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Real stops (T-stops) tested for a few lenses
  • nomadnomad

    I had the opportunity to put a cine Distagon, the Arri/Zeiss 40mm 2.1 T, on my GH2 today and thought I'd check a few of my stills lenses against it for exposure.

    The results are interesting: while I thought that all marked F-stops would be slower than the marked T-stops, this is wrong!

    All lenses were slower in T-stops when wide open, but starting from the first stop closed down, the values were constantly identical to the F-stops (within an error margin well below one third stop).

    One example: the Minolta Rokkor 58mm f1.2 (aka Bokeh-Monster) has a T-stop of 1.4 (true speed) wide open. At f2.0 the value is identical to the lens with the marks in T-stops. This was the same on all the lenses I tested, a few Minoltas, Zeiss Contax and one Voigtländer. OK, not a scientific test, but good enough for me. For most practical purposes I can consider my lenses closed down as marked like T-stops, only the WO values are exaggerated.

    Voigtländer 25mm @ f0.95 is a T 1.1.5 Minolta Zoom @ f3.5 is a T 3.8 Both Rokkor PG and Contax Zeiss @ f1.4 are T 1.6 Even the Rokkor 28mm @2.0 is a T 2.1

    Thought some might find this useful…

  • 1 Reply sorted by
  • Ralph_BRalph_B

    Very interesting report. One thing people need to keep on mind about T-stops/F-stops is that even though a lens has a slower T-stop than it's F-stop, depth of field is still determined by the F-stop. In other words, if you have a lens that's only measured in T-stops, it's important to know it's F-stop as well, in order to make accurate depth of field calculations.

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