Re: Canon Digital Rebel Questions
From: Chris L Peterson (clp_at_alumni.caltech.edu)
Date: 12/28/04
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Date: Tue, 28 Dec 2004 17:49:24 GMT
On Tue, 28 Dec 2004 17:15:45 -0000, J C <JC@xxx.com> wrote:
>I'm debating about whether to buy one.
>What is the longest useful exposure one can use at room temperature(around
>20 C)?
I've gotten some good results at 10 minutes, but overall I try to limit exposure
times to 5 minutes.
The camera has read noise and dark current noise. The read noise is fixed, and
significant, and is the reason why you want to make long exposures (to reduce
the total number of exposures, and therefore the total read noise). Dark current
noise increases quadratically with time (double the time, the noise increases by
the square root of 2). From a noise standpoint, it doesn't matter how long the
individual exposures are, only the total time. A single 10-minute image has the
same dark current noise characteristics as ten 1-minute exposures. The problem
with long exposures is that the dark current signal also increases with time,
which reduces the total dynamic range of the image (this signal is subtracted
off the raw image by the Digic logic in the camera). The camera has a bit better
than 11 bits of dynamic range for a short exposure. I figure that drops about
one bit (or a factor of two) for every 5 minutes of exposure.
>Are relatively short exposures (30 sec) noisy like uncooled modified
>webcams?
The Canon CMOS sensor is quite a bit less noisy than any of the sensors I've
seen in webcams.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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