I'm trying to build a small slide projector but (being a lens noob) I'm not able to get a proper focused image.
I've got a 20mm (biconvex?) lens with a focal length of 50.8mm so when calculating the distance I would need a distance of about 81,28mm but whatever tweaking it does not focus.
Apparently I'm doing something wrong here or do I need a different type of lens?
Thanks in advance!