From DXOMark: More pixels offsets noise3

Started May 30, 2009 | Discussions thread
bobn2 Forum Pro • Posts: 61,122
Re: Another possible issue..

Steen Bay wrote:

bobn2 wrote:

Steen Bay wrote:

bobn2 wrote:

Steen Bay wrote:

John Sheehy wrote:

Steen Bay wrote:

I actually wondered a bit about that, because 40D only has ISO 3200
when "H", "ISO expansion" is activated. Is ISO 3200 a 'real' ISO on
40D, or just a 'pushed' ISO 1600? Doesn't that make a difference?

The actual exposure is what makes the real difference.

The 40D shot is 1/400 and the 50D shot is 1/500, so, unless something
in the setup gave different illumination, the 50D may be exposed 1/3
stop less.

Yes, that would be a disadvantage for the 50D, but the brightness
level seems pretty much the same. If anything the 50D image actually
seems a tab brighter. But suppose it's possible that 50D uses a
different tonecurve.

Not in this case, the images are from raw and the tone curve applied
in dcraw. It's the default straight line, hence PIX's comments about
the less than optimum conversion.

When comparing the detail/sharpness from high resolution cameras like
40D and 50D, then it's pretty important that the focus plane is
exactly the same, which (looking at the comparisons in the DPR
reviews) seems to be a bit difficult to achieve.

The time taken to set up a perfectly comparable test could be long.
Interesting to note, however that both the 50D/40D and D3/D3x
examples clearly show the detail advantage of the downsampled high
res camera over the native low res camera. I'm sure every such test
would do so - you'd just have to ignore the noise differences of very
different sensor technologies.

Yes, I'm sure that the 50D is a better 10mp camera than the 40D, as
you put it in another post, but it's also possible that you could
find another crop from the same IR images, that wouldn't show that.

The choice of crop is quite important. PIX was basing his assessment on the teacup part, where the localised exposure is quite different due to differences in the set-up of the test. The background, where he was judging the noise, was much darker for the 50D. Moreover, the shot noise we're comparing is differently visible for different brightnesses. In the bright areas, the photon count is high enough that the signal masks the shot noise, even though it is high. In the very dark areas, the shot noise is very low, because it's the root of the number of photons. Therefore, the important areas for this assessment are the mid to dark areas. Of course, you can't judge detail where there is no detail to judge, but I can't think of a part of the image where the 40D would appear more detailed, and that crop is, I think, the one with most visible shot noise, so I don't think there are any crops which would reverse the order (whatever your own order is, as I said, I think they're too close to call in terms of noise at 10MP).

All these are the pitfalls of subjective assessment, which is why I prefer quantitative evaluation - if you know how to use it, you can gain a godd idea of how things will come out much quicker.

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