear, life-like images result from superb control of chromatic aberration. That's the one feature that distinguishes it from all the others. My ED may or may not transmit better than the other models, I have no way to measure that, but it always seems to be brighter even when I'm not engaged in side-by-side comparisions. In other words, apparent brightness, clarity, and realism all impress me as being part of an integrated perceptual response. From what you've mentioned on other threads, I gather that your most "life like" binocular is the 8x42 FL, which may share this CA control property, and therefore produces similar perceptions.

Anyway, I'm just throwing in my two-cents again.

Regards,
Ed

Ed,
We should be surprised you prefer ED glass?
John

 

[Steven Astley]

You can't beat the laws of physics by putting fancy badges on

 

[elkcub]

Ed,
We should be surprised you prefer ED glass?
John

John,

I have no preference for ED glass, per se, but simply believe that exquisite life like images can result from successful efforts to correct CA. The 804ED Audubon is a good example of such success, but probably not the only one. It's got excellent apparent brightness but I doubt that it sets the record for physical light transmission.

Regards,
ED

 

[Pileatus]

John,

I have no preference for ED glass, per se, but simply believe that exquisite life like images can result from successful efforts to correct CA. The 804ED Audubon is a good example of such success, but probably not the only one. It's got excellent apparent brightness but I doubt that it sets the record for physical light transmission.

Regards,
ED

Ed,
Allow me one more try.
Ed enjoys ED glass.
John

 

[henry link]

ksbird,

Thanks for all the links. I'm afraid I didn't find anything that changed my view. We're probably just not going to be able to agree about these numbers. I would point out that I think you misread Ed's comments in the CN review. What he said was that transmission with modern multicoatings is "99+% for each surface". A conservative figure I certainly agree with. Most of the claims I've seen fall between 99.5% and 99,8% for each glass to air surface, not each group. Unfortunately the maketing folks can claim anything they want.

There are also other light losses. The light path through glass for a binocular like the Prostar is between 100 and 125mm, most of it through BAK4. There will be about a 2% loss from that alone and the reflections at the prism faces (even with TIR) will not be completely free of internal scatter. I'm sticking with 90-94% as a more realistic estimate for peak transmission in the Prostar. That's about what I see when I compare the Prostar to a simple telescope with no erecting prism which I estimate really does have about 97% transmission. The Steiner Commander is a simple binocular (10 surfaces) so it, and other binoculars with 2 or 3 group eyepieces may do a bit better than the Prostar if their coatings are up to it.

Ed,

The old fashioned way to achieve low longitudinal CA (and SA) under daylight conditions is to use a very large exit pupil binocular like a 7x50. Yesterday as an experiment I stopped down one barrel of an old Leitz 7x50 Marseptit to 35mm and examined my standard sunlight CA target (a white card on a dark background) with the magnification boosted to 35X. The 50mm f/4 barrel had plenty of LCA, but the 35mm f/6 barrel had very little, even a little less than the 8x42 FL. I'm sure you see where I'm going with this. Most of the time any ordinary 7x50 with a crown and flint objective is stopped down by the eye to less than 35mm in daylight, so there just isn't much LCA in the image. The Marseptit is very sharp and comfortable to look through, but I wouldn't describe it as vivid or life like. It probably has transmission of about 75% so the image looks a bit dull. I think the smaller the exit pupil of the binocular the more benefit there will be in daylight from using some form of ED glass in the objective. From your description I think you must have lucked into the best Swift Audubon ED they ever made. As I recall those binoculars are fully multi-coated so they probably do have very high light transmission.

Henry

 

[elkcub]

Ed,
Allow me one more try.
Ed enjoys ED glass.
John

John,
I get it!
ED

 

[Otto McDiesel]

You can't beat the laws of physics by putting fancy badges on

No, but you can win hearts and customers by designing and building with the minutest of details in mind. Binoculars are made of much more than just lenses.