Pinhole doodads and reciprocity thingies
Hi guys. I'm trolling this one. I've posted this at three other forums including f295, but HERE IS WHERE I BELEIVE I WILL FIND MY ANSWERS. Here goes.
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 what 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 very much.
Sunny 16 and TMX sez f-16 and 1/100
To move that to f-256 :
(256 / 16) ^ 2 = 256 (duh)
256 x 1/100 = 2.56 seconds without reciprocity.
Now at this point you could just look at the chart/table for TMX make the adjustment for time. Or if you prowl around on some of the astronomy sites, (http://www.robertreeves.com/filmtest.htm) you might find the "p" coefficient for the Schwatzchild equation. For TMX, at least according to Mr. Reeves, p=0.81
This equation works like this:
Tnew = (Told + 1) ^ (1/p) - 1
Tnew = (2.56 + 1) ^ (1/0.81) - 1
Tnew = 3.8 seconds
Lets just call it 4 seconds.
Yes, this does seem like a lot of work but to me it isn't. I'm quite comfortable with the math and to be frank, just pre-compute some of these things and keep them on a cheat sheet. Something like sunny-16 and then +1 stop, -1, -2, -3 just like the tip sheet that came with film long, long ago.
f16 1/200 -> 2sec
f16 1/100 -> 4sec
f16 1/50 -> 8sec
f16 1/25 -> 19sec
f16 1/12 -> 45sec
Some films suggest in their reciprocity charts that you change the developing time. So far, I've just skipped that part because I do enough testing with the darn LF camera. I'd rather just whack together the exposure chart and go play with my latest Frankenstein'd camera.
I once scanned from a book a circular calculator which has around the edges all the f-stops up to ... a lot! Anyway, you would utilise it by reading your meter for f-16, set the cardboard calculator at the shutter speed for f-16 and read around to f-256. You could probably buy one from a place like Pinhole Resource (USA) or Zero Image (Hong Kong). Also years ago I once took notes from a book in a shop which had details of reciprocity for each brand of film... they were all different. Very exhaustive tests. I didn't write down TMX as I didn't use it but I copied TMY and I'd expect they're in the same ballpark. Here are the figures: They're all in order. If the light meter says in seconds; 2,4,8,12,16,24,32,45,60,90,120 the the new reciprocity time for TMY will be; 3,7,18,32,54,90,140,206,300,550,800 >>> if your times are longer than this, there is enough of a pattern to work out the difference. Hope this helps, both have served me well.
Or you could download and install Pinhole Designer and use its exposure table calc to do the math for you, and even figure the reciprocity for many different films. I'm sure TMX is listed and has reciprocity data.
But having the math is good too, escpecially for films that are not listed in Pinhole Designer like Efke 25 and 50.
My Weston Master IV (dial style and sweet) goes up to f/32. So I just take the time and multiply by 2 to the 6th for recommended time. I got a formula from a guy on f295 and worked it out kind of this morning. But the figures are off from what is provided by blokeman and I wanna check out rw's math and see what comes up. I might use these to see what geometric series is closer for my camera. Thnak you all for the ideas. now to the testing.
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Blokeman said he used TMY & assumes it's the same. (He's probably right; I think
they were both in the same datasheet).
He's also in a different time zone .
The formula I gave you was fairly close to the curve Excel plotted for a 3rd order polynomial fit. And there are only three data points given in the TMX data sheet.
If you compare the difference logarithmically (for f-stops), the difference becomes trivial.
You will find many different approximations as well as people who are unhappy with manufacturer's reciprocity data. You'll probably find the widest range of opinions on the older films like Tri-X, Plus-X, FP-4, HP-5, oh, and those Foma and Efke ones are way out there.
There are many people who do not correct anywhere near as much as mfr's suggests.
Luckily TMX is a modern T-grain film and not as bad as the old ones.
You'll be close. If you look at too many opinions you'll have to guess which one you want to use! :O)
My f512 meter:
Seems to work well as long as exposure times are short enough to get away from reciprocity.
Nice piece, Greg. I love homemade pieces like this. As soon as I have the opportunity to take some decent pics, I'll post them of my all-new homemade 75mm Wollensak Anistigmat f/4.5 MD mount bellows style adjustable perspective control lens.
Actually TMY recommends 300 seconds for 100 seconds meter recommendation. TMX is 200 seconds. And TM- films require no extra development. The Tri's and Plus's do, however. I thought I had it figured out and even told someone, sorry Murray. Looks like I gotta back up and punt. '4th and 5. Roby at the 30. . .'
Originally Posted by blokeman
OK. Processed some trial negatives. Exposure called for two minutes. Exposed 30 seconds. One minute. One and a hlaf minute. Two minutes. Three minutes. Four minutes.