# Focus points - who cares? (honest question)

Started 8 months ago | Discussion thread
 Like?
 Re: Focus points - who cares? (honest question) In reply to lepton, 8 months ago

I do what you do for the exact same reason. This bothered me enough (given the issue of focal planes that everyone brings up) that I calculated the maximum angle one can rotate as a function of distance from the subject before one loses the DOF. The practical upshot and this is what you are intuiting as long as you don't go crazy focus recompose tends to work pretty well. The 5D2 (which is what I have now and had at the time)'s AF points are too close to the center to make any real difference quite honestly.

I did not do this in all our old cameras where we had eye control focus (7e and eos3) but here this is the easiest..

The y axis is angle in degrees and the x axis is distance in meters

Here are a couple of focal lengths at wide aperture:

200 @2.8:

200 mm @ 2.8 maximum angle one can focus/recompose at and still be within ones DOF [200mm has a FOV of 5 degrees from center to left edge, 3.5 degrees from center to top edge and 6 degrees from center to diagonal edge]

x axis = meters
y axis = angle (in degrees)

At 2.5 meters you can center recompose w/ no issues w/ DOF, At 1 meter or so you can go half way to the edge (which is still further than the 5d2's AF point).

35 @1.4:

35mm @ 1.4 maximum angle one can focus/recompose at and still be within ones DOF [35mm has a FOV of 27 degrees from center to left edge, 19 degrees from center to top edge and 31.7 degrees from center to diagonal edge]

x axis = meters
y axis = angle (in degrees)

At 4 meters you can center recompose w/ no issues w/ DOF, At 1.0 meters or so you can go half way to the edge (which is still further than the 5d2's AF point).

50@1.4

50 mm @ 1.4 maximum angle one can focus/recompose at and still be within ones DOF [50mm has a FOV of 20 degrees from center to left edge, 13.5 degrees from center to top edge and 23.4 degrees from center to diagonal edge]

At 4 meters you can pretty much center recompose w/ no issues w/ DOF, At 1.0 meter or so you can go half way to the edge (which is still further than the 5d2's AF point).

Complain
Post ()
Keyboard shortcuts: