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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Steen Bay Veteran Member • Posts: 7,418
Re: A red herring

thomasnb wrote:

Steen Bay wrote:

Crocodile Gena wrote:

Steen Bay wrote:

Also, If we're looking at deep shadows at ISO 1600, then we're in the read noise affected/dominated area where the 50D has a relative advantage, since its read noise is much lower than the 40D's (something like 2.5 vs 4.5e- I think), and things like that (and others) becomes rather important when we're comparing images with such relatively small differences in theoretical noise levels (about 22%, or so).

In terms of read noise, that's just about right. That is, we would expect that for the same technology that the read noise is proportional to the pixel area. Since the pixel area is proportional to the pixel count (for the same size sensor), then 4.5 e- x 15 MP / 10 MP = 3 e-, which is pretty much on the mark.

The point is that "The area read noise scales with the square root of pixel density", like ejmartin explains in the link below. That means that the 50D only needs to have factor 1.22 lower per-pixel read noise to maintain the same per-area read noise as 40D, but 50D's read noise is much lower than that, which gives 50D an in this context (a general pixel density debate) 'unfair' advantage when comparing read noise affected/dominated high ISO (3200?) images, like we're doing here (an advantage that 50D wouldn't have at lower ISOs where the shot/photon noise is dominant).

  1. You're arguing this back to front. The read noise naturally scales down with the pixel area. So what you appear to be saying is that, for this to be a fair comparison, Canon would have had to endow the 50D a relatively worse pixel (lower pixel SNR) than the 40D? If so, how is that 'fair' in any sense?

I said "in this context". Normally it's of course a good thing if the read noise gets lower in new cameras. The less, the better. (Just wish that Canon would make it lower at base ISO too)

  1. Your argument about which kind on noise is 'dominant' is wrong in any case. It is at high ISO's where most of the noise you see is shot noise. Remember shot noise increases with exposure, but so does the SNR. At low ISO's the (high) shot noise and the (high) read noise is masked by the high level of signal, but usually the read noise is much higher than at high ISO's and apparent in the deep shadows.

Isn't it a fact that 50D, and even more so the 7D, have better high ISO (3200+) performance than 40D when compared at the same scale? (e.g. 8mp like DxO does) And isn't that because 50D and 7D have less per-area read noise than 40D? (since all three cameras have pretty much the same QE)

  1. The issue of read noise is in any case irrelevant to what you're being asked, which is the perceptual quality of the two noises. Which one is slightly higher in the deep shadows is an irrelevance.

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