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Archive 2015 · Loss of light due to lens coating absorptions

Oscarsmadness
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p.1 #1 · p.1 #1 · Loss of light due to lens coating absorptions

We love lens coatings because they combat glare, flare, and other optical "junk." However, I'm suspicious that these coatings are eating or reflecting a not-negligible amount of light.

I've shot with minimally coated Nikon lenses from the days of film, and I have used today's industry-standard 24-70/2.8G, the 70-200/2.8G VR, and other lenses featuring like technologies. I often find that my old, minimally coated lenses produce brighter images than the modern, enhanced lenses using the same exposure parameters.

Does anyone else notice this, and if so, how much are you noticing it? Does anyone have an idea of how much light (in actual numerical values) the modern lens coatings are absorbing or reflecting?

Notes:
1. Some of you may know that cinema lens terminology favors the t-stop, which is similar to the f-stop.The t-stop describes the amount of light transmitted through the lens. Here is a very short explanation of the t-stop: "T-stops are the f-stop of the lens corrected for its absorbance and reflectance. The T-stop is the true speed of the lens, calculated by compensating for its light absorbance and reflectance" (copied from http://www.lavideofilmmaker.com/cinematography/f-stops-focal-length-lens-aperture.html). If we take a superbly coated Nikon lens and assign t-stops to it, would the t-stops be very different from the f-stops?
2. Since my experience with Canon, third party, and alternative lenses is very limited, I'm posting this under the Nikon forums. But feel free to draw from your experience with other manufacturers.
3. The advantages of coatings are outside the scope of this topic. I want to know specifically about absorbance and reflection.

Oct 01, 2015 at 01:56 AM

Lance B
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p.1 #2 · p.1 #2 · Loss of light due to lens coating absorptions

Actually, the reason there are multi coatings is that it actually transmits more light than without. In other words, if there weren't any coatings, you would have less light transmission due to reflections from each glass surface. Pentax quote 4% per glass element per side is reflected and thus in a 5 element design that would mean 40% loss of transmission - 5 elements and 2 per side = 10 x 4% = 40%. Apparently, they talk about .2% loss by reflection using their Super Multi Coating (SMC) technique and thus a loss of only 2% in the above example. These are older figure, I am sure the new coatings are even better.

An excerpt from Nikon's info on Nano Coating:
"Glass generally reflects 4 - 9% of the light at its surface. Since light passes through two surfaces (front and back) for a single lens, as much as 8 - 18% of the light is lost. The purpose of coating technology is to minimize this loss of light through reflection; coating has a major influence on lens performance. Coating is used not only on the elements of camera lenses but also on the lenses of eyeglasses and the windshields of cockpits of airplanes to reduce reflection and make objects easier to see through them."[/]

Edited on Oct 02, 2015 at 03:48 AM · View previous versions

Oct 01, 2015 at 03:11 AM

Two23
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p.1 #3 · p.1 #3 · Loss of light due to lens coating absorptions

I am a collector of "ancient" lenses and own lenses from every decade starting with the 1840s. Lance is correct. Modern coatings help more light to go through the lens. Think of it this way. Instead of about 10% of light bouncing off the glass in the lens, coatings act as a sort of "sponge" that help absorb the light and help it on through the glass elements. Some of my favorite lenses are uncoated--Petzvals, DAGOR, Heliar--but I like them because they give a soft, low contrast look to the image. Before coatings, lenses had to be made with very few glass elements because the passage of light through them was very inefficient. The first photography lens was the achromatic doublet and it had two elements. Second one was the Petzval with four elements. The first anastigmats had three elements, and the ground breaking Tessar had four. Once coated lenses were invented in the late 1930s, lens designs stated to become much more complex, to correct for aberrations.

Kent in SD

Oct 01, 2015 at 08:28 AM

MalbikEndar
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p.1 #4 · p.1 #4 · Loss of light due to lens coating absorptions

> Actually, the reason there are multi coatings is that it actually transmits more light than without.

Yup.

