What effect does the ISO have when using a digital SLR?

Okay, so I know about ISO's, f-stops, exposure - etc.

I know with film, ISO was the film speed and also affected the grain on the photographs.

I'm confused really about ISO with digital cameras? There's no film speed... and ISO doesn't affect grain with digital...

So is it just a third factor to get the amount of light you want? I use it for instance, when I'm trying to get a certain shutter speed, I'll change the ISO faster or slower depending on what I need...

So do you set the ISO as often as you do shutter speed and aperature? I tend to use it when I have too much light, I'll change the ISO to 100. If I have too little, I'll use 800, etc.
But usually I leave it on around 400.

Still, not sure if there's any affect on the result...and I'm not sure what's actually happening inside the camera when I change it. With film, I understood ISO, changing ISO, pushing film, etc.

Sorry, I'm sure this is a remedial question... thanks for taking the time to read and/or respond.

Lisa

It will affect noise which is the digital equivalent of grain. The higher the ISO the greater the noise. It just depends on where you are shooting and what feel you are trying to get. 400 is a good ISO to keep it on, but when you head outside I generally shift to 100 or 200 depending on the camera. If I'm inside and don't want to use flash I will bump up to 800 or higher depending on what I am doing.

Z

Ah, thanks! Noise... does that mean blur, graininess??

Well, I feel a little more informed now.. I was quite confused! Thanks! :)

noise is basically grain, but it is the parts of the photo that random colors are replaced in for the true color. it occurs most in dark areas, and keeps the file size very large because the file can't be compressed as heavily as a jpeg because the compression algorithm can't interpolate the different colors as well. the more you underexpose and try to bring it back with Post processing, the more noise you will see

ISO 3200
http://img85.imageshack.us/img85/772...074copy2jz.jpg

ISO 200
http://img85.imageshack.us/img85/415/img0813copy4kd.jpg
both taken with the digital rebel.

notice the difference (other than the motion blur)? one has a lot smoother color, while the other one has a lot of random colors thrown in.

Wow, awesome explanation Brent and Z. Brent I appreciate the examples as well. I appreciate y'all taking the time to answer my questions.

no problemo. that's what I'm here for. now if only a camera store would hire me so that I could get paid to do it...

Higher ISO speeds also seem to have more contrast - but this is also true of high speed films. For example, Ilford Delta 3200 compared to Tri-X... Underexposing will lead to more grain or noise (same for film & digital), so your exposure has to be accurate.

Brent, that ISO3200 example is a 100% crop, isn't it?

yep, both of them are 100% crops. I have a couple photos that have even better examples of a lot of noise, but theyre on CDs, and the reason higher ISO has higher contrast is because you lose dynamic range as you increase ISO.

ISO 3200 with a DRebel? Is that the 300D or the 350D? I didn't know they could go that high. I thought 1600 was the max. Is there a custom function that I missed on my 300D? Not that I would use it since the noise at 400 drives me nuts.

300d. you can hack it. if you overexpose by 1-2/3 of a stop at ISO 3200, it actually is still usable.