Can a clip filter distort images ?

There is a slight change of angle when imaging through a plane filter in a converging cone of light, depending on optical thickness of the filter and equivalent refractive index

Yep, that sounds like the amount of shift that you might expect due to a plane parallel plate with a relatively large FOV.  Stopping down won't change anything.  It's all about the Chief Ray.  It should look like a very small magnification change and most registration algorithms should take care of it.

John

andrea tasselli:

Peter Robertson:
Thanks for that. It sounds like stopping down might reduce the issue then? I usually shot wide open. On a Canon EoS Ra the clip doesn’t “ click” in place the same way it dies in a Rebel,  So I was wondering if slight variations in the seating is also might contribute if it is no exactly parallel to the sensor.

As John said, not really. It is not a function of the field angle at all. I don't think very slight variations in position are going to change much of anything, to be honest.

Andrea,
Perhaps you didn't understand what I said.  When a tilted ray passes through a plane parallel plate, it gets displaced.  The bigger the tilt, the more the displacement.  Here's a calculator:  https://www.edmundoptics.com/knowledge-center/tech-tools/beam-displacement-calculator/.  For a typical 3 mm filter, with the Chief Ray at 5 degrees, the displacement would be about 90 microns.  That's easily enough displacement to notice when compared to an image taken without the filter.  The OP didn't say what lens he is using but the field angle at the edge of a 42 mm image circle with a 135 mm lens would be 8.8 degrees so the displacement would be even larger.

John