CPA, a way to increase film speed without increasing granularity - why is this so obscure?

[Karim Ghantous]

<p>CPA stands for 'concurrent photon amplification'. I found out about it only a few days ago after buying a few old photography magazines from an op-shop. This is from Popular Photographpy, June, 1976, p.61:</p>

 

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<p>... the new process enhances film speed by irradiating the sensitized emulsion crystals with measured doses of weak light added during the actual camera exposure. Effective speed gains of... one to two f-stops - are claimed for the new device, and were amply substantiated in Popular Photography's exclusive tests... Four tiny lamps are mounted in sets of two at either side of the camera chamber, approximately 34mm in front of the film plane.</p>

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<p>I wonder two things: why I have only just found out about this; and why no camera that I know of includes this feature. A Google search brings up mostly academic papers.</p>

[lex_jenkins]

<p>Sounds like a variation on pre-flashing film and paper, an old technique with real but limited usefulness. To be effective the film (or darkroom printing paper) must be exposed immediately after pre-flashing and processed ASAP. With color film there's some risk of color shifting, although the technique has been used for this effective in cinematography (most notably in "McCabe and Mrs. Miller"). Cinematgrapher Roger Deakins talks about it <strong><a href="http://www.rogerdeakins.com/forum2/viewtopic.php?f=1&t=1205">here</a></strong>. And <a href="http://en.wikipedia.org/wiki/Photographic_hypersensitization">this wikipedia article</a> summarizes various film hypersensitizing techniques. As with gas hypering, any post-manufacturing techniques to increase the true speed or dynamic range of light sensitive emulsions have very limited useful working time.</p>

[Hector Javkin]

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<p>why I have only just found out about this.</p>

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<p>No way that I can answer that.</p>

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<p>and why no camera that I know of includes this feature.</p>

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<p>Probably because if the concurrent flashing is not done just right (too much, not enough, or unintentionally, because the setting for it was left on) it may ruin the photo.</p>

[pics]

<p>Like Lex says, it sounds similar to pre-flashing film or paper. The major negative side effect I see, would be a reduction of contrast and dull highlights. This could be ok in high contrast situations (one purpose of preflashing,) but for most other situations it would pose more problems than solutions, all of which would probably negate the benefit of a higher effective film speed. Different film brands and ISO speeds would require flashes of different durations and intensities. I assume this could never be easily perfected electronically in the camera.</p>

[michael_darnton2]

<p>The Bowens Illumitran, a 35mm slide duping rig in the 70s, used this principle. http://members.bitstream.net/tlmartin/copiers.html There was a tiny light with a milky cap on the back of the lens board--Bowens called it a Contrast Control Unit-- so that you could flash the film at the same time as taking the exposure. It was helpful in that case, because dupes without special duping film were invariably higher contrast than the originals, and this brought the contrast back down.</p>

<p>The idea was to flash just below the threshold of the film so that ANY light that it got contributed to exposure, not threshold, but blacks, receiving no additional light, would remain beneath threshold. Harman's Direct Positive Paper benefits from the same strategy and users of that do it. It's more useful for lowering contrast than increasing speed.</p>

[Bill C]

<p>I imagine the results would be close to "flashing" your film, but in principle it's more like adding lens flare. Since flare is "concurrent" with the actual exposure whereas "flashing" is not. But I doubt that the result would be very much different.</p>

<p>You can experiment with "flashing" on individual frames if your camera allows double exposures. One way is to point your camera at a white wall (put it out of focus), take a meter reading, then reduce the exposure about 3 or 4 stops (try different exposures to see the effect). What you'll likely find is that shadow details will become visible or more pronounced, and this is the real essence of film "speed." But when you overdo it, the afffected zones become objectionably flat. (I came up with this idea on my own, as a kid, but later found that it was well-known, in fact Ansel Adams even expained it in one of his books.)</p>

