What happens if you stick A4 size sheet on a pinhole camera instead of butter paper or tissue paper?

Answer 1

It depends what density of A4 paper you were using.

At above 60 gsm you would see not very much. However at 60 gsm A4 paper is getting thin enough (like tissue paper) to be slightly transparent and so allow an image possibly to be seen. Tissue paper is used as it is thin enough to be transparent enough to see the projected image.

If you were to make your pin hole camera a camera obscura - a box big enough for you to physically get inside - you could use a piece of ordinary 80gsm A4 paper onto which to project the image from the pin hole and view it from the same side as the pin hole: in this case the light does not need to penetrate the paper "screen" to be viewed and so thicker, non-transparent paper can be used. The camera obscura was used in this way with the occupant tracing the image projected onto their paper to create a good drawing of the scene they wanted to draw.