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I calculated the gain values, and then measured the readnoise from a bias frame. I measured gain by subtracting pairs of images from each other and then using the resulting Sigma values, as per Buil.
OK, assuming that the D40x clips the blacks like other Nikons, this underestimates the read noise substantially. I have looked at the D300 read noise histogram and it is actually clipped above black by a substantial amount; if one used the above method to measure the D300 read noise one would get essentially zero (for instance in a D300 black frame at ISO 200, over 90% of the pixels have raw value zero). See for instance
Oops, I should know better than to calculate under editing time constraints. Forgot to take a square root. The clipped gaussian is narrowed by 1.71 (the square root of 2.93), or .77 stops.
That matches what is posted here:
No, Buil is talking about some sort of filtering of the raw data for exposures> 1s (this also occurs on the d200, which I have, and I suppose also other nikons). This is not something particularly noticeable from "normal" images, unless you specifically test for it like Buil does, and it's not terribly relevant for normal photography. Martinec's talking about serious NR done on the jpegs (of the sort done by nikon capture, neat image etc).
EJ Martin has raised questions about your calculation of the read noise, but I find that your determination of full well is questionable, because your value is greater than Emil found for the D3 whose pixel area is twice that of the D40x.
OK, assuming that the D40x clips the blacks like other Nikons, this
underestimates the read noise substantially. I have looked at the
D300 read noise histogram and it is actually clipped above black by
a substantial amount; if one used the above method to measure the
D300 read noise one would get essentially zero (for instance in a
D300 black frame at ISO 200, over 90% of the pixels have raw value
zero). See for instance
http://theory.uchicago.edu/~ejm/pix/20d/posts/tests/D300_40D_tests/
I have some blackframes from a D200 but haven't had time to analyze
them; eyeballing them it looks more like they are clipped at zero
rather than above zero. Approximating the noise as gaussian
(standard bell curve), clipping at the middle of the histogram pushes
all the values below zero to zero and effectively narrows the noise
fluctuations by hand and giving a falsely low measure of the read
noise. If you chop the gaussian exactly in half, the spread is
narrowed by a factor 1.66, the read noise is underestimated by about
.7 stop and the DR by the same amount.
(edit: if you take a gaussian and replace all the negative values by
zero, which is what clipping does, the spread is narrowed by 2.93, or
slightly over 1.5 stops)
--
emil
--
http://theory.uchicago.edu/~ejm/pix/20d/
Here's what I'm talking about: a blackframe of the 40D looks like this:
The ISO at which the full well capacity is reached is a measure of the sensitivity of the sensor to light, but the full well is the full well, regardless of ISO. Pending verification of your data, I am skeptical of your measurement. If anyone else has data on the D40x, please post it.
I don't think we know full well for the D3 yet. Emil has calculated the capacity that corresponds to the max ADU value at base ISO, but I think full well is higher, reachable at an ISO slightly lower than 200.
I understand what you are saying, but the effect of that would be be reduce the D40X DR to somewhat less than the 350xt, yet as we can see from the IR DR data, this isn't the case, in fact this would cripple Nikon DR, in general and have a noticable impact on IQ, yet, again I don't observe this to be the case. If we increase readnoise to 30 electrons at ISO100, the DR drops to absurdly low levels, well below the D200, as per Clark, in fact.
I have no problem with you being skeptical, but the 20D, with 6.4 micron pixels for example, has a measured full well capacity of
Hmmm. I suppose I'll have to make time to analyze the D200 test images I have. I suspect the same issue I am harping on may afflict Clark's measurements as well, though I don't know for sure. He simply states that the analysis he uses is the same as he used for the Canon 1D2, ie subtracting two bias frames and measuring the standard deviation. That will under-report the read noise for the reasons stated in my previous post. Clark makes no mention of any compensation for clipping of the blackframe noise.
