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| p.194 #14 · Official: Sony A7 and A7R Fullframe Mirrorless |
Wonk Alert! I've done a light transmission test (t-stop) on my A7r for all the 35mm to 50mm lenses I have on hand. The purpose of the test is to see how much light is reached at the sensor when the lenses are wide-open, which accounts both for lens transmission (t-stop) as well as sensor micro-lens transmission loss (DxO Article). This information can be used to make weight/size/price decisions for those considering a purchase of these lenses and wanting the best effective light transmission (ie, best low-light performance).
The test was performed by shooting a raw image of an X-Rite Color, with each lens at the largest aperture supported and the chart centered in the frame to prevent vignetting from affecting the results. All raws were loaded into CS6 with identical white-balance. The Leica Noctilux f/0.95 was used as the reference lens since it is the fastest. The 2nd darkest gray patch on the color chart (3rd patch on top row from left) was sampled in CS6 to obtain the reference luminance. I then loaded the images from the other lenses and adjusted the ACR exposure compensation for each so that the 3rd patch matched the luminance from the Noctilux reference...the delta from reference to the f-stop shows the relative transmission of each lens.
Side note: By comparing the light transmission of the Canon 50L f/1.2 and f/1.4 lenses mounted on a generic EF adapter (no electrical contacts) vs those same lenses mounted on a Metabones Smart Adapter (w/electric contacts, so f/stop data passed to camera), I determined that the A7r does a "silent" ISO gain of .35 stops for f/1.2 and .33 stops for f/1.4. The values in the chart for EF lenses exclude those gains (ie, show the real EV difference).
How to read the chart:
* The "Expected Exp Comp" column shows the expected EV difference between each lens tested vs the Noctilux based on the nominal aperture difference. For example, f/1.2 -> f/1.0 represents 1/2 stop nominal difference. For ease of viewing I've rounded the Noctilux up to f/1.0
* The "ACR Exposure Comp Necessary" column shows how much positive exposure compensation was required to match the brightness of the Noctilux. This value is expected to be less than or equal to the nominal f/stop difference.
* The "EV Diff" shows the exposure compensation necessary vs the expected exposure comp. A positive value indicates that the amount of ACR exposure comp was less than indicated by the nominal f/stop difference. A negative value indicates that the amount of ACR exposure comp was more than indicated by the nominal f/stop difference.
If you want to see the effective transmission difference between two lenses, compare them in the "ACR Exposure Comp Necessary" column. For example, the Sony FE55 @ f/1.8 is 1.30EV darker than the Noctilux @ f/0.95.
http://horshack.smugmug.com/photos/i-Sc9xZ3B/0/O/i-Sc9xZ3B.jpg...Show more →
The above quoted post measured the light transmission of various large-aperture lenses on the A7r. This was done to measure both the amount of light loss at the sensor microlenses and the relative light transmission of the lenses tested. I've done a new light transmission test but this time testing the relative transmission of the same lens (Leica Noctilux f/0.95) on all three A7 body types. Per DxO's f/stop blues article, higher-density sensors typically exhibit more light loss at very large apertures vs lower-density sensors, as a function of the light loss at the microlenses. My new test sought to compare this loss across the bodies. Here are my results:
A7s: f/0.95 -> f/1.4 lost 0.20 EV
A7: f/0.95 -> f/1.4 lost 0.27 EV
A7r: f/0.95 -> f/1.4 lost 0.39 EV
The above results were obtained by shooting a color checker chart at both f/0.95 and f/1.4 on all three bodies, then measuring the relative EV difference on each body between those two apertures on the gray patch of the chart. The A7r loses .19 EV more light at f/0.95 vs the A7s (ie, the A7s collects 1/5 stop more light vs the A7r at f/0.95), whereas the A7 only loses .07 EV vs the A7s.