ISO and shutter speed question.

[bloodhound31]

If I take a photo of the sky for 5 minutes at ISO 800, then another for 10 minutes at ISO 400, would the result be the same? Would the image be smoother/less noisy at the lower ISO with the longer shutter speed?

Baz.

[dannat]

In theory the pic should have the same result, in winter I reckon the longer exp at half iso would have less noise but in summer it may be reversed, though ind camera chips vary a lot..some handle iso 1600 but others struggle and are noisy at 800

[mithrandir]

Comments from our professional photographers welcome, but I don't think DSLR's concept of ISO has much to do with film's concept of ISO.

With film, higher ISO comes from larger grain in the emulsion.

With digital, it comes from how much amplification is applied to the readout from the CCD/CMOS. Double the "ISO" and the camera roughly doubles the readout value from each pixel.

You don't want to overfill the pixel wells and too long an exposure could do that.

With longer exposures you are probably subject to more amp glow.

[troypiggo]

You could tell us the answer in 15 mins ;)

[bloodhound31]

;) Turned out 10 minutes through my 16-35 was too much anyway. It was very washed out.

[naskies]

In general, lower ISO results in less noise than higher ISO. Similarly, shorter exposures result in less noise and hot pixels than longer exposures. I'm sure there's an cross-over point somewhere depending on ambient temperature. If you're stacking multiple subs, then noise becomes less of a concern.

I remember reading an article that someone wrote where they systematically tested ISO 100, ISO 200, etc with multiple stacked long exposures on a Canon dSLR. From memory, I think the bottom line was that ISO 800 and ISO 1600 was the best option for them (trading off exposure time versus noise, etc). I can't recall the link, sorry.

[SkyViking]

I think I read that too somewhere,. It seems to depend a lot on the actual camera/chip. Personally I use a pretty basic Canon S5IS for widefield shots and it's my experience that lower ISO combined with longer exposure gives a lot less noise. My understanding is that ISO for digital cameras is pretty much the gain applied to the signal, so in theory one would be better off with lowest possible ISO. But it's probably best if you take some test shots for your particular camera and then decide.

[Phil Hart]

The ten minute exposure has twice as much information, so will be better! Of course, if it's overexposed then you shorten the sub-exposure time.

The more objective question is whether a five minute exposure is better at ISO400 or 800 and the answer is it makes bugger all difference. All you are really doing is changing the 'gain' setting on the camera.. the photons/electrons being counted at the sensor are the same.

Somewhere around ISO400/800 is about unity gain for most DSLRs (in 14 bit RAW), ie. at that point every electron is being converted to a data number in the file so increasing ISO above that does not make much difference, although you do start to sacrifice well depth/dynamic range.

http://philhart.com/content/astrophotography-test-canon-20d40d-iso-setting-sub-exposure-length

Check out ClarkVision site for really in-depth info: http://www.clarkvision.com/articles/digital.sensor.performance.summary/

cheers
Phil