Curious about noise

[EricL]

I was thinking, will a faster ISO produce more noise than a longer exposure? Or does the longer exposure produce more noise? I'm talking exposures of around 7-10 seconds.

[braduardo]

I think you'll be better off with a longer exposure than higher ISO. If I'm going to have my shutter open that long I'll normally drop to ISO 100 or 200 and just l et the shutter be whatever. Not much difference between 15 seconds and 30 seconds really. They are both just kinda long.

[hejl]

I think you'll be better off with a longer exposure than higher ISO. If I'm going to have my shutter open that long I'll normally drop to ISO 100 or 200 and just l et the shutter be whatever. Not much difference between 15 seconds and 30 seconds really. They are both just kinda long.

ISO is amplification of the data off the sensor. Higher amplification means higher noise. Long shutter does not make noise, it prevents it (it allows more light into the sensor - so sensor gain isn't needed). As you noticed, a step in ISO is the same as a step in shutter (twice as long). But... the difference between ISO 100 and 200 is going to be REALLY small when it comes to noise.

[Curtis N]

Long shutter does not make noise, it prevents it.That statement is somewhere between misleading and false. Some cameras have a specific function to reduce noise from long exposures. It's a reality, and the OP's question is legitimate, since both high ISO and long exposures result in noisy images.

To answer the OP: I have taken shots as long as 45 minutes with my 20D and there was definitely significant noise. I haven't known it to be a problem with exposures under 30 seconds. So if you have the luxury of choice, lower ISO and longer exposure will generally yield less noise than high ISO and shorter exposure.

[hejl]

That statement is somewhere between misleading and false. Some cameras have a specific function to reduce noise from long exposures. It's a reality, and the OP's question is legitimate, since both high ISO and long exposures result in noisy images.

To answer the OP: I have taken shots as long as 45 minutes with my 20D and there was definitely significant noise. I haven't known it to be a problem with exposures under 30 seconds. So if you have the luxury of choice, lower ISO and longer exposure will generally yield less noise than high ISO and shorter exposure.

I'll go with misleading. Yes, if you keep the shutter open long enough, hot pixels on the sensor can generate their own noise (which is different than on chip amplification artifacts). In my experience, this doesn't come into play (on my 5d) until the shutter is open longer than 10 minutes or so. After that, noise reduction can be enabled, a dark exposure is created (with the shutter closed) and all the hot pixels are subtracted off the original image. But, short of that (pathological) example, it is generally true that a longer exposure with a lower ISO will result in less noise.

[PhotosGuy]

Generally, the better the exposure, the less the noise. High ISO doesn't have to mean a terrible pic, but you will get noise in the shadows even when the highlights look good:
ISO 200-6400 20D Tests (http://photography-on-the.net/forum/showthread.php?t=269964)