We build a similar camera in the photo class I teach. We use a mini Pringles can, so the focal length is about as wide as a pop can. This camera is used to teach students that you don't need a $500 camera to make pictures so we don't fuss much about focal length and exact pinhole size. I have found, however, that the push pins with the handle on them, when pushed almost all the way up the sharp end (not big enough for the metal pinhole to slide up the shank of the pin) is a perfect hole size for a 4-5 second exposure and fairly crisp negatives. We use pop cans for pinhole creation, poke 'em and then sand the front and back to get the burrs off. Spray paint the inside of the can, tape over the lid and cut some multigrade paper. Most importantly have fun!
You can change the contrast of the print by using the contrast filters on your enlarger when you project light onto the neg/pos sandwich with multigrade paper. No need for in the camera.
Attached is a photo of a camera my daughter (who was 5 at the time) and I made as well as negative made from said camera.
"focal length" is probably the most practical phrase, despite pinhole not being as critical as a lens. One could argue that it is easier to measure for a pinhole camera than figure out exactly where it's measured from on a lens camera!
If you consider any alternative terminology in place of "focal length", they may be arguably correct but raise more questions than they clarify.
The terms image distance and object distance correctly (from an academic standpoint) define the distance from pinhole to film(paper) and item being photographed (subject or object) to the pinhole, but are used less often because fewer people are interested in the physics than are interested in the art.
If better understanding of pinhole results from an occasional cognoscenti utilizing alternative language or discussion about it, then it serves some purpose. Otherwise...not to worry.
Say, that word "cognoscenti" sounds like it should mean "I know that smell" in some classic language, doesn't it?
Goros, it's best to just jump in and try it! I'd recommend making your pinhole in a thin sheet of brass shim or other metal, and mount it to the tin using black electrical or gaffer's tape, so that you can replace the pinhole if you decide to at a later date.
When I'm building a new camera, I measure the 'projection distance' and then refer to a pinhole calculator for the optimal size. I try to stay with millimeters in measurements, because I usually measure my pinholes using a loupe and a millimeter scale. I place the pinhole next to the scale and back light the hole, then use the loupe to guestimate what fraction of a mm the hole size is. For instance, I recently made a mini 'spy cam' with brass tubing, that had a projection distance of about 1cm. So I figured F/100 was pretty close to optimal. That meant the hole had to be ~1/10 mm in diameter.
The quality of the pinhole - how round and smooth it is - is at least as important as proper size, IMO. So when I'm drilling my own pinholes I will often reject 3 or 4 before arriving at a smooth, round hole near the optimal size.
I like contact printing paper negatives; the image gets reversed in-camera at the time of exposure, due to the optical projection properties of pinhole lenses. Then it gets reversed back to the 'correct' orientation when contact printed emulsion to emulsion. You can also scan the paper negatives and flip horizontally in PS, for quickie online posts.
I like your idea of a water drop lens. Try it, and tell us how it worked. The thing is this: don't ever lose your creative urge to explore new ideas. That's what keeps pinhole fun.
One last thing: try grade 2 RC paper as negatives. Better control of contrast than multigrade.
Thaks for your comprehensive reply.
Of course I will try. As Steves (both of them), Birger, Jim, Murray and yourself have said, I'll just do it (I'm not related to Nike or similar brands) and enjoy it.
Regarding using Grade 2 RC paper, I think I will put a Grade 2 filter at the back of the pinhole and change it according the light conditions.
The water lens is an idea I got from a friend of mine who uses to do macro photography of water drops showing in the drop the scene that was behind. For this purposes I think refined oil will be better than water (evaporation matters). I will let you know.