Another thread prompted this query, whic for me is just a matter of curiosity. The possibility of substituting a shorter focal length lens and moving closer was suggested as a way to increase DOF. I was sure of having read that the trade-off doesn't work that way, and that years ago I had confirmed it with my 35mm and MF lenses' DOF scales . Checking, I found that Steve Simmons is relatively clear, though his exact language leaves wiggle room, and I thought I had seen it from Adams and others, but couldn't find it easily.
Turning to an online calculator (https://www.pointsinfocus.com/tools/...ens-calculator), I ran two examples for my two lenses, 135 and 210. Starting with the 210, I set a subject distance, selected a constant aperture (f/16), noted horizontal field of view (HFOV) and total DOF. Substituting the 135, I got as close as possible to the HFOV, and noted distance and total DOF.
My examples are copied below. I used portrait sorts of distances, because that's my primary interest.
(To avoid confusion, please note differences between decimals in feet and inches, e.g., 14.9 feet, 5 feet, 2.5 inches.)
There is clearly one or more other variables operating, and I am curious about it, IF it can be described without presenting optical formulae.
Thanks.
210
HFOV 9'
Distance 15.6'
TDOF 5'8.9"
135
HFOV 9'
Distance 10'
TDO 5'11.8"
Example 2:
210
HFOV 2'7.3"
Distance 5'
TDOF 6.19"
135
HFOV 2'7.4"
Distance 3.22'
TDOF 6.24"
Bookmarks