ISO offenders… The worst kind! This is quite possibly my biggest “pet peeve” when it comes to other photographers. You know who you are. You use high ISO like it is going out of style. Not enough light? Everyone jumps to bump up the ISO. Now, I am not saying using your ISO is bad. When used properly, it can work to your advantage. I often simulate film grain by shooting at 1600 and then changing the photo to a black and white. If you are shooting in daylight, like at dusk, then, fine, use a higher ISO (AS LONG AS YOU HAVE THE CORRECT EXPOSURE!!!) You might think that natural light is best. Well, it is… if you have some. In dark situations you will need to use some sort of lighting, most likely in the form of an on or off camera flash unit (strobe). If you are shooting in very dark or dimly lit situations, use your flash and keep your ISO to 100 with a shutter speed of 1/4 to 1/30 depending on what you are shooting. Your main subject should be lit by the flash and your background will be exposed with the longer shutter speed. As long as everyone or everything is fairly still you will not get much blur (tungsten blur) at shutter speeds from 1/8 to 1/30. The below shots are examples of what I am talking about.
The above was shot hand held at 8:12pm, 15 minutes after sunset, so it was quite dark outside to the naked eye. It was shot at ISO 100 f/4.5 1/8 sec.
The above was shot with a flash at ISO 100 f/4 1/6 sec. The photo has slight Tungsten Blur because the couple was moving in for a kiss.