Reciprocity Failure, color film, pinhole

<i>Mod note: Moved from b&w film forum.</i>

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<p>I call again upon all you fine people for some advice. I am diving into the world of pinhole photography at the moment. I purchased a lovely little camera from Zero Image, and the f/stop is about f/235. Yeah...</p>

<p>Luckily the camera has a built in guide that allows you to match up a given exposure reading, say f/5.6 @ 125th of a second, and then read the correct time for the pinhole. Then you must take reciprocity failure into account. One photo said 4 minutes... so I shot for a bit longer. Turns out according to a chart I found later, that I should have shot for 40 minutes. Crazy. I don't mind, I just wish I could know how long to do it for. Now that black and white doesn't seem to be an issue with the chart I found, I am wondering about color.</p>

<p>I have a lot of Kodak Portra 160 NC & VC. From all the googling I have done, I see that no corrections are needed up to 10 seconds. They don't say anything about past that. I wanted to try some really long exposures, but I have no idea what the correction factor is. I could stick to lighting situations that require no more than 10 seconds, but I'd like to know for other circumstances if possible. Does anyone have information that could help me?</p>

<p>Thanks for any advice.</p>

<p>Cody</p>

<p > Reciprocity:</p>

<p >E=IT is the equation called the law of photographic blackening or the law of reciprocity.</p>

<p > </p>

<p > This simple equation tells us that the amount of blackening of film upon exposure, and development will be a result of the intensity of the exposing light "I" multiplied by the time of exposure "T". The product of these two actions results in exposure "E".</p>

<p > </p>

<p >Now film is sensitive to light energy and when light plays of the film a chemical change occurs. The developer is selective in that it has the ability to distinguish unexposed sliver salts from exposed silver salts. </p>

<p > </p>

<p >To explain, we resort to the corpuscular (particle) theory of light. Thus when the shutter opens the film is bombarded with photons. The working ISO relates to how many photon hits will be required to render the silver crystal developable. The published ISO value applies only to moderate shutter speeds in the range of 1 second to 1/1000 second. In other words, the published ISO assumes that the photon hits will occur over a typical time span. Normally these hits occur in rapid succession. Should the exposure be quite lengthy, the photon hits are spread out, over time. If true, there are delays between hits. These intervals of no activity afford some “healing” time before the next hit occurs. Thus, films undergoing unusually long exposure times require more exposing energy than the reciprocity law equation discloses.</p>

<p > </p>

<p >Conversely, if the shutter speed is fast, 1/1000 second or faster, the photon hits happen in quick succession. The full impact of each hit is not finished doing its deed before another hit occurs. Thus, the film will require more exposing energy then the reciprocity law equation discloses.</p>

<p > </p>

<p >Therefore, we are talking about applying a correction factor for reciprocity law failure. </p>

<p > </p>

<p >Each film has different characteristics thus different correction factors apply. </p>

<p > </p>

<p >Try this table:</p>

<p >1/100,000 sec. + 1 stop</p>

<p >1/10,000 sec. + 1/2 stop</p>

<p >1 - 1/1000 sec. none</p>

<p >10 sec. + 2 stops</p>

<p >100 sec. + 3 stops </p>

<p>Notwithstanding the fact this forum is oriented towards B&W film processing, it should be noted that the corpusclular theory espoused by Newton is no longer an accepted theory. While he was definitely on the right track, there are a number of problems with the theory. </p>

<p>Note also that the term "photon" was not used by Newton. His particles were called "corpuscles". Photons are associated with the modern wave-particle theory of light and represent "packets" of energy.</p>

<p>Thanks for that explanation Alan. I didn't even think to post this elsewhere, I forgot this was for black and white. Should I move this somewhere more appropriate?</p>

<p>The issue with a fixed stop is one cannot or may not want to open it up.<br>

(Cannot :is the pinhole cameras concern ie focus. may not: maybe an astrophotographers settup; the lens is already wide open)</p>

<p>Thus the issue is one might have a meter to say shoot at iso 125 in bright sun:</p>

<p>1/125 @ F16;<br /> this is 1/60 at F22;<br /> 1/30 at F32;<br /> 1/15 at F45;<br /> 1/8 at F64;<br /> 1/4 at F90<br /> 1/2 at F128;<br /> 1 second at F 180;<br /> 2 seconds at F256<br /> Your Bright sun exposure is thus about 1 or 2 seconds</p>

<p>****In shade or on a dark day the meter may say 10 seconds with your F235 pinhole; but the reciprocity table says open up 2 stops. If you use the same F235 and use 4 times more time; ie 40 seconds; one has the double wammy of more reciprocity. !!!</p>

<p>Instead of 40 seconds at F235 one might have to expose for 2 to 3 minutes!</p>

<p>Each film layer has different reciprocity; one gets color shifts to</p>

<p >When it comes to explaining complicated phenomena of photo chemistry and optics I allow myself the poetic license to switch back and forth between the particle theory, the wave theory and the unified theory. Maybe Alice said it best. "I think I should understand that better, if I had it written down: but I can't quite follow it as you say it." Lewis Carroll </p>

<p>Hmmm.....Alice on quantum mechanics! (I think Grace Slick would appreciate)</p>

<p>Color film with not so much reciprocity failure: Fujifilm Provia 100F.<br>

B&W film with almost no reciprocity failure: Fujifilm Neopan 100 Acros.</p>

<p>Here's <a href="http://home.earthlink.net/~kitathome/LunarLight/moonlight_gallery/technique/reciprocity.htm">a good reference</a> for these two.</p>

I've done long exposure work, and reciprocity failure charts tend not to cover the sort of times that you are talking about. The only way to do it is to experiment. So, in bright sun, try, say, 10 seconds, 50 seconds, 5 minutes, 10 minutes, 40 minutes. And see what happens.

 

Cheers

 

Alan

<p>Cody, there's an excellent little exposure calculator (amongst other functions) downloadable at David Balihar's pinhole site. Check it out:<br>

http://www.pinhole.cz/en/pinholedesigner/<br>

Laurence</p>

<p>Reciprocity really is a factor for B&W film. Neopan 100 Acros has almost none, Kodak TMAX 100 has alot. I shoot TMAX 100 in 35mm, 120 and 4x5 sheet film, and the reciprocity really comes up when my exposure time is more then 2 minutes. An example on one negative, the exposure time was metered at 7 minutes, but it really needed 28 minutes, that's right 3 times more. So here's my rule of thumb: If my meter says expose for 1 minute or less, that's what I use and it's creates a "very good" negative to print in my darkroom: if it's timed for 1-2 minutes I shoot at that time and half again, example 75 seconds I expose for 112 seconds: if my meter tells me to expose for "MORE" then 2 minutes I expose it 3 times more. I have used Acros film before and I'm sure I'll use Acros again, but I prefer TMAX. Have you looked at www.f295.org... the world best pinhole website? If not, please do.</p>