 Christopher A. Walrath |
Pihole doodads and reciprocity thingys OK. Quandry. I have been on the periphary of pinhole photography for sometime but, now that I am finally able to develop my own B&W I'm going for it. I took an old folder (6x6 Wirgin) and removed the bellows/lens assembly to make a 35mm perspective control lens and I was trying to come up with another fate for the folder body than the trash receptical. Then it dawned on me. Medium format pinhole. I reassembled the innards (roll film brackets, film tensioners) and then set about to building the front of the camera. I cut cardboard from the back of a Steno pad about 10mm larger than the old lens window and cut a hole in the center of that and then covered it with black electrical tape. I then cut a 5cm square piece of aluminum foil, rubbed it flat and taped it to the inside of the pinhole board. Then I mounted that to the front of the body and marked the center on the inside. Took a regular sewing needle and applied very light pressure while rolling it slightly in my fingers until the needle tip just pierced the foil. Then I cut out another square of cardboard, smaller this time and covered it, taped a hinge onto it and mounted it over the pinhole. I also fix a couple loops of thread so that I could more easily open the cover. Put a piece of tape over the tripod hole inside the camera body, loaded film, tied the lid closed with a thin black shoe string and christened it the Walra-Lux 2008. Now I have made a couple of exposures. But I need to test this camera a little bit. I have TMX-120 loaded and the aperture is around f/256 (needle tip miked at about .2mm and the focal length is about 27mm). This would require any meter reading calling for an exposure at EV16 about one second exposure. Which means EV 10 would need a little over a minute mathematically. How would I figure reciprocity adjustments? Since Kodak publishes an additional 5 seconds if a meter reading requires ten seconds, I'm figuring that you should add about four seconds to a reading that would call for an 8 second exposure. And since that same publication says to adjust a 100 second required exposure by doubling it so probably about the same for a two minute exposure, expose for four minutes. But waht about in between? Do I do what Kodak did and just bracket it until I get the matching exposure and publish it or has anyone here already done pinhole and reciprocity with TMX and can shed some light on the subject (no pun intended)? Any comments would be greatly appreciated. Thank you
|
|
|
|
||
|
Alan N. Marcus |
Hi Chris, Reciprocity law failure is a curve that can be grafted. However I think you will find this table adequate for the task. 1/10,000 sec. = + ½ stop i.e. shutter speed times 1.5 Alan Marcus
|
|
|
|
||
|
Alan N. Marcus |
For theses who are curious: The sensitivity of film to light (exposure) is rated by a method established by the International Standards Organization (ISO). Film is coated (emulsion) with a myriad of light sensitive silver crystals (salts of silver). A numerical value (speed) is assigned based on how much light energy is required to cause the average crystal in the emulsion to be rendered developable. This value is a variable based on circumstances. While many factors cause this value to alter, shutter speed has the most pronounced effect. ISO plummets for very short exposures, such as the type employed when recording super fast motion like bullets or humming birds in flight. ISO also plummets when time exposure is employed. It begins to drop when the speed exceeds 1 second. Photographers must take this behavior into account and apply exposure compensation. In a nut-shell, reciprocity failure is a disobedience to the published ISO. It occurs when the shutter speed is set outside the customary range. In other words, exceeding long or exceeding short exposure results is a plunge of the rated ISO speed. Why? Silver salts react to light via an internal chemical change. If the exposure is exceptionally short the silver salt will not have sufficient time to wholly absorb and react. In other words, the sliver salt is under “chemical inertia” a force that tends to keep the crystal stable. Exceeding short exposure demands additional light energy to cause the crystal to be exposed. Should an exposure take place over a longer than normal interval, the photon hits, doing the job, come in serial fashion meaning there is a time lag between hits. During the dwell periods between hits, the crystal has time to restore itself somewhat. The net effect is more exposure than one would calculate is required to render the crystal developable. Alan Marcus (marginal technical gobbledygook)
|
|
|
|
||
|
Christopher A. Walrath |
Posted by Alan Marcus "Should an exposure take place over a longer than normal interval, the photon hits, doing the job, come in serial fashion meaning there is a time lag between hits. During the dwell periods between hits, the crystal has time to restore itself somewhat. The net effect is more exposure than one would calculate is required to render the crystal developable." Ya know, there are questions that never occur to me until the great guru tells me something I didn't know I wanted to know. You rock, Alan. Thank you
|
|
|
|
||
|
Christopher A. Walrath |
I came up with these general numbers. As f/256 at one second is EV16, here is what I have. EV - recommended time - adjusted for reciprocity This fits Kodak's recommendations for TMX in their literature which states 'expose TMX for 15 seconds when exposure calls for 10 and expose for 200 seconds when exposure calls for 100'. Works so far. Thanks
|
|
|
|
||
|
This old forum is now archived. Use improved Forum here
Report this Thread |
||