Hi there,
I have built many pinhole cameras in the latest 6 years, and I am now building my first telephoto one.
I basically cut a 2" PVC pipe, lined it with black velvet and glued a 35mm body cap on one end, while on the other end I put a pipe fitting with a an aluminum foil with a pinhole - that way I can change pinhole size easily. Internal reflection is completely absorbed by the velvet, and the tonal range is really good .
I made a 1.2mm pinhole for a 250mm tube, which is about 1.7 times larger than the optimal size recommended by Pinhole Designer and other manuals (0.7mm).
The results are very, very blurry, so much that I can barely distinguish my subjects.
I have made pinhole cameras with much larger holes compared to their optimal sizes, and they never looked so fuzzy.

Logically reasoning I thought that this might be due to the narrow FOV.
If I used a, let's say 4x5" film instead of a 35mm, while keeping the same hole diameter and distance from the film, the circle of confusion would have the same size but on a larger image, thus making the image appear less fuzzy. I could then say that a narrow FOV will give a fuzzier image to begin with, even with an optimal aperture.

I haven't built an optimal pinhole for this length yet, but I don't think it will be useful to my project. I might just go ahead and try just for testing purposes.

Has anybody experienced a similar issue?