> Instead of about 10% of light bouncing off the glass in the lens, coatings act as a sort of "sponge" that help absorb the light and help it on through the glass elements.

Um. No. A single-layer coating causes a reflection at the lens-coating interface that cancels the reflection at the coating-air interface.

Oct 01, 2015 at 01:49 PM

CanadaMark
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p.1 #5 · p.1 #5 · Loss of light due to lens coating absorptions

As mentioned above, the coatings actually improve light transmission among other things. Pretty much all Nikon's lenses are SMC, but Nano takes it a step further.

Oct 01, 2015 at 02:23 PM

Keith B.
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p.1 #6 · p.1 #6 · Loss of light due to lens coating absorptions

Although you have requested the discussion be about AR coatings, it is important to remember that the number of lens elements is the big factor at play here, and that for the differences between the lenses you mention, the difference between the claimed f stop and the actual f stop is not insignificant, the difference between the f stop and the T-stop is not mentioned by the still camera lens manufacturers, and with multi element zooms the increased number of elements further drives the transmission down.
In short, the lens manufacturers lie about the f stop, and don't dare tell us the T-stop.
Your 15 element f/2.8 zoom is probably delivering T/3.5, and that's with the very best, most modern AR coatings available.

Oct 02, 2015 at 01:27 AM

Lance B
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p.1 #7 · p.1 #7 · Loss of light due to lens coating absorptions

Keith B. wrote:
Although you have requested the discussion be about AR coatings, it is important to remember that the number of lens elements is the big factor at play here, and that for the differences between the lenses you mention, the difference between the claimed f stop and the actual f stop is not insignificant, the difference between the f stop and the T-stop is not mentioned by the still camera lens manufacturers, and with multi element zooms the increased number of elements further drives the transmission down.
In short, the lens manufacturers lie about the f stop, and don't dare tell...Show more →

Strictly speaking, the f stop is just the focal length over the diameter of the lens aperture, or if you like, the ratio of the lens's focal length to the diameter of the entrance pupil. So, I don't think that you can make a broad brush statement that all lens manufacturers necessarily lie about the f stop especially as it's a bit difficult to have it exact at one end of a zoom compared to the other. You make it sound like they have all made a conscious effort to "diddle us out of our money" somehow. Yes, the T-Stop may be different to the actual f stop, but it's not a big deal in the grand scheme of things.

Oct 02, 2015 at 03:45 AM

Two23
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p.1 #8 · p.1 #8 · Loss of light due to lens coating absorptions

Oct 02, 2015 at 08:00 AM

jimmy462
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p.1 #9 · p.1 #9 · Loss of light due to lens coating absorptions

Hi Oscarsmadness,

One of the tests the folks over at DxO do on the lenses they test is for the Transmission (T-Stop) value. For instance, the 24-70/2.8G and 70-200/2.8G VR you mention in your original post come in at T 3.1 and T 3.4, (approximately -1/3 and -2/3 stops) respectively. Sources...

Nikon AF-S Nikkor 24-70mm f/2.8G ED mounted on Nikon D300 : Tests and Reviews:
http://www.dxomark.com/Lenses/Nikon/AF-S-Nikkor-24-70mm-f-2.8G-ED

Nikon AF-S VR Zoom-Nikkor 70-200mm f/2.8G IF-ED mounted on Nikon D3X : Tests and Reviews:
http://www.dxomark.com/Lenses/Nikon/AF-S-VR-Zoom-Nikkor-70-200mm-f-2.8G-IF-ED-mounted-on-Nikon-D3X__485

So, to your point about light loss...yes, light loss due to coatings and optic absorption will be specific to any particular lens design and, may or may not have to factor into one's exposure choices.

One often sees this come up in discussions on photo boards more in the realm of, oh say, something like this, "my 24-70 seems to under-expose 2/3 stops", and had the photographer checked the T-Stop rating on their Canon 24-70mm f/2.8L USM they would have discovered the actual T-value on that lens was, in fact, 3.4...just a hair off the 3.5 value they were guesstimating! Source...

