OK. Let's take a practical example:
- the Noctilux 50mm/f1 wide open at 1m focus distance
- the FOV is around 46 degrees (across the diagonal)
- if you do a typical 1/3rd composition, you focus in the center and then rotate the camera by 46 degrees / 6 = 7.66 degrees.
- assuming that it's perfectly straight (which for the Noctilux, it's not ...) the focal plane will have moved to the back a little, and is now from the camera, at distance 1 m / cos(7.66 degrees) = 1.009 m.
Meaning, you have about 0.9 cm focus error. The DOF of the Noctilux at that distance is around 2cm and covers the error well.
Best,
Roland.
Well, in a close-up, a centimeter is the difference
between focusing on eyes and focusing on temples.
You see this mistake all the time, especially in the
close-up portraits of Rolleiflex newcomers -- they
use the focusing aid in the middle of the screen to
focus, then swing the camera to compose, and the
eyes are soft and the hair by the temples is in
perfect focus.
I had the same problems shooting tight portraits
with the M3 and the 50/1.4 Nikkor until people here
recommended the focus-and-slide approach. Since
using it, I've been able to focus my portraits at
wide apertures without many focusing errors.
Here, for example, is a portrait shot with the
Nikkor 50/1.4 at f/2:
Melanie at the Cheyenne Diner
Do it a few times and it will feel second-nature.
Sanders