D3 sensor efficiency

Started Nov 18, 2007 | Discussions thread
DuncanM1 Contributing Member • Posts: 817
Re: D3 sensor efficiency

I used IRIS to give me stat summary of a bias frame. These values must be correct or the DR would be substantially less than that reported by the IR 40D DR summary. I have used the D50 extensively for astro work. Nikon does not clip data, but they simply use a 0 offset. Any signal in a bias frame is preserved.

Duncan

ejmartin wrote:

DuncanM1 wrote:

ejmartin wrote:

Interesting. What methodology did you use to measure the read noise?
Did you correct for the clipping of blacks in Nikon's treatment of
the raw data?

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.

Duncan

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/

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