Photons Missing In Action

Started Nov 24, 2009 | Discussions
Marianne Oelund Veteran Member • Posts: 7,788
Photons Missing In Action
2

This thread is all about those unfortunate photons which wander into your lens, but fail to reappear at the other end - even though they were on the right path to do so.

Over the past few months, I've been collecting data about their mission success, and compiling it for your enjoyment. Camera used for testing was the D3, and the test results give the image sensor's view of arriving photons, as represented directly by RAW file data; no RAW conversion software or firmware is involved.

The information breaks down into several categories, each of which will be discussed in a separate part to follow:

  1. Lens transmission efficiency, or T-stops

  2. Aperture control accuracy (sometimes you get more photons than expected)

  3. Lens color cast, or does photon color matter?

Apologies in advance if your favorite lens isn't represented in the set that I've tested. Not all of us are as well endowed with Nikkors as Bjorn.

It will probably take me several days to finish all of the posts, so be sure to check back again later.

OP Marianne Oelund Veteran Member • Posts: 7,788
Part 1: Lens T-stops
3

In photography, we generally use the f-stop of a lens to tell us how bright our images will be. Using f-stops to compare the brightness of our various lens options would be perfectly accurate, if it were not for variations in lens transmission efficiency. A convenient way to include the effect of light loss in a lens, is to use T-stops (transmission stops), which are equivalent to the f-stop of a lossless lens that projects the same image brightness.

Some people like to estimate this, by counting the number of air-to-glass transitions in the lens design, and applying a "reasonable" loss factor for each. As it turns out, this can be wildly inaccurate. There is no substitute for actual measurement.

All of these test results are for the green channel only, as that makes up the greatest share of the apparent image brightness. Measurements were made at the center of the image, thus do not include any vignetting effects. Red and blue channel data was also collected, but this will not be discussed until part 3, about lens color cast.

When using autoexposure, the camera's metering takes account of light losses in the lens, so this information is not relevant to metering errors or issues unless you use an external meter. It simply tells you how close your lens comes to its ideal image brightness. You will see that the values span a surprising range, from as high as 96%, down to less than 70%.

For each lens, I give the measured green-channel transmission efficiency in percent, and then the lens T-stop. For example, an f/4 lens with an efficiency of 85% has a T-stop of 4/SQRT(0.85) = 4.34, so its T-stop is written T/4.34. In making these calculations for lenses with nominal f-stops which are not integers, I used the following 3-digit values for the base f-stop: 1.41 for f/1.4; 1.78 for f/1.8; 2.83 for f/2.8; 3.56 for f/3.5.

Prime Lenses

AF DX 10.5mm f/2.8D: 85%, T/3.08
AF 14mm f/2.8D: 74%, T/3.29
AIS 28mm f/2: 83%, T/2.19
Zeiss ZF 35mm f/2: 85%, T/2.17
AF 35mm f/2D: 92%, T/2.09
PC-E 45mm f/2.8: 90%, T/2.99
AIS 50mm f/1.8: 95%, T/1.83
AF 50mm f/1.4D: 91%, T/1.48
AF-S 50mm f/1.4G: 91%, T/1.48
AF Micro 60mm f/2.8D: 83%, T/3.11
Hartblei 65mm f/3.5 T/S: 96%, T/3.62
AF 85mm f/1.8D: 81%, T/1.97
AF Micro 105mm f/2.8D: 82%, T/3.13
AF 135mm f/2D DC: 86%, T/2.16
AF-S 200mm f/2G VR: 79%, T/2.24
AF-S 400mm f/2.8G VR: 76%, T/3.25

Zoom Lenses

AF-S 14-24mm f/2.8G: 90%, T/2.99 (meas. at 20mm)
AF-S 17-35mm f/2.8D: 88%, T/3.01 (meas. at 24mm)
AF-S 24-70mm f/2.8G: 88%, T/3.02 (meas. at 50mm)
AF-S 24-85mm f/3.5-4.5G: 83%, T/4.39 (meas. at 35mm f/4 nominal)
AF-S 28-70mm f/2.8D: 80%, T/3.16 (meas. at 50mm)
AF 35-135mm f/3.5-4.5D: 72%, T/4.73 (meas. at 66mm f/4 nominal)
AF-S 70-200mm f/2.8G VR 1: 70%, T/3.38 (meas. at 200mm)
AF-S 70-200mm f/2.8G VR 1 plus TC-14: 67%, T/4.90
AF-S 80-200mm f/2.8D: 70%, T/3.39 (meas. at 200mm)
Sigma 120-300mm f/2.8: 72%, T/3.34 (meas. at 200mm)
AF-S 200-400mm f/4G VR: 61%, T/5.11 (meas. at 300mm)

