The optimum pinhole size is 0.4mm but I can't make it smaller than 1mm.
This is the last test: 10min
When I've made pinholes, I've used very small sewing needles (quilting ones are labeled by size) and not gone all the way through. I then smooth the area from each side using emery cloth (cloth based sandpaper intended for metal). I use pieces of soda cans as my material. I usually color both sides with either spray paint or Sharpie marker. To measure, I have a magnifier with a scale built in, but a magnifying glass and a metric ruler should work fine. Mine are usually around 0.5mm - 0.6mm, though I can make smaller ones if I try hard.
When I build (converted, actually) mine, I used the pinhole calculator already posted and drilled the hole in brass shim stock by hand with a tiny drill bit (#79, 0.0145") and a pin vice. For exposure times, I've had pretty good luck with two Android apps: LightMeter for the initial EV, and Pinhole to calculate the exposure for the pinhole camera. The latter app allows you to save the details of your camera so you can simply call it up and consult the exposure tables when you need them. Both are free and available through Google Play (no personal interest, just a satisfied user).
If you cannot make your pinhole smaller than 1mm then make your camera film to pinhole distance bigger, or buy a correct size pinhole.
By the way I agree with KS Klain that the first one is interesting the way it was. I believe you could make some very interesting photographs with that bigger pinhole.
I make mine the same way Bethe does: a square of aluminum form a soda can ( you can cut it easily with scissors ) a sewing needle, and emery paper. I use 600 grit paper. I spin the needle with light pressure until it just pokes through, then sand on both sides a lot, then repeat. The hole is always way too small to begin with. It gets bigger as I keep working on it and with some practice you'll get an eye for when it's about right.
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I am making the holes in a hard black paper. But I will try using aluminium from soda can.
I just made a 0.5mm pinhole. Very close to 0.422mm. I will test tomorrow but I am sure it will be good and sharp.
Thank you all for the information and help. I love it more than my brand cameras.
Now I am thinking about to load 4 sheets of paper in the camera. Maybe place a black thick paper between each of them just to make sure tha the light is not going to get though the first paper and expose the seccond one behing it. Like this I can go to the city or woods, and after each exposure I can put the exposed paper behind of those not exposed yet. Of course I woud do this changing in a change dark bag.
Do you think it woud work? Or any of you have a better idea to reload and take more than one shot without have to go back home to reload the camera?
I think that could work. I sometimes keep envelopes for exposed and unexposed paper in my changing bag and carry it.
I thought of two more things:
1) If you are shooting paper, you can aim to make your pinhole slightly smaller than "optimal" because most of the pinhole calculators use "daylight" and the paper will be more blue sensitive.
2) If you use a little water when you are sanding you can avoid breathing the aluminum dust.
And most important of all: have fun!
By the way, I am using Ilford MGIV RC Satin and I really like it as paper negative. But I am not sure yet about the ISO. I did the maths all wrong. rsrs.
But I guess it is ISO 25 about.
I rate photo paper normally at ISO 5.
"Have fun and catch that light beam!"
Bert from Holland
my blog: http://thetoadmen.blogspot.nl
my Linkedin pinhole group: http://tinyurl.com/pinholegroup
* "So much time and so little to do. Wait a minute. Strike that. Reverse it. Thank you." (the original Willy Wonka: Gene Wilder, 1971)
* My favorite cameras: Leica SL, Leica M7, Russian FKD 18x24, Bronica SQ-B and RF645, Rolleiflex T2, Nikon F4s, Agfa Clack and my pinhole cameras
It's is sharp now:
It is f200, 45sec exposure.
I think the white part was a light leak which I can fix easy.
And as the test show it is ISO 6.