plate holders, film inserts, plane of focus ???

I'm mostly asking about the single sided metal plate holders. OK, a glass plate is pushed up to the proper plane of focus by the flat springs inside the holder. When using a film insert, it is also pushed up by the flat springs in the bottom of the holder.
But, do the folded over parts of the insert that hold the film prevent the film from getting all the way up to the proper plane of focus? Seems like there is a difference due to the thickness of the metal edges holding the film.
Am I crazy?
Thanks,
Robert

I once measured and calculated that the difference in height in the cases you describe are within the depth of focus of the typical (f/6.8 or slower) 9x12 camera lens.

Regards,
Jason

The 9x12 film sheaths I have in my Voigtländer plate holders don't have any folded parts where the film is pressed against the "frame" inside the holder, so the film is held exactly in the same place as a glass plate would, so the focus plane is the same.

I have some holders (Kodak and unbranded ones) with a hinged frame and pressure plate inside so you can use either plates or sheet film without adapters, but the end with the hinge is deeper inside the holder than the upper part, so with these holders there is a noticeable difference in focus. Worst are the 6,5 x 9 holders of this type. On the negative you see a focus difference of 2-3 feet when you compare the top and the bottom of the frame when focusing at about ten feet.

I used Kodak 4x5 Sheaths in the old Folmer slotter plate-holders. I have noticed some focus issues with my Graflex RB with these holders, but I think that's mostly due to the focus being too loose on this camera.