I suspect that last entry may cause some shock, but yes it's true, our $6K f/4 lens is only T/5.1. As consolation, it's extremely color accurate, as you will see later.

Tom Christiansen Senior Member • Posts: 2,239
Re: Part 1: Lens T-stops

Thank you, Marianne. I'm looking forward to the rest of your study.

Your findings seem to confirm what others have suspected about the 200-400 being "dark" for an f/4 lens.

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Duckie Senior Member • Posts: 2,188
Re: Part 1: Lens T-stops

Thanks for the thoughtful post and all the hard work behind it. I do look forward to the coming instalments. I have always been complaining that some lenses are producing "broken" images. Now it can clearly be shown that some photons actually die inside the lens!

Just one question. What aperture was used for the measurement? The lenses are so different. I just wonder what if we measure all at say F5.6 and see how much they differ by.....

Needless to say I am a "photon fan" and I like light unmolested.

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Ravncat Senior Member • Posts: 1,109
Re: Part 1: Lens T-stops

Interesting
looking forward to the rest
thanks for taking the time to post your data (and do the tests)

your findings certainly confirm what I suspected about my851.8
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OP Marianne Oelund Veteran Member • Posts: 7,788
Aperture for testing

Duckie wrote:

Just one question. What aperture was used for the measurement? The lenses are so different.

All lenses were tested wide open.

I just wonder what if we measure all at say F5.6 and see how much they differ by.....

That would introduce yet another variable, which is the subject of Part 2, coming soon.

Bob GB Senior Member • Posts: 1,826
Part 1: Lens T-stops?

Marianne Oelund wrote:

AF-S 200-400mm f/4G VR: 61%, T/5.11 (meas. at 300mm)

I suspect that last entry may cause some shock, but yes it's true, our $6K f/4 lens is only T/5.1

Interesting thread of yours

The only question I have so far is if you have measured the pupil and the focal length of these lenses? I suspect that the nominal f/stop and focal length for a given lens is just approximate values from the vendor.

If the 200-400 is f/4.2 in reality your percentage would be quite different.

I know it is more work.

drip01
drip01 Veteran Member • Posts: 3,053
Any Canon lenses?

I suppose people would be interested to see these head to head:
200/2VR vs 200/2IS and/or 200/1.8
70-200VR vs 70-200IS
24-70 vs 24-70 ... etc

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jhinkey
jhinkey Senior Member • Posts: 2,814
Re: Part 1: Lens T-stops?

Bob GB wrote:

Marianne Oelund wrote:

AF-S 200-400mm f/4G VR: 61%, T/5.11 (meas. at 300mm)

I suspect that last entry may cause some shock, but yes it's true, our $6K f/4 lens is only T/5.1

Interesting thread of yours

The only question I have so far is if you have measured the pupil and the focal length of these lenses? I suspect that the nominal f/stop and focal length for a given lens is just approximate values from the vendor.

If the 200-400 is f/4.2 in reality your percentage would be quite different.

I know it is more work.

Yes, this is good stuff - very interesting. Also, maybe I missed it, but at what focusing distance was the measurements taken? Doesn't focal length and effective aperture change with focusing distance with some lenses?

John

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OP Marianne Oelund Veteran Member • Posts: 7,788
T-stops are the primary result

Bob GB wrote:

The only question I have so far is if you have measured the pupil and the focal length of these lenses? I suspect that the nominal f/stop and focal length for a given lens is just approximate values from the vendor.

The % efficiency figures are based on the nominal maximum aperture, thus as you pointed out, could change somewhat if the actual aperture differs.

However, I consider those values to be of only secondary importance. The primary data is the T-stops, which are still accurate, as that is what my tests actually measured.

I know it is more work.

