Photons Missing In Action

Started Nov 24, 2009 | Discussions thread
Marianne Oelund
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Part 1: Lens T-stops
In reply to Marianne Oelund, Nov 24, 2009

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.

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