calculations – Holga 120 WPC exposure times

Why would you commit any more film until you have had the first roll from any film camera developed? Get the first roll developed and see what it looks like.

If it’s massively over or under exposed, then dedicate the next roll to test shots where you systematically start at a longer exposure and then decrease the exposure time about one-half stop each frame or two. Don’t forget to write down the exposure times for each frame so you’ll know what you did for the frames that come out properly exposed.

Doing it systematically for one roll will almost certainly get you where you want to be with less “wasted” film that trying to hit the nail on the head haphazardly for several rolls before you get lucky and hit it.

EV15 at ISO 100 and f/133 is around 1/2 second. This should be proper exposure for a brightly sunlit scene. The problem with calculating an f-number from the absolute size of a physical aperture to determine exposure with a pinhole camera is that “focal length” isn’t exactly defined in the same way as it is with refractive lenses.

Having said that, most 120 cameras have more than 39.9mm between the lens board and the film plane. Does your Holga only have about 40mm (1.6 inches) between the pinhole and the film plane? That’s the focal length that figures to f/133 with a 0.3mm entrance pupil (aperture).

Another consideration is that with an aperture that narrow, an appreciable amount of the light going through it will be scattered due to the effects of diffraction. Much of that scattered light will fall outside the area of your negative.

Don’t forget that with film, any exposure longer than about one second or so will be subject to the Schwarzschild effect.

Most film manufacturers publish data sheets for each of their films that outline development times for shooting the film at different speeds as well as for developing the film when it is shot at the advertised sensitivity. They also include data regarding exposures longer than about one second (for most films) that are affected by the Schwarzschild effect, also known as reciprocity failure. Each film has different characteristics, and how much compensation must be made for long exposures can vary significantly from one film to the next.