>>3295123The top of the cone is where the pinhole is. It gets shoved into the DSLR camera body past the mirror and keep the mirror in the up position. This allows for a shorter focal length. In this image, you can see where the pinhole is being placed inside the camera body. That is why I call it, "Invasive". The "Recessed" pinhole cap does the same thing, but it doesn't interact with the mirror because its focal length is much longer so it doesn't enter so far into the camera body.
>>3295111Thickness and sharpness of the edge of the hole. A thicker material reduces the angle of view. Cheap thin foil, will be about at thin a material you can fine. The ability to cut a perfect hole without a ragged edge can be easier using metal. However, it depends on what type of tools you have access to. The best are chemically etching and laser burning. Whatever material you use, it should be black. The dark color helps prevent problems with ambient light bouncing around inside the camera body. It'd be nice if aluminum was black inside the hole too, that can be very difficult if the focal length is very short since the hole will be ultra tiny.
Long focal lengths need larger holes. Once you get past a certain hole size the type of material you use doesn't matter so long as it is black and light tight everywhere else. Cutting larger holes is super easy and plastic or cardboard is the easiest to cut holes in unless you have a hole punch that does 0.5mm+ size punches.
