As a follow-up to this question, I was thinking about how one might build a pinhole camera with extremely long focal length to photo the Sun with a DSLR.
Imagine that pinhole is at the end of a long piece of black PVC which is well sealed to the camera.
A 1500mm tube and 1.6mm pinhole is a combination produced with an online pinhole calculator. This would produce an image that would mostly fill the frame of an aps-c DSLR.
Wikipedia seems to indicate that the circle of confusion is the same size as the pinhole, and 1.6 mm is far too large circle of confusion to be useful on a DSLR.
Can a smaller (0.1mm) pinhole be used to improve the resolution even at long focal lengths?