Lily, when we did this at a school where I worked(I am a Science Technician) we found that the cleaner (and within a cerain range of small sizes)and more circular the hole the better and cleaner the image, we solved part of the problem by making a larger hole in the container and gaffer tapeing a piece of aluminium foil over it and made the hole in the foil using a needle, NOT a pin, as the profile of the point on the needle is better. a focusing screeen was also very useful we made one from a piece of tracing paper stretched on a frame and placed in the position the pahotographic paper would occupy. We also had a supply ( I dont know from where) of photographic paper that produced a positive image on developement so there was no need to make a negative first. it was fun and