ISO setting doesn't do anything

[rkstudio]

The ISO setting on my camera was working perfectly a few days ago, now it's not working at all. When I change it to 100 or 400 or 1600 or more, it makes no change in the display or the image when I upload it.  I don't know if I made a setting change or something when I was using it, but all I know now is that it's not working.

I have a Powershot sx40 HS.

[diverhank]

Can you explain a bit more on what the problem is?  

What mode are you shooting at? Auto, P or some others?

What makes you want to change the ISO?

To oversimplify, an image brightness (exposure) can be controlled by 3 factors in any camera - Aperture (Av), Shutter Speed (Tv) and ISO.

Most cameras have various auto-modes that will automatically take care of 1, 2 or all 3 of these factors.  If you set the camera to Auto mode or P mode or even Av mode or Tv mode...changing ISO alone is not enough to affect the brightness of a picture because the camera can still change Av or Tv or both to keep the picture brightness the same.  As long as there is enough Av or Tv values for the camera to change, the brightness is not changing.  This is what you probably are seeing.

The only time you will clearly see the effect of changing ISO is to put the camera in Manual mode (M) and keep Av and Tv to fixed values.  Now when you change ISO, you will see the picture brightness varies.  Hope this makes sense to you.

[ScottyP]

Unless you are shooting in full M Manual mode, changing the ISO won't change how bright the image is.  This is because the camera will simply change the shutter speed and/or the aperture (f/stop) to keep the exposure correct.

If you raise the ISO the camera gets more sensitive to light so it will speed up the shutter and/or narrow the aperture to get rid of the extra light and prevent an overexposure.  The opposite happens when you lower ISO.

If you look closely you will see the side effects. Higher ISO makes more of the ugly noise and grain. And then if the camera narrows the aperture opening to deal with that higher ISO then your depth of field in focus will get deeper.