Getting perfect parallel alignment between subject and sensor

I am a simple man and I like to have simple solutions.

I would not find parallelism. I would find the center.

You need a nonelastic thread (cotton for example), one tip pen and one assistant or two.

1. Paste one end of the thread next to one corner of the painting. Or have another assistant holding it.

2. Put the assistant holding the spool of thread on the opposite corner of the painting and start to unroll the thread from the center to the front of your camera.

3. Cut the thread and find the exact center of it folding in half and mark it with the pen.

4. Do the measurements again but this time the camera lens needs to be exactly on the marking.

5. Do the measurements again with the same cord but the other two corners of the painting and adjust the position of the camera if needed.

You now have the lens at the "exact" center of the painting.

6. Frame and level the best you can.

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You can, of course, use a laser meter or something, especially for big artwork, just do not fire the laser to the painting, especially if it is an antique.

Put the cap on your lens and fire at it. Paste a sheet of paper on it so the laser reflects nicer on it.

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An additional tip: use the longest focal length you have or the longest one you can use in the room you are in.

A quick and easy method is to put a mirror where the target will be (you can also possibly cover the target with aluminum foil), and scribble a cross at its center.

You then look into you viewfinder, and put two things at the center:

1. the reflection of your lens in the mirror
2. the cross on the mirror

#1 alone guarantees parallelism. #2 centers the camera FOV

Maybe the most reliable way would be to shoot the artwork on the floor anyway.

I mentioned in the question that it wouldn't work for larger artworks. However, thinking about it, mounting the camera on a giraffe boom (or whatever the actual name is) would allow to photograph quite large artworks.

Having both the canvas and the sensor parallel to the ground allows to make full use of the two axis of a spirit level. There may still be an angle between the sensor and the subject, but this angle being around the lense axis it's the easiest to correct by eye.

An additional advantage of this method is that it's easy to put a white or colored cloth behind the painting and therefore maybe allow autocropping in post.

However, this method might not be very convenient for large artworks unless using a wide angle lens. A painting with a length (not width) of 1 m would require a distance (i.e. height) of 1.40 m with a 50 mm lens on a full frame sensor, or 1.70 m with a 40 mm lens and an APS-C sensor, which would be impracticable without an articulating screen.

Note that I didn't try this method yet.

A tube placed on the surface of the artwork can provide alignment. If the far end of the tube (smaller) is perfectly centered within the opening of the tube (larger); then the camera must be perfectly aligned with the tube, and therefore with the surface the tube is on.

We have one of these https://www.versalab.com/PARALLEL.html

It works pretty well for getting things aligned but the laser is not super powerful. Currently looking at options for something better.