Yes, and I may get around to looking at some of these, at least. If so, I will add the information to this thread.

OP Marianne Oelund Veteran Member • Posts: 7,788
Focusing distance

jhinkey wrote:

Yes, this is good stuff - very interesting. Also, maybe I missed it, but at what focusing distance was the measurements taken? Doesn't focal length and effective aperture change with focusing distance with some lenses?

Thanks for asking - I obviously forgot to mention this. All lenses were set to infinity focus for the tests.

OP Marianne Oelund Veteran Member • Posts: 7,788
Sorry, I don't have any of those (nt)

- - - - -

luxor2 Contributing Member • Posts: 612
Marianne, Lens T-stops
1

Marianne Oelund wrote:

All of these test results are for the green channel only, as that makes up the greatest share of the apparent image brightness. Measurements were made at the center of the image, thus do not include any vignetting effects. Red and blue channel data was also collected, but this will not be discussed until part 3, about lens color cast.

I tried Google to find just how these measurements are made, but no luck. Could you describe the instrumentation setup necessary to make t-stop measurements? Thanks.

OP Marianne Oelund Veteran Member • Posts: 7,788
Part 1 Addendum: 70-200 VR II
1

One of the reviewers of this new lens model showed an example of the VR II's improved brightness, so this was one of the first checks I wanted to perform on the new lens.

Rather than going back to the original test setup (it's the wrong time of day for that), I did a quick relative measurement between the VR I and VR II.

The results indicate that the new model is indeed brighter, but not by nearly as much as the reviewer had thought: Green-channel efficiency is improved to about 78%, versus 70% for the VR I. This translates to a T-stop for the new lens of T/3.19, against T/3.38 for the VR I - or about 1/6 stop better.

Another welcome improvement is better color neutrality (more about this in part 3). The red skew of the VR I has been literally cut in half, so this means a reduced red cast with better blue saturation.

noirdesir Forum Pro • Posts: 13,753
Re: Photons Missing In Action

Marianne Oelund wrote:

This thread is all about those unfortunate photons which wander into your lens, but fail to reappear at the other end - even though they were on the right path to do so.

Stupid question, where do they end up?

  • some will be reflected back out

  • some will be absorbed by the inner lens tube and baffles

  • some will be absorbed by the glass itself

  • yet some will end up on the sensor as haze, if you shoot a homogeneous target you won't be able to distinguish between those photons which stayed on course and those which took a detour (but I don't think the amount is very large in relative terms). If you measure a small bright target in front of dark background, you can minimize that error.

Robert Tolputt Regular Member • Posts: 322
Re: Photons Missing In Action

This is an interesting discussion. Lenses are just devices which select out those photons which will produce the required image. Being quantum entities, photons will take all possible paths - only some of these are usefully image forming. Lens manufacturers prefer the theoretical f-stop measure of their products' efficiency of course.

As to what happens to the "missing in action" photons - its true that most will captured by the lens itself by absorption. For those interested in the natural history of photons - have a read of Richard Feynmann's "QED" and prepare to be amazed !

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OP Marianne Oelund Veteran Member • Posts: 7,788
Part 2: Aperture Control Accuracy
2

The efficiency issue discussed in Part 1 gives us an idea of how our lenses perform relative to their nominal apertures. This factor, however, is completely compensated for by the camera's exposure metering system, so will not affect the exposures we obtain for our images.

Over time, I've noticed some surprising differences in exposure between certain lenses, when they were used stopped down slightly. The reason is inaccuracies in the aperture control linkage between the camera body and the lens diaphragm.

It's important to understand that the camera's meter calculates exposure from the light level it sees when the lens is wide open. The camera then calculates the expected light level when the lens stops down for the exposure - it does not actually measure it. Thus, if the lens doesn't stop down precisely by the expected amount, the exposure will be incorrect.

The data that I'm discussing here can only be considered accurate for my particular copies of the lenses. Yours may differ, and results can vary simply by using a lens on different camera bodies. The point of this post, is only to demonstrate how much of an effect the aperture inaccuracies can have. Often, when reading about others' difficulties with exposure, I wonder if their lens aperture control is the culprit.

