I hope this doesn't get dumped as off-topic, but it is about 35mm lenses.

A friend of mine plans to do some whale watching, but he has only a modern follow-on to the fabulous Olympus Pen model with a non-film light receptor 18 x 13.5 mm. He is looking at longer lenses than he has, but is choking on the prices of the new auto-everythings. I made him aware that all the old manual OM lenses would still work on this camera. I am wondering how the shorter distance to the "film plane" and the smaller size of the non-film receptor affects the operation and performance of the lenses. Does the adaptor for these lenses correct for the distance? Does it then just act as if the camera has a mask down to 18 x 13.5?