Tim: I've often read to up the ISO in order to be able to keep the shutter speed lower, does this fit into the equation anywhere?
Thanks!
Not directly. But there are some side-effects.
There's definitely a relationship between the focal length of the lens, crop factor of the camera, and shutter speed. And since the camera's crop-factor "is what it is" (it doesn't matter if you change lenses, zoom, change the f-stop, change the ISO, etc.) it really boils down to the relationship between the focal length and shutter speed.
But there is a nuance... if you crank up the ISO, you get image "noise". If you shoot JPEG, the camera has internal software which attempts to reduce the impact of that noise. To do this, it "averages" pixels by comparing each pixel's values to the values of it's surrounding pixels. This does work for reducing the "noise" but it has the side effect of "softening" an image that might have been much sharper without the de-noising. That means the lens and camera might have done a great job and capturing a "sharp" image, but the noise-reduction "softened" the image. By the time you see it, you're thinking it wasn't actually a sharp image (even if it was sharp before the de-noising was applied.)
This happens in "tiers". I'll pick on my old Canon T1i (I don't own that camera anymore.)
At ISO 100 it looked great.
At ISO 200 it still looked great.
At ISO 400 it looked good... but you could see some noise.
At ISO 800 it actually had LESS NOISE than at 400... but technically not as sharp. What happened here is that the camera left everything alone up through (and including) ISO 400. But by 400 you could actually see some noise (it was moderate.) At ISO 800, the engineers who wrote the firmware decided the noise was getting strong enough that they needed to do something about it... so the camera firmware was programmed to start applying noise-reduction as soon as you hit ISO 800. If you use small-ish images you wont see that "softening" effect. It's when you really zoom in to see the 100% pixel level detail that you'll notice it's not as sharp. And of course if you REALLY crank the ISO... the camera REALLY cranks the de-noising in response (which has a much greater effect on image softening.)