• I'm not a pinholer, but as an engineer I think simple optics is the problem.

With a 1.2mm pinhole diameter, even a subject at infinity will only resolve to a 1.2mm diameter circle on film. This gives you a resolution of about 30 x 24 on 135 film. Closer subjects will reduce resolution further. In order to improve resolution, you need to reduce the diameter of the pinhole; but at some point you will run into problems with diffraction, since the long focal length will magnify the ray divergence due to diffraction.

 OK, I did the maths and I think you will become diffraction limited with a pinhole diameter of around 0.4 mm. At this point, the pinhole diameter is the same as the airy disk diameter and the total uncertainty (calculated as the RMS of the airy disk and pinhole diameters) is minimum at 0.55mm. That still only gives you resolution of 66 x 44 or so on 135.

Calculations were for green light (500 nm) but blue light doesn't gain you much - it only reduces the RMS circle of confusion to 0.52mm.

So the problem with long focal lengths is that the f-number is small (f/625 in our example with a 0.4mm pinhole) which puts a lower limit on pinhole size (due to diffraction) that is still too large for good resolution (due to geometric optics). This is a fundamental physical problem, nothing you can do about it (except to reduce your pinhole size to 0.4mm to get the best possible resolution at that focal length).