ISO : Exposure Length

[Guest Mr T]

What's best longer exposure length with lower ISO setting or shorter exposure with higher ISO?

I suppose if you are not autoguding then it is best to use high ISO as your exosure times will be shorter because of trails. But what if you are autoguiding and can ake longer exposures?

Would say 20 x 6 minute subs at ISO 400 be better than 20 x 3 minute subs at say ISO 1600?

The higher the ISO = more noise, but do longer exposure lengths produce more noise?

I use a Canon 450D DSLR camera.

[Sheila]

iso 800 is normal for what i do and longer exposure i am told. The higher ISO introduces more noise as you say.

Mine is a modded canon 40d and iso 800 and the longest exposure you can should see you right, and of course the more exposures the better as more

data.

Sheila

[andyboy1970]

More exposures = better signal (image) to noise ratio.

I found iso800 and lots of 3 minute exposures works best for me.

[Guest Kheldar]

As above, 800

[Guest Mr T]

Thanks everyone. Consensus of opinion seems to be ISO 800 is the optimum (which on my Canon 450d is 2nd highest) and more exposure time is better than a little...which makes sense.

This leads me on to ask , is there a point when adding more exposure time makes little real difference to the image, and if so how do you know, are there any guides, or is it more is better?

[Brantuk]

All depends on the object. Some objects have even brightness accross the frame - but typically most are dimmer in parts and brighter in others. Take M31 in your avatar Mark.... too long on the core will burn it out ..... not long enough on the dust lanes and they won't show up.

You often find a range of exposure times and ISO settings are used to get individual aspects of an object so they can be masked and layered together to produce a composite shot. Have a look at Ollys collaborative effort on M31 - 45hrs of data from different scopes. Then factor in the different wavelengths captured and filters used.... I doubt there's a fixed formula.

[Guest Mr T]

It was the Olly and Co superb M31 picture which got me thinking about it. I guess there's no shortcut to learning to taking great images, its down to experience, trial & error and learning from ones peers. I don't think I have the patience to spend 40 of hours on one target, but I'm glad Olly and his friends did.

[Brantuk]

It sure does make one think - I have the patience - if only I had the time and weather to engage fully in the "dark arts" lol - maybe when I'm retired (if I get that far). But you can run off quite acceptable shots with a basic set up and a minimum of knowledge - it's very absorbing and you do get sucked right in as money flies out the window though - just like a black hole lol

[Sheila]

yes I know bout black holes and astro imaging equipment. Credit card will never recover and has gone into hiding LOL

sheila