It is easy to check your own lenses. Make sure you are working under a steady light source (for example, typical household fluorescent lights must not be used). Using manual exposure, take a shot with the lens wide open, and again with the lens stopped down one stop, at half shutter speed. Continue to narrower apertures in this manner, if desired. Then check the image histograms; if they are all essentially identical, your lens is accurate. If you notice significant left-right shifts, then there is an accuracy control problem, which you may want to use exposure compensation to correct.

Here are some examples of aperture errors among my own lenses:

AIS 28mm f/2: Very accurate; slight underexposure up to 0.1 stop at f/22.
Zeiss ZF 35mm f/2: Slight overexposure, about 1/6 stop across all apertures.
AF 35mm f/2D: 0.4 stops overexposure to f/8, tapering to 0.3 stops at f/22.

45mm PC-E: This one is interesting since it's the only electronically-controlled aperture Nikkor that I own. Shows 1/6 stop overexposure up to f/5.6, then gradually dropping to 1/3 stop underexposure at f/32.

AF-S 50mm f/1.4G: Very accurate at f/5.6 and f/8, but otherwise overexposes 1/4 stop to 0.4 stop.

AF Micro 60mm f/2.8D: Very accurate through f/8, then gradually builds up to 1/2 stop overexposure at f/32.

AF 85mm f/1.8D: Overexposes 1/6 stop at f/2, then 0.4 to 0.5 stop through the rest of the aperture range.

AF 135mm f/2D: Overexposes 1/2 stop through f/5.6, tapering off to 1/3 stop at f/16.

AF-S 200 f/2G: Very accurate through f/5.6, then underexposes 1/6 to 0.3 stops through remainder of range.

14-24mm f/2.8G: Very slight underexposure up to 1/8 stop at f/22, except overexposes 1/6 stop at f/8.

24-70mm f/2.8G: Underexposes 0.3 stops through entire range, except 1/8 stop at f/8.

70-200mm f/2.8 VR I: Accurate through f/8, then gradually increasing underexposure to 0.4 stops at f/22.
AF-S 80-200mm f/2.8D: Overexposes about 1/2 stop through entire range.

As you can see, the most common errors are overexposure, and can definitely be enough to affect your images. The moral: Check your lenses .

For those few lenses which exhibit a constant error across the aperture range, I'm considering experimenting with an adjustment - but I'll need to check that their behavior is consistent between camera bodies first!

atigun Contributing Member • Posts: 828
Re: Part 2: Aperture Control Accuracy

Should not the exposure be correct wide open, since that is where the light is metered? (If not, the body's metering could as well be involved). So when you write "Overexposes about 1/2 stop through entire range" do you mean all apertures except wide open?

I wonder ho much light falloff towards the edges wide open influences metering in different modes. Here is a visual example I performed a while ago with my D200, if possible with aperture controlled both from the lens and the camera. It needs updating as I have repeated it later with different bodies and more lenses, but not had time to put it together:
Some more info here:
http://otoien.zenfolio.com/p626313451/eac136c2

At 24mm my 12-24 only stopped down 1/3 stop at f/5.6 (overexpose +0.7 compared to wide open) and is going back to Nikon for adjustment/repair. In search for a correctly centered copy of this lens, I encountered several copies showing the same pattern if not quite as bad.

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OP Marianne Oelund Veteran Member • Posts: 7,788
Wide Open vs. Stopped Down

atigun wrote:

Should not the exposure be correct wide open, since that is where the light is metered? (If not, the body's metering could as well be involved). So when you write "Overexposes about 1/2 stop through entire range" do you mean all apertures except wide open?

Yes, all of the descriptions for the lenses are referring to stopped-down apertures only.

I wonder how much light falloff towards the edges wide open influences metering in different modes.

That's certainly another factor to consider, especially since it would add to any overexposure. For my tests, however, I used manual exposure.

Here is a visual example I performed a while ago with my D200, if possible with aperture controlled both from the lens and the camera. It needs updating as I have repeated it later with different bodies and more lenses, but not had time to put it together:

That's a very nice sample set. I see some familiar patterns there. Thanks for sharing!

Lucinda Senior Member • Posts: 1,284
Thanks for sharing.

It's amazing what I can learn on these forums. Thanks for taking the time to share your extensive testing. I have a lot to learn.

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