Sorry if this is already on here somewhere, but I did a quick search and didn't come across anything. I'm looking for a f/stop chart that goes down to ridiculous aperture numbers (in the 3 or 4 hundreds). When using my pinhole camera I've just been taking a random exposure time guess and then developing - then making revisions on exposure times based on that. A chart would be nice though. Any help would be great, thanks.
I've no link to a chart, but I'll instead describe the method I use to determine exposure with pinhole. I use a handheld light meter that goes up to F128. I set the meter to the exposure index that I rate the film or paper to, then meter the scene, then reference the recommended exposure time opposite F128. I then use this formula:
Tc = Tm * ( Fc / 128 ) ^2
Tc = corrected exposure time
Tm = metered exposure time at F128
Fc = focal ratio of pinhole camera
If you're shooting with the same camera all the time, the values for "( Fc / 128 ) ^2" become a constant that you can write down and carry with you, and then just multiply this constant by the metered time at F128.
If you don't have a light meter that goes all the way to F128, you can meter a scene for F16 and just plug "16" in place of "128" in the formula.
NOTE: This formula does not take into account reciprocity failure, which is common with most B/W films, but less so with Fuji Acros, and essentially nil with paper negatives. There are other formulae for compensating for reciprocity failure, which would do you well to research. But in general, exposures from 1-10 seconds I double the calculated exposure time.
Is "focal ratio" the same thing as focal length? The rest makes sense to me. Thanks for the formula, I'll definitely try that out next time. I'm using paper negatives so I don't think I have to factor in reciprocity failure.
Originally Posted by Joe VanCleave
Just keep doubling the values every other stop, and you have it.
From 1.4, doubling: 1.4, 2.8, 5.6, 11, 22, 45, 90, 180, 360, 720......
From 2.0, doubling: 2, 4, 8, 16, 32, 64, 128, 256, 512.......
Merge them together in the right order, and you have a list of whole stops from 1.4 to however high you would like to go.
Focal ratio is the so-called "F-stop" of your pinhole camera, which is calculated by dividing the distance from pinhole-to-film by the diameter of the pinhole. You want to use the same units for both, of course. It's most convenient to use millimeters. For instance, if my camera has a 120mm distance from pinhole to film, and the diameter of the hole is 0.3mm, then the camera's focal ratio is: 120/0.3 = F/400.
Originally Posted by aaronmichael
It's common to find the pinhole-to-film distance referred to as "focal length," although technically you can't focus a pinhole (or rather, pinholes are fixed focus). The distance is usually measured from the pinhole to the center of the film plane.
This is also a good time to mention that there are formulae for optimal pinhole diameters. I believe it was Lord Rayleigh who figured out that for a given "focal length," there's an optimal diameter of pinhole aperture for a give wavelength of light. Any larger and the image gets softer because of simple geometry, while any smaller and the image begins to get softer due to diffraction of the light around the edge of the aperture. There are people who obsess over this to the point of missing the bigger picture, which is that it's pinhole, and it's supposed to be (somewhat) soft focus. A more important point is to ensure your pinhole is as round and smooth as possible, and "in the ballpark," size-wise.
Anyway, I look forward to your images. Keep us posted.
Just popped in to read this topic and I have to say this is one of the best explanations I've come across...I think I can understand this one. Thanks!
Thanks for the explanation about the focal ratio. The camera I'm using now has a .46mm aperture, and a focal length of 190mm - works out to f/413 (I think you saw one of the images made with it in my other thread about the size of the camera). The pinhole should be larger but I found that when I used the optimal pinhole size for the focal length it became too unsharp for my liking. I swapped it for another pinhole I made for a shorter focal length camera.
Originally Posted by Joe VanCleave
Sorry - forgot to ask one more thing: I'm assuming the "metered exposure time" is in seconds?
I suppose shooting, evaluating, and re-shooting is still a good method. The problem with paper negatives is that they all vary in ISO based on brand (or I have read). I just did some reading on it and some people rate it at ISO 6 and others at like ISO 25 (even when using the same paper?) I wish there was a standard.
Just wanted to say thanks again to Joe for that formula. Had some time to spare while I was at school the other day and decided to lug my 8x10 wooden pinhole camera out for a shot. I bike to school so it was a hassle carrying the camera to where I went and riding at the same time, but having the formula in hand gave the assurance I wasn't going out and making a guess. Here is the final product:
I metered for the sky and then added an extra stop of time because I tried using a yellow filter for the first time.