depth of field

[gazza333]

Ok I thought i understood this but now ive just confused myself on holiday. I know when i take portraits at f2.8 i can blur the background and i understand that. But I was taking landscapes in low light so if i used f2.8 at 50mm and the subject was a long way away so i manually focused at infinity.

So am i correct to say if im focusing at infinity, depth of field really doesnt come into it as everything will appear in focus (unless i have something in the foreground of course). So whether i use f2.8 or f8 or f16 it makes no differance to depth of field in this situation ( i know it affects shutter speed).

to be more specific i was photographing the opera house and harbour from the apartment window i was staying in. This was from accross the harbour so quite some distance away 200m plus.

 

[Scarecrow]

DoF is not only a function of aperture size, but also distance from camera. Depth of field (for the same aperture size) will be much greater at 200m than it will be at 2m.

For proof of this try your hand at macro where f11, which under normal shooting would give a good DoF, will only get you a millimetre or so if you are lucky.

 

[fitzer]

According to DOF Master a 50mm at 2.8 on a 40D focused at 200m would have a depth of field from 37.7m to infinity, at F16 it would be 7.9m to infinity, so for your given scenario it wouldn't make any difference (practically) which aperture you used, from a DOF perspective.

 

[gazza333]

thanks fitzer thats what i thought but i managed to convince myself otherwise. I was taking shots at night too and was using f2.8 to get a quicker shutter speed to try to prevent the ferrys giving a trailing light effect. ( i was deliberately doing the trailing lights in other shots) .