<p>Another similar idea was called "latensification." It was similar to flashing, but was done with very weak light, so that the exposure took roughly 30 minutes or longer. The general idea is that this light is too weak to build up an exposure (the latent image "leaks" away before it becomes stable), but it can reinforce a sub-visible latent image.</p>

<p>The weakness of these methods is that they are only visible in the weakest part of the image, and overuse gives odd-looking shadow areas. No effect will be noticeable above the shadow areas. I'd be interested in hearing what the article has to say - did they notice such an effect? I've been aware of the technique (CPA), but have never read any articles with results.</p>

[lex_jenkins]

<p>Incidentally, one of the originally advertised purposes of the ExpoDisc, back in the 1990s, was for pre-flashing film. I was too cheap and used the same trick I used for converting TTL camera metering to makeshift incident meter - a white styrofoam coffee cup over the lens.</p>

<p>I tried the trick on a few rolls of consumer grade Agfa color negative film. The results were interesting: slightly shifted but vivid colors, easily corrected by the then-good minilabs; slightly less contrast, which suited my preferences for midday shooting on sunny days. The result was a sort of poor man's Fuji NPC or Reala. I might try that trick again on some long expired rolls of color negative and slide film, just to see what develops.</p>

<p>Not sure why I never tried that trick with b&w films. Might be interesting with films like TMX and Pan F+, which I always found a challenge in midday use. Diafine resolved my issues with Pan F+ for contrasty lighting; as did normally exposed TMX in Microphen.</p>

[dave_s]

<p>I've read the old time press photographers used to call this 'waking up the film'. They would just give it enough pre-exposure, using an out-of-focus sky, to get it off the toe of the H & D curve.</p>

<p>I used this a few times with B & W about ten years ago, but the intent was to get the contrast under control. </p>

[Mike Gammill]

<p>I recall reading that article. It sounded promising but I never got around to trying it. </p>

[Karim Ghantous]

<p>Just to clarify, this has nothing to do with flashing whatsoever, except superficially. I wish I could reproduce the article here, which would answer some of the points raised above. Suffice to say that the CPA devices were fitted to cameras just like motor drives, and they had settings for three different types of b&w film (so Daniel's point was well made).</p>

[mike dixon]

" this has nothing to do with flashing whatsoever, except superficially"

 

The similarity is not superficial; it's fundamental. The dissimilarity (adding additional light during the normal exposure

rather than before) is superficial--it makes no difference in the effect of the added exposure.

[lex_jenkins]

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<p>"I wish I could reproduce the article here, which would answer some of the points raised above."</p>

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<p>Post it on Google Docs or, if it's scanned to image files, to any image host, and post a link. Ditto the academic study if you can find a full copy. So far I can only find the abstracts.</p>

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<p>"Just to clarify, this has nothing to do with flashing whatsoever, except superficially."</p>

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<p>Previous web discussions by folks familiar with the technique indicate that's exactly what it was - an effort toward more precisely controlled pre-flashing. Note this comment by Joseph Wisniewski (who also occasionally haunts photo.net) from a <a href="http://www.dpreview.com/forums/post/16531450">2006 dpreview thread on the same topic</a>:</p>

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<p><em>"If you want to get this to work even better, make the white light exposure happen simultaneously with the main exposure. In latensification, you typically try to pre-expose the film to a level just below fogging. At this point, the silver grains are in an unstable state, and many of them will collapse back to an unexposed state (wasting the preflash) before you get around to actually taking the picture. If you feed in the latensification light simultaneously with the main exposure, there's no "forgetting" factor. It also "cures" reciprocity failure on long exposures.</em><br /> <em>"The first fellow I read about doing this back in 1976 called it "concurrent photon amplification". We used to add grain of wheat bulbs or LEDS in the mirror chamber to add controlled amounts of latensification. The best CPA system I ever built used fiber optics to conduct the latensification light into the film chamber, and the light source was a small, hot shoe mounted flash unit. The Xenon strobe was daylight balanced, so it could be used with daylight color film. The incandescent (grain of wheat bulb) systems needed to be set to a different brightness at every shutter speed, pulsed Xenon or pulsed LED eliminated this requirement.</em><br /> <em>"If you're still interested in this technique, I'd consider investigating pulsed white LEDs as the latensification source. Or continuous LEDs if you're going to put something like that in your swing lens Horizon. You'll have to filter the LEDS to proper daylight balance, but it's a heck of a lot easier than filtering grain or wheat incandescents."</em></p>

