ISO

[Glider]

Last night I had a play with the ISO on my Canon 1000D. In the past I've always used ISO 1600.

Under my sky this means I can get 300s exposure before skyglow starts to turn everything orange and the histogram display for the images reaches the right hand side of the display.

Last night I tried ISO 800. Now I can expose for 500s before I get to the same orange fog limit.

Now, have I just actually captured nearly twice as many photons and therefore improved my signal to noise ratio no end?

Will I be able to bring all of this out using CS3 levels and curves?

[Photosbykev]

iso 1600 @ 300 seconds is the same exposure as iso 800 @ 600 seconds given the same aperture. So at iso800 @ 500 seconds you are capturing less data by a small amount

[barkis]

The theory is, that the higher the ISO setting you use, the higher the sensitivity, and as such, you can use shorter exposures. The downside is, the data may contain more noise.

It's desirable to have a happy medium between ISO speed, and exposure times whilst imaging faint objects such as DSO's And Nebulae. At least that is my understanding of it.

Those with more experience may advise you further.

Ron.

[Glider]

Am I right in thinking that the camera chip sees the same light photons whatever the ISO setting? Changing the ISO simply changes the gain of a subsequent amplifier?

If so then in terms of photons on detector 300s at ISO 1600 isn't the same as 600s at ISO 800.

The later sees twice as many photons but only amplifies the resultant signal by half as much before digitisation.

Is that right?