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Digital "noise" - effect of shutter speed

 
 
Neil Bell
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      07-16-2003
Mt question is probably simple for most of you:

Do you get more noise if you use longer shutter speeds - say 6 or 8
seconds for example? It seems to me that I do...

Any help appreciated.
Neil Bell

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Luke
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      07-17-2003
There are actually a lot of factors that come into play here, but looking at
it in the most simple terms:

If you have a long exposure you are letting less light in (either because
there is less light available or you have incresed the f number).

If you have less light coming in the signal detected in dark areas of the
image will be closer to the noise floor in the detector (known as dark count
or dark current). This gives more noise in dark areas or otherwize you lose
the dynamic range in the image.

With longer exposures a greater signal builds up from the dark current this
limits the dynamic range available for the image signal so any errors
(noise) on the pixels represents a greater part of the of the dynamic range
available for the image.

There is more to it but these are the basic problems.

So in short you are right you do tend to get more noise with longer
exposures.

Many cameras will employ noise reduction above a certain exposure time. This
is like taking an exposure without opening the shutter, it allows the camera
to better determine where the black level should be but it doesn't get round
the fact that you are left with less dynamic range available in the detector
to store the image in.

Luke


"Neil Bell" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> Mt question is probably simple for most of you:
>
> Do you get more noise if you use longer shutter speeds - say 6 or 8
> seconds for example? It seems to me that I do...
>
> Any help appreciated.
> Neil Bell
>
> For e-mail replies remove the "notreally."
>
> |------------------------------------------------------|
> |"The greatest peril faced by all people is a failure |
> | to question and challenge religious dogma" |
> |------------------------------------------------------|



 
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Mxsmanic
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      07-17-2003
Neil Bell writes:

> Do you get more noise if you use longer shutter
> speeds - say 6 or 8 seconds for example? It seems
> to me that I do...


Yes, usually. Noise in a digital camera is a function of temperature
mutliplied by the duration of the exposure, so if the temperature goes
up or the shutter stays open longer, noise increases.

In early cameras, and still today in many cheaper cameras, thermal noise
is bad enough to make long exposures problematic. High-end cameras with
large sensors (which are less vulnerable to noise) do much better.


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MarkH
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      07-17-2003
Neil Bell <(E-Mail Removed)> wrote in
news:(E-Mail Removed):

> Do you get more noise if you use longer shutter speeds - say 6 or 8
> seconds for example? It seems to me that I do...


On a Canon 10D the best bet is to use ISO100, I get no noise visible, even
for a 30 second exposure.

Here’s a 15 sec exposure:
http://arrdee.net/%7Eflyinass/markpi.../IMG_2699a.htm






--
Mark Heyes (New Zealand)
See my pics at http://homepages.ihug.co.nz/~markh/
"There are 10 types of people, those that
understand binary and those that don't"

 
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