Chris,
The formula to calculate the pinhole is:
Pinhole diameter =
√distance from the pinhole to the paper (milimeters) 28
So, for a 5 meters room pinhole diameter is
√5000 = (not exactly, but close to) 2.5mm
28
For the pinhole camera (for paper):
for a 10cm deep box - 0.4mm
for a 20cm deep box - 0.6mm
This formula is to calculate the optimal pinhole size. To speed up times for students you may find that you need to make it up to 50% larger.
For making the pinhole, use a plate as thin as you can find (a can?). 0.3mm to 0.5mm tick is fine.
The box for the pinhole camera is simple, of course. For a school, maybe making a wood box is a good idea. It must be light-tight, use small hinges and a lock for opening it for loading the paper. You can use tape on paper corners to hold it in place. Use black tape to cover the pinhole until you take the picture and after that too. Tape is fine to use as a curtain for a pinhole camera but of course you can manage any sort of cap to cover the pinhole.
Finding the exposure time:
It could be calculated if I know the paper ISO, but the "try-error-try" way works fine.
Start with, for example, a 15sec exposure and develop it.
If too white double exposure time on next try. Keep trying, always using twice the previous exposure time until you're happy with results. (15sec - 30sec - 60sec...)
If too dark do the opposite. Use half the exposure time for the next try.
Made a search on Calumet UK products and what you need is:
- Photo BW paper 5x7":
http://www.calumetphoto.co.uk/item/136-513V/- BW paper developer:
http://www.calumetphoto.co.uk/item/139-347E/- Fixer:
http://www.calumetphoto.co.uk/item/139-402D/- A darkroom lamp. It is the only light source that you can turn on while you load the photo paper sheet on the camera, and when removing it from the camera after exposure and during paper development. Here is one:
http://www.calumetphoto.co.uk/item/167-652B/- 4 plastic trays for chemicals and for washing the paper prints.
http://www.calumetphoto.co.uk/item/167-570R/- latex or vinil gloves
- and a graduate to measure chemicals for mixing it with water:
http://www.calumetphoto.co.uk/item/167-554D/Mixing chemicals with water:
Read instructions on developer and fixer bottles to find dilution ratios.
If 1:5 it means 1 part of developer (or fixer) plus 4 parts of water (total=5)
If 1+5 it means 1 part of developer (or fixer) plus 5 parts of water
Recommended processing times are also printed on instructions on bottles of each chemical.
Always mix developer first and clean the graduate very well after use with each product. Otherwise chemical contamination will happen. Mark one tray DEV and other FIX and always use the same tray for each product. Again, to avoid chemical contamination.
The developing process:
Dispose trays on a bench on this order:
1- Developer
2- Water (for a quick wash between chemicals, 30 seg is ok)
3- Fixer
4- Water (for a final wash, changing the water several times. Usually on fixer instructions is referred the recommended time for the final wash, as it is to remove all fixer residuals from the paper)
Wear latex or vinil gloves when working with chemicals.
And that's it. Then let the prints dry.
Hope all this is clear to you, but feel free to ask again.
Jose
