Using a mirror allows to perfectly align the subject and sensor planes. If you see the center of the lens in the center of your image, alignment is done and perspective distortions are none.
In order to get minimal distortions caused by misalignment of object and image planes, the following procedure gives best results:
Point the camera to where the negative is going to be
Put a mirror in place of the negative
Adjust camera and backlight so that the image of the lens is centered in the shot. Adjusting is easier with LiveView and with mirrorless cameras.
[slightly off]
In this shot, the lens is not centered vertically. Horizontal offset is minimal.
[almost there]
In this shot, …
This item helps to adjust the camera easily:
[Screen 2021-09-14 um 15.21.40]
I came across this thing on amazon.
The device allows to point the camera precisely (to center a mirror image as shown here ) by turning the knobs so that the camera swings in two planes, left-right and fore-aft in a vertical setup.
The photo shows the device with max swing in both planes. The base is Arca-Swiss compatible, the screw sticking out at the top has a standard 1/4 inch thread. Max dimensions are 4 in approximately, max load 6 kg.
Here’s an image of…