This post was from before the big reset. I keep coming back to this post to recompute the magic number for various lenses. I hope somebody finds this useful.
The magic number for Canon 5D mk II with 40mm 2.8 STM is 53 which is twice that of X100 with 23mm.
Once upon a time, I stumbled upon this concept called hyperfocal distance. For each given focal length, aperture and size of the circle of confusion, you can calculate a special number called the hyperfocal distance. When you focus the lens at the hyperfocal distance, everything from half the distance to infinity will be acceptably sharp. This maximizes the depth of field. This is useful for landscapes or groups of people.
There is only one problem. I have to memorize a table of values for each focal length and aperture. Yucks! Sure, I can carry a little pre-calculated table of values. Better but still yucks! Or use a phone app that will calculate it on the fly. That's too slow. Still yucks.
The X100 only has one focal length, 23mm. That's easy. I only need to memorize one chart:
(computed online using DOFmaster)
Oh, wait. If I multiply the aperture number by the hyperfocal distance, I get a number slightly above 26. All I need to remember is the number 26! That I can do!
H = f * f / N / cwhere
(ignoring the irrelevant +f)
H is the hyperfocal distance
f is the focal length
N is the aperture number
c is the size of the circle of confusion
Using 0.02mm for circle of confusion, 23mm for focal length, I reproduced the table for X100. That checked out. Since f and c are constant, the formula reduces to:
H = C / N
C = f * f / c
For the X100, C = 23mm * 23mm / 0.02mm = 26450mm = 26.45m!
So, to find the hyperfocal length for any aperture, just divide 26.45m by the aperture number! Piece of cake!
Also notice that if you double the focal length, the hyperfocal distance goes up by 4.
For 5D Mk II with a 24mm wide angle lens, the magic number is 19.2m. For 50mm normal, you won't be too far off if you guessed it is about 80m (50 * 50 / 0.03 = 83.33m).
Interestingly, if I'm shooting at f/5.6 on the X100, everything from 2.3m will be in focus if I focus at 4.7m. I can just pre-focus at 4.7m, switch to manual focus to lock it and forget about focusing altogether! This will also be great for videos. The X100's movie mode does not track faces and it tends to shift the focus unnecessarily especially if the background has some high-contrast items like lights.