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| p.5 #7 · DxOmark reviews Sony FE 55mm 1.8 ZA: Outstanding! |
How about f/1.7?
Regarding the sensor - it can only reduce the effective transmission, not increase it, so that wouldn't explain how a > T/f1.8 lens was measured as T/f1.8 by DxO.
The evidence that it's extremely likely to be related to the sensor/camera body and not the actual F-stop is to be found looking at the behavior of other specific lens T-stop changes when used on different camera bodies at DxO...the only variable being the camera body/sensor. For instance, the same Otus 55/1.4 ZE has a T-stop of 1.4 on a Canon 5D but only a T-stop of 1.7 on a Canon 450D/ Rebel Xsi at Dxo. We know that Otus does not have an actual T-stop of 1.4 if the F-stop is 1.4..and I completely don't buy the idea that Zeiss avoided calling the Otus an F1.2 lens if that was the actual calculated F-stop (which would be an easy calculation for them).
There are many attributes of a camera that could influence what DxO calls "effective" T-stop - mount, mirror box, shutter accuracy, mis-stated sensitivity, sensor design/ toppings and so on. I don't know what caused the inflated T-stop but I think the evidence strongly points to something in the camera body rather than Sony marketing not taking advantage of calling the FE 55 a F1.5, 1.6 or even 1.7 lens rather than an F1.8 lens when the F-stop is known to them (if one is suggesting they are being conservative with stating the F-stop). That just doesn't make sense to me.