Originally Posted by Daniel Browning
So I chose to pull out my handy-dandy iPhone and bring up DoF Plus. It's set to the Canon 40D at the moment.
Field of view, 35mm at 10m to subject, is 6.32m wide by 4.21m high.
Field of view, 70mm at 20m to subject, is 6.32m wide by 4.21m high.
Depth of field, 35mm at f/4 (8.75mm iris) at 10m to subject, is 6.17m to 26.19m (-3.83m and +16.19m).
Depth of field, 70mm at f/4 (17.5mm iris) at 20m to subject, is 15.27m to 28.94m (-4.73m and +8.94m).
Depth of field, 70mm at f/8 (8.75mm iris) at 20m to subject, is 12.35m to 52.38m. (-7.65m and +32.38m)
So, if I keep the same aperture, I get different DoF. If I compensate for iris diameter, I get double the DoF. I get the same numbers using the link found elsewhere in this thread. Am I mis-interpreting something? I'm not seeing the DoF staying the same with unchanged iris diameter.