Which ISO Should I Use? 

I often get asked this question. There is no easy answer.
You start with low  ISO's  for bright light  and as the light gets lower  you can increase the ISO to maintain your shutter speed and aperture. 

Then you factor in the down sides of higher ISO's. Low ISO's give you the highest quality image with the best detail and maximum tonal range. As the ISO increases these features reduce. Today the software modifies the noise to make the images look pretty good but as the ISO gets really high and if you enlarge your image you can see the compromises that have been made.

So ... the aim is to keep your ISO as low as feasible, keep your f-stop at  the desired point for your chosen depth of field, which only gives you the shutter speed to alter to get the correct exposure. This usually means a low shutter speed which of course means using a tripod. If your subject doesn't move, this is fine.

If you can't have a very slow shutter speed. then it is a juggling act to be able to keep the depth of field you want and have an ISO that is not too high!