f-number equivalent between m43 and FF

Started Mar 25, 2014 | Discussions thread
Great Bustard Forum Pro • Posts: 40,847
SNAP! There it is!

xpatUSA wrote:

Oops, the focal lengths were typed back-asswards, should be:

70mm zoom: mean 67.185, s.dev 8.551, min 32, max 104

35mm zoom: mean 77.853, s.dev 16.98, min 32, max 140

. . . making the wider-angle shot the noisier.

Confirmed by a test I just did at HI res (avoids binning obfuscation):

70mm: mean 49.7, s.dev 11.9

36mm: mean 48.4, s.dev 24.4

These last figures straight from RawDigger, whole image, no exporting involved. Numbers are un-scaled raw data values. Much mo' simple!

The text I highlighted in bold was exactly what I needed to know.  Excellent!    However, I am a bit surprised.  That is, which the 70mm photo has about half the noise as the 35mm crop, as predicted, it should have had a tad more than half the noise, not a tad less.

The reason is that the 35mm crop will have been made with 1/4 the number of pixels, and thus had half the read noise.  And while photon noise is dominant, that still should have eaten into the noise advantage of the 70mm photo.

One possible explanation is that the lens had a lower t-stop at 70mm than 35mm.  That is, you took both photos at f/8.  Let's say the t-stop at 70mm was t/8, but the t-stop at 35mm was t/8.5.  The slightly increased exposure from the lower t-stop at 70mm might then offset the lower read noise from the crop.

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