Pinhole Exposure Times
Hi guys and gals,
I'm about to venture into the (hopefully) wonderful world of large-format pinhole photography, and I need a little help from all you fine people here.
First off, I'm using an old 5x7 Kodak View Camera No. 2-D (not that it matters), with a bellows extension somewhere between 6" and 14". I bought a set of 3 brass pinholes on eBay. They range in size from 0.34mm to 0.63mm.
In order to not waste too much film, I'm trying to figure out a base exposure time for Bergger BFP200. Using the old sunny 16 rule, and calculating my effective F-stop with a bellows extension of 8" as somewhere around f/320 (0.63 / 25.4 = 0.025 inches, 8" / 0.025" = 320), it seems to me that my exposure time in bright sun is between 1/8 and 1/4 second. Even if the true effective speed of the film is closer to 100 than 200, we're still talking about sub-second exposures.
Does that sound right to you pinhole guys? I had been assuming that my pinhole exposure times would be in excess of a second, so that I wouldn't need a shutter (just use some sort of opaque material in front of the "lens", remove it to expose, then replace it), but my calculations indicate otherwise.
I'd appreciate any advice/confirmation/refutation you can offer, and thanks in advance!
Try www.mrpinhole.com for an online calulator. Also, the this little tool, which runs localy, http://www.pinhole.cz/en/pinholedesigner/ can export charts that include film reciprocity...
A quick way to calculate exposure for f/320 -- multiply your f/16 exposure by 400: two stops from f/16 to f/32, and then a factor of 100 from f/32 to f/320, so with a range from 1/100 to 1/200 at Sunny 16, you'd have two to four seconds, not counting reciprocity failure (depending on the Bergger reciprocity curve, you might need real exposures anywhere from four to twenty seconds, and could also need N-1 to N-2 development to tame the contrast rise).
Photography has always fascinated me -- as a child, simply for the magic of capturing an image onto glossy paper with a little box, but as an adult because of the unique juxtaposition of science and art -- the physics of optics, the mechanics of the camera, the chemistry of film and developer, alongside the art in seeing, composing, exposing, processing and printing.
Thanks for setting me straight, guys.
I, ummmm, screwed up by, ummmm, making a, ummmm... simple mistake, namely calculating the difference between f/16 and f/320 as 4 1/3 stops, when it's actually more like 8 1/3 stops. :rolleyes:
2-4 seconds in bright sun, that sounds more like what I was expecting. Whew!