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<p>That statement confirms my hunch about how CPA would work. Ideally it should occur at a level proportionate to the film and exposure, and as near as possible to simultaneously with the exposure for greatest effect with the least risk of undesirable fogging from excessive pre-flashing.</p>

<p>Clever and, like the Contax SLR with vacuum seal film pressure plate, an elegant solution in search of a problem.</p>

<p>Joe is a very tech savvy fellow, so you might email him and ask if he knows of any other sources of info on this technique.</p>

[pics]

<p><strong>"Just to clarify, this has nothing to do with flashing whatsoever, except superficially" </strong><br /> <br />Well technically it's not "pre"flashing because it is happening as the exposure is being made, but it's more or less the same thing.</p>

<p><strong>"irradiating the sensitized emulsion crystals with measured doses of weak light"</strong><br /> <br /> As far as I can tell that is just a really fancy and scientific way of explaining flashing the film emulsion.</p>

<p>As was mentioned above, when you flashed film or paper, you had to use it relatively soon afterwards for best results. The film would fog/go bad easily due to it being very close to the "threshold" of exposure. I believe that is the main reason for this contraption being invented. Instead of flashing a roll ahead of time and being forced to shoot it right away (or risk the image quality being compromised over time), you could achieve the same effect but do it at your leisure since each frame was flashed at the moment of exposure. (or that was the hope anyway.) The technique is finicky though and outside of the handful of films and situations they tested for in a lab, it was most likely next to impossible to get repeatable and desired results over a broad range of films and shooting conditions.</p>

[detlef_pr_fert]

<p>I have a Canon F-1 with orig. CPA Modifikation !!!<br>

also i knew that ther whwere also Nikon´s !!<br>

regards Detlef</p>

[michael_przewrocki2]

<p>I also have that article from 1976 back. pity i didnt buy it. had a different camera-system. it would be wise if someone could upload the article. could scan it but have little time right now. maybe the other person can do it.<br />results with b+w-shots are really amazing. yes speed will also be increased.<br>

1. i would like to see how the bowens illumitran-system is built. maybe i can add it to an existing bellows-system.<br />German Storz has fiberoptical-systems for medical use. one of these is a cheap pen-like mobile-system. pls search how its working.<br>

2. i did prefog, also afterfog using different methods. one is shooting actual subject once with filter then without.<br />i also tried it picture after picture with rotation-lens-swinglens-cams like widelux, horizon, and horizonts.<br />the other method is shooting grey concrete not in the sun. or if possible greycard, always underexposed by a least 2.5 stops(maybe its 3.5 must recheck my data). best is if one is selfdevelopping c-41.<br>

I was using milkglass-filter. kamerawerke noble made me a special filter from an original uv-filter for their 150(6x12)-camera.<br />there is a special procedure of underexposure.<br />3. most effective contrast-reduction is resulting by using outdated colornegative films, overexposed by 1 stop and push-processed by 15%.<br>

I was also prefogging whole films(mostly colornegatives but also tried slidefilm once-is a bit delicate finding the right exposure then). But i call my method secret....!<br />There must be some informations how old filmcracks had prefogged film by using flashlight.<br />maybe in one of the hundred popular-photographys iof 70/80ties which i must remove this summer.<br />maybe there is an index somewhere and then i could search for it.<br />I only know that the editors of modern photography or a different company had the option to get old issues.<br>

e.g. i couldnt find an article anymore by simon nathan about widelux 120(6x12 version-later 1500 which i have now- had a big quality-drama in 1990. Herbert Keppler gave me the adress where to get it. I knew i didnt throw away. but cannot find.</p>