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04-10-2013, 08:09 AM   #1
Pentaxian




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Samyang/Rokinon 85mm - is it really f1.4 lens?

I took some test shot at f1.4 with newly acquired 85/1.4 and it seems to underexpose by say half stop at f1.4. Exposure seems to be correct for the smaller apertures (f2 & smaller). A few other people using it with Nikon also seems to have observed the same.

So wondering, are those lenses are really f1.4? To me, it seems like a f1.7 lens.

04-10-2013, 08:49 AM   #2
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Maybe the discrepancy is due to the difference between T-stops and F-stops. a test of depth of field could tel you whether it is really opening to 1.4. It is highly likely the the aperture is 1.4, but the T-stop is 1.7. In fact, I think the cine versio the cine version lists a t-stop of 1.5.
04-10-2013, 10:03 AM   #3
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F-stops (aperture) is just a mere physical attribute of the lens, the relation between the focal length and the optical diameter, and actually doesn't tell how the light transmittance is. Though it does the you the depth of field and similar instead. For light transmittance you have to look at T-stops instead, commonly used for video equipment.
04-10-2013, 10:48 AM   #4
arv
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Can't it be just vignetting? According to lenstip.com it is 0.8 EV.

04-10-2013, 12:09 PM   #5
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Why not do a real test.

Take a series of shots against a block wall, and meter manually at an intermediate point like F5.6 and then take shots at each full stop from F1.4 thought minimum aperture, each time adjusting the shutter speed accordingly. Then take a photo editor that can measure the greyscale value inside a selection and plot greyscale vs aperture for the entire range.

If it is truly F1.4 you will have a flat line at about greyscale of 115-120 across the entire range.

If it is not F1.4 when wide open the greyscale value will drop below this. A 1/2 stop, as your implied 1.7 would be a greyscale 20-25 points below the central portion of your plot.
04-10-2013, 07:38 PM   #6
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Probably T-stop difference.
Noticed it when comparing Canon EF50/1.8 vs A50/1.7 and S.Tak 55/1.8 on the same camera too. (the Canon always was 0.5-0.7 stop darker for the same exposure setting)
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