We all have read about "set the ISO to the lowest possible setting", to avoid that dreaded noise. So, we sacrifice sharpness, for example, because we can't get a fast enough shutter speed at ISO 100. My experience is that today's DSLRs are very good at ISO's well above the traditional "limits". I shoot a lot of motor sports, and shooting a car coming at you at 60-70 MPH, or more, requires a pretty good autofocus and/or pre-focus, and lots of depth of field to get the whole car sharp. ( Note: I find the Servo A/I on my Canon is good enough to track the cars.) Well, the formula for that is high shutter speed to stop the vehicle, and stopped way down to get depth of field. So, even in broad daylight ISO 100 just won't cut it. I've run ISO as high as 1600 with great results. The only caution is don't expect to crop heavily. But if have the glass you need, you should get great results. For the head on shots I use a Canon 5D II with a 400/5.6. I set ISO to between 800 and 1600, and get f/11 or so. If the lighting's right, you can read the helmet brand.