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canon dslr best ISO


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Good questions, to which I will be waiting for an answer also.

Having just purchased a DSLR (an older 300d) I'm not used to f stops ISO etc! I used to be more of a auto focus hocus pocus type of photographer!

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It depends on sky conditions and the luminosity of your target, is the short unhelpful answer.

High ISO settings make the "read noise" of your camera amount to fewer photoelectrons (which is good) but it reduces your dynamic range (which is bad).

Low ISO makes the "read noise" amount to more photoelectrons (which is bad) but increases your dynamic range (which is good).

If you have ample luminosity, your main source of error is "photon shot noise" so you can put up with the greater "read noise" of low ISO as it won't make much of a difference and you'll gain dynamic range.

If you don't have many photons coming in, low ISO can make your "read noise" the dominant source of noise. So you up the ISO until it's not.

The advice is "always be shot noise limited" for the luminosity of your target.

f-stop: we need the lowest setting (widest aperture) so that we collect more light but not so wide that lens aberrations ruin the image (look at the shape of stars near corners as you reduce the aperture, they should improve).

Edited by themos
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It depends on sky conditions and the luminosity of your target, is the short unhelpful answer.

High ISO settings make the "read noise" of your camera amount to fewer photoelectrons (which is good) but it reduces your dynamic range (which is bad).

Low ISO makes the "read noise" amount to more photoelectrons (which is bad) but increases your dynamic range (which is good).

If you have ample luminosity, your main source of error is "photon shot noise" so you can put up with the greater "read noise" of low ISO as it won't make much of a difference and you'll gain dynamic range.

If you don't have many photons coming in, low ISO can make your "read noise" the dominant source of noise. So you up the ISO until it's not.

The advice is "always be shot noise limited" for the luminosity of your target.

f-stop: we need the lowest setting (widest aperture) so that we collect more light but not so wide that lens aberrations ruin the image (look at the shape of stars near corners as you reduce the aperture, they should improve).

i know with the wider aperture it's also a bit harder to focus, And that the aberration depends ont he quality of the glass. I'm just curious if anyone can give me a guideline.

like if i can shoot 2minute+ exposures is it better to use a high iso to reduce noise.

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Where that can of worms smiley when you need it... canofworms.gif

I personally tend to use ISO800 with the 1000D and 500D and will occasionally use ISO1600 and for very bright targets or Clusters I may drop to ISO400

Billy...

Edited by Psychobilly
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like if i can shoot 2minute+ exposures is it better to use a high iso to reduce noise.

Depends on your target and your sky.

If you are trying to tease the finest, faintest wisps of light that are just above the sky background, go for high ISO. If you're trying to capture a range of colours and luminosities, go for a medium ISO.

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Where that can of worms smiley when you need it... canofworms.gif

I personally tend to use ISO800 with the 1000D and 500D and will occasionally use ISO1600 and for very bright targets or Clusters I may drop to ISO400

Billy...

can of worms for sure. lol i know how to use all the settings in daytime and studio settings (im classically trained in photography, darkroom, etc etc) but this astro stuff is a whole nother ballpark.

what kind of targets do you set at 400, just as an example?

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There is NO right or wrong. The settings you use on your camera all boil down to the level of light pollution you have in your locale.

It really is all down to trial and error to find the settings that you are happy with.

Edited by LukeSkywatcher
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I've tried at 200, 400, 800 and 1600... If the conditions allow and the optical settings can get me super fast, then I'll use lower. Otherwise, I tend to stick to 800. As for the F stop, this, if using prime focus is defined by your scope. Otherwise, it'll depend on the optics. The Canon EF50II for instance, whilst it's super fast at f/1.8 the distortion on stars is frankly appalling. Stop it down to f/4 or f/5 and it's much better. The 18-55 kit lens seems to work very nicely at f/3.5 at 18mm, but it's not a prime lens so the resulting image quality is a little compromised. You have to work out what works for your setup, and I know that's not a helpful answer, but it's all we've got I'm afraid.

I'd suggest starting at ISO800 and just see how you get on.

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For spectroscopy I use ISO200 as the best compromise for me....
I am surprised. I would have throught spectroscopy was the one place where read-noise was definitely an issue, and (allegedly) the read-noise at ISO200 in Canons is much worse than at ISO1600.

NigelM

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We measure the SNR in the spectrum using a program called DER_SNR which seems to give consistent results.
So have you ever tried measuring the read-noise at different ISOs? It would be interesting to see if low ISO really is worse. I guess this info comes from a couple of well-known websites which claim to have measured it (on some Canon models anyway), but independent verification is always a good thing.

NigelM

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No not really.

I was more interested in finding a reliable measure of SNR.

Yes, we've all seen the formula but do the actual results give the "theoretical" SNR????

DER_SNR allows the calculation (in spectra) of near as %^&* the real actual SNR.

( I was trying to determine if a stack of say 10 x1min exposures gave a better/ worse SNR than a 1 x 10min exposure - Answer; no better!! A single long exposure will always beat a stack!)

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I think I've got the same philosophy as Psychobilly on this one. If good clear nights weren't at such a premium I think I'd go for lower ISOs - I do like shooting at ISO400 but most of the time I'm on ISO800 and occasionally ISO1600. Guiding is also a factor - if guiding is working well I'll tend to go for longer subs at lower ISOs - If I'm having guiding woes, I'll go for higher ISOs and shorter exposures so I don't have so many wasted subs.

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