How does the D3 achieve such high ISO?

Started Feb 17, 2008 | Discussions thread
Marianne Oelund Veteran Member • Posts: 7,788
Black clipping

Peter Carmichael wrote:

Does the Nikon implementation in the D3 clip the black level in the
NEF files?

Yes, it does. There is no bias applied ahead of the A/D converter, so negative noise values are truncated. On the D3, truncation occurs very close to true zero; statistically, I see about a 52-55% zero-value pixel content at moderate ISO settings. This is what one would expect, since in addition to negative signals being represented as zero, there will be some fractional-count positive ones as well.

On the D300, there actually seems to be a slight negative offset, as the zero-value pixel proportion is in the 85-90% range. (Note: These D3 and D300 measurements were made from 14-bit NEF files.)

For this reason, it is necessary to use bias frames rather than black frames to measure read noise on these cameras. Some astro users seem to be concerned about the implications for astrophotography, but whether or not this is an issue will depend on the degree of sky glow present. For long exposures, it takes an extremely dark sky indeed to lack adequate bias.

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