[audiobomber]

I think that f-number is the primary limitation based on the way phase focus detectors work. Slop in lens gears, mechanical overshoot, etc must contribute a bit. Finally, sharpness & contrast must also play a role.

If anyone knows a source that discusses these things in detail I'd love to see it.

I believe you're correct. All I can say is, sometimes my 18-250 hunts and can't find focus in dim light, whereas the 18-135 is almost always quick and sure.

[JinDesu]

Wouldn't longer focus throw be a factor in hunting as well? Or is it the opposite?

[RioRico]

ObTopic: A 17-70/2.8 or something in that vicinity would be a better indoor zoom IMHO. Or, for rather less money, some fast primes; manual-focus if you can stand it, as I suggested above.

[newarts]

Wouldn't longer focus throw be a factor in hunting as well? Or is it the opposite?

I'd guess not exactly focus throw but the gearing ratio for the screw or motor drive. If the gear drive is too slow it'd take forever and if too fast night overshoot before detection.

Oops, sorry to be off topic!

[audiobomber]

ObTopic: A 17-70/2.8 or something in that vicinity would be a better indoor zoom IMHO. Or, for rather less money, some fast primes; manual-focus if you can stand it, as I suggested above.

While I agree with your thought, I would just point out that no one makes a 17-70 f/2.8. There's a 2.8-4.0 and an older 2.8-4.5. There are several 16/17/18-50mm f/2.8's.