Holga 120 WPC exposure times

Question

I'm struggling to reconcile the recommended exposure times of a Holga 120 Wide Pinhole Camera as suggested by Holga, vs the values I am getting via an app like Pinhole Assist (or even when computing times from the EV captured by my Sekonic L-408 - which are in the same ball park as the app)

The specs of the Holga state that its aperture is f/133 (for a 0.3mm pinhole). And the film plane is 40mm/1.6" from the pinhole.

The Holga recommended exposures for ISO 100 is:

   Fine Weather 7-9 seconds
   Overcast     10-12 seconds 
   Morning/Dusk 13+ seconds

But when metering a +15 EV outdoor scene, both the Pinhole Assist app and my calculations give a shutter speed of around 1/2 seconds (which is impossible to accurately time anyway). This is so significantly different from the Holga value that it confuses me.

Can anyone explain why I am seeing this difference, or even if it is critical? (Or is EV 15 considered way more than a "Fine Weather"?)

I have only run 1 roll of film through the camera and haven't yet gotten it processed. But I want to feel comfortable that I am sort of in the right ball park before committing a lot more film to it, but also don't want to waste film/$$ characterizing the exposure of my camera.

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And finally, yes I know it's a Holga (I also have a non-pinhole Holga 120 that I have used and enjoyed) and that part of the "fun" is that it isn't a perfect camera, but there is a difference between capturing something interesting and a negative that is either fully clear or totally black.

Answer 1

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.

Answer 2

Unless you are using color negative film that can absorb a large amount of overexposure, I would disregard the 'shutter' times that are shown on the camera. I use an orange Wratten gelatin filter glued on, which with Acros film means ISO 25. Using f/150 on my light meter, I usually expose in the range of 2-8 seconds for daylit motifs. In this range, with Acros, I do not worry about reciprocity.