Getting down to the nitty-gritty about noise and it's effect on IQ Locked

Started Apr 15, 2010 | Discussions thread
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thomasnb Regular Member • Posts: 456
Re: I just took a look, and...

Crocodile Gena wrote:

except for low-light ISO where the 7D enjoyed a substantial advantage (854 vs 696 for the 50D and 703 for the 40D), and that's because it does have significantly less read noise than the other two.

The DxO low light ISO rating determines the ISO (according to DxO's way of measuring it, which is not the same as the International Standards Organisation's) at which the normalised ('print', scaled to 8MP) SNR is 30 dB. This test relies on a single intercept, so doesn't say much about the relative performances of the cameras at any other ISO rating, because the shape of the SNR/ISO curve may well be (and often is) different. In the case of the read noise, if the slope of the curve is very shallow around the intercept (which it usually is) a small change can make a big difference in the rating. It's another case of not assuming a measured quantity has a direct relation to the perceptual content of the image you actually see.
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thomas

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tko
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