more pixels are better!

Started Dec 14, 2008 | Discussions thread
Andrew dB Contributing Member • Posts: 970
Re: more pixels are better!

Graystar wrote:

boels069 wrote:

Is the noise independent of shutter time?
(ISO 6400 @ 1/8000s has the same noise as ISO 3200 @ 1/4000s ?)

As with all these kinds of technical nuances, the true answer is long
and complex. However, I'll try to keep it simple.

First, a change in ISO changes everything because you're changing the
actual exposure to light. So I'm rewriting your example to match
what I had written...f/2.0 @ 1/8000s vs. f/2.8 @ 1/4000s.

Simply stated, even with different shutter speeds, shot noise (noise
from the light itself) will be the same as long as exposure is the
same. Read noise (noise added by the electrical going-ons in the
sensor) will be different. That difference in read noise is
practically negligible when comparing two such fast times. It gets
worse as the shutter speed gets slower. At 1.3 seconds, Canon
compacts are already performing a dark-frame subtraction to get rid
of the extra sensor noise. I haven't seen any tests of a sensor's
noise across the more typical shutter speeds (1/30s to 1/1000s.) If
anyone knows of any, post a link. Should be interesting.

Read noise ought to be the same regardless of exposure duration since the read is an event occuring at the end of the exposure. Thermal noise will become more significant as exposure length increases and for constant temperature is linear but increases by a factor of 2 every 6-7 deg C (in CCDs, not certain if CMOS follows the same rules). Unless you are dealing with exposures in multiple seconds or minutes, thermal noise shouldn't be an issue.

Read noise for CCDs also increases with temperature and increases as readout clock frequency is raised to reduce read times. For this reason, low noise scientific cameras often have very slow readouts that take many seconds to read each image from the chip.

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