Originally posted by: CFster
Originally posted by: randomlinh
Originally posted by: CFster
Your eyes have to refocus - they just do it so fast you don't notice it.
To get everything in focus with a camera you need to close the F-stop to have increased depth of field. The trade off is this cuts the amount of light coming into the camera by half - sometimes more. So either you use a flash (which is only really good for close up to medium distance objects), or you have an SLR with very "fast" and expensive lens.
er... i've always known a "fast" lens means you can have a huge aperture.. which would be the opposite of what the OP was asking.. no?
A fast lens means it lets in a lot of light. Usually they have bigger optics. So, while a one 200mm lens might have a fully open aperature of f4, a more expensive lens might have a aperature of f1.2. BTW, the difference in cost could be in the thousands...
The other benefit to this is they'll let more light in across the whole range. So at f16 (if you choose, for a deep depth of field) you have more available light, enabling you to use a faster shutter speed which eliminates a blurry picture.
Edit: Scratch that. I think I'm wrong about that last part. I think f16 is f16 regardless of the lens - somebody tell me if I'm wrong. The benefit of a faster lens is the maximum aperature available.