Canon EF 24-70mm f/2.8L USM mounted on Canon EOS 1Ds Mark III : Tests and Reviews:
http://www.dxomark.com/Lenses/Canon/EF24-70mm-f-2.8L-USM

Also, for those unfamiliar with this topic of discussion, this video from Matt Granger offers a great primer on the matter...

F Stops vs T Stops - what is the difference? - YouTube:

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Hope this was helpful!

Oct 02, 2015 at 12:45 PM

Oscarsmadness
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p.1 #10 · p.1 #10 · Loss of light due to lens coating absorptions

Thanks guys, everything you've said is helpful. Thanks for pointing out sources like DXO and whatnot who study light transmission; I was unaware that those existed until now.

I have a theory that my old clunker (but perfectly good) lenses seem brighter because they have fewer elements. Those elements are coated, but not with the state of the art modern stuff. What do you think?

Oct 02, 2015 at 03:48 PM

JohnBrose
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p.1 #11 · p.1 #11 · Loss of light due to lens coating absorptions

I doubt they are brighter, that's why there are standards like iso/asa and f/aperture numbers so things can be consistent. Some films or digital sensors are slightly more or less sensitive to light than the expressed iso etc. but it's going to be pretty close to the given amount. Any light benefit in the fewer elements in the older lenses-if there are actually fewer elements-would be nullified by the lesser quality in the coatings and in the glass itself. I think your mind is just glamorizing the use of the older glass. There are plenty of wonderful manual lenses, I really doubt they transfer any more light though.

Oct 02, 2015 at 04:09 PM

Two23
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p.1 #12 · p.1 #12 · Loss of light due to lens coating absorptions

Oscarsmadness wrote:
I have a theory that my old clunker (but perfectly good) lenses seem brighter because they have fewer elements. Those elements are coated, but not with the state of the art modern stuff. What do you think?

Yes, I would agree. Modern lenses can have something like 18 glass elements. All of that takes a toll. Consider this as well: the Nikon 24-70mm f2.8 has 15 elements. My vintage 1845 Ross Petzval (6 inch FL) has 4 elements and is also f2.8.

Kent in SD

Oct 02, 2015 at 04:12 PM

Oscarsmadness
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p.1 #13 · p.1 #13 · Loss of light due to lens coating absorptions

and the new Nikon 24-70mm F2.8E has 20 elements! I'm sure it's brilliant, but not exactly in the way I am interested [pun intended]. can't wait for DXO to test that one...

Oct 03, 2015 at 04:36 PM

johnctharp
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p.1 #14 · p.1 #14 · Loss of light due to lens coating absorptions

Light loss due to number of elements isn't going to be static; the lens design itself is going to affect this number, as will vignetting depending on how exposure is measured.

Further, note that coatings do reject some unwanted light, reducing flaring; this is subject and lighting specific, of course, but it figures in.

But I'd generally look at DxO's and PZ's measurements as listed above to get an idea of how a lens is going to perform.

And do note that while transmission does affect exposure, it does not affect depth of field calculations; also note that there may be differences in selected aperture and physical aperture, especially among lenses manufactured decades apart, due both to differences in manufacturing tolerances and in wear and tear.

Oct 03, 2015 at 04:52 PM

MalbikEndar
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p.1 #15 · p.1 #15 · Loss of light due to lens coating absorptions

>and the new Nikon 24-70mm F2.8E has 20 elements!

Not fair to compare zooms and primes.

> my old clunker (but perfectly good) lenses seem brighter

And why exactly would this matter? I care about DOF, resolution, etc. etc. A minor increase in exposure- even if necessary- is no big deal these days with really excellent sensor performance at higher ISOs.

If you just LIKE having an older lens, or find its characteristic aberrations more appealing, that's cool. But you won't find much to criticize about the optical performance of lenses that benefit from computer ray-tracing, modern coatings, and better glasses.

Oct 03, 2015 at 06:24 PM


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