I'm aware of the fogging technique and its benefits - I've done it with chrome films like Fuji Velvia sometimes. My Nikons are quite exact in film transport, so I have only to mark the film when I insert it in the camera first time.

The question is: is there some physical difference, if the fogging is made before or after the image forming exposure?

Sakari

-- Sakari Makela (sakari.makela@koulut.vantaa.fi), August 18, 1999

Answers

This is a very old and possibly defective memory, but I recall that effective film speed can be increased by fogging the film slightly before exposure. Makes sense, as it would move the shadow detail up on the curve. I don't think it works the other way 'round, as the initial exposure simply didn't register enough for the fogging exposure to add to. Maybe some of the real techies here have more info.

-- Conrad Hoffman (choffman@rpa.net), August 18, 1999.

Here's some more...

A quick look through "Photographic Sensitometry" by Todd & Zakia lists two effects that might be of interest.

First is the Intermittency Effect. They are somewhat ambigous about it, stating "In general, if a continuous exposure is divided into portions, the photographic response differs from that to be expected from the continous one, even if the total energy incident upon the emulsion is the same." It's related to reciprocity law failure, but they seem to be thinking more of flash (stroboscopic) situations.

The next one is the Clayden Effect. They say "Two exposures are non- additive in their effect if they are used in the sequence:

1. high-illuminance, short time. 2. low-illuminance, long time.

The effect may be that of reversal..." This is not the same as solarization, and they then go on to say "The Clayden Effect and variations of it have found practical application in the enhancement of the visibility of such photographic images as the recording of oscilloscope traces." I'd forgotten about this (the electronic world is digital now) but when taking 'scope photos, it's common to prefog the film so that the dim and/or fast CRT trace shows up adaquately.

They talk about both the Clayden and Herschel Effects, then state "In both effects, the sequence of the two exposures is significant: It appears that the effect of an initial high-energy exposure is to some extent diminished by a following low-energy exposure. The effects are not found if the sequence is reversed."

I don't know if this clairifies things or makes it worse! The book is 1969, and has an SBN number of 87100-00008. I don't know if that's the same thing as an ISBN number or not. Morgan & Morgan probably still has it. (I took M&P (materials & processes) from Todd many years ago at R.I.T.)

-- Conrad Hoffman (choffman@rpa.net), August 18, 1999.

Ansel Adams devotes 5 pages to pre-exposure in "The Negative", but makes no mention at all of "post-exposure". This in itself says something!

HOWEVER - I vaguely recalled having seen something about post exposure in an old "Darkroom and Creative Camera" and amazingly enough I found the magazine at the bottom of one of my "filing piles"! It's Nov 1993, p43. It is by David Vestal and concerns Relph Steiner's Latensification technique, which is basically the exposure of a latent image to a very dim light for a very long time. Example: Green filter on 7W bulb 10ft from the film, 15 to 20m exposure for Tri-X with paper masks to cut down the light.

It is claimed that this produces more dark tone (shadow) detail AND it increases film speed. It does not work pre-exposure. Claims of 3 or 4x film speed increase are made. The fog level is said to change very little.

Sound like an fruitful area for photographers who find the normal zone system too limiting! Clearly a lot of experimenting would be required to get optimal results.

-- Bob Atkins (bobatkins@hotmail.com), August 18, 1999.

I've tried pre exposure for taming contrast in B&W. I don't use it much, but what I do is expose a gray card for 1/3 of the gray card exposure reading, with the lens focused at infinity and the gray card very close to more than fill the frame. Obviously you will need a camera that allows multiple exposures and TTL metering is a big help. This approch avoids the need to pre expose an entire roll of film prior to loading. I refined this technique when I was experimenting with Kodalith for pictoral photography. I gave up on this as it became too much of a hassle.

-- Gene Crumpler (nikonguy@worldnet.att.net), August 19, 1999.

I have pre-exposed slides with success in the same manner as Gene. You can slightly affect the color of shadow areas by varying the color of your card. Expose FIRST with 1/3 or 1/4 of gray card reading. There was just a lot of talk about this in some of the B&W forums recently.

-- Jan Eerala (jan.eerala@itameri.net), August 19, 1999.

When you are talking about "1/3" or "1/4" exposure, are you refering to -3 or -4 stops from a grey card reading or -2 stops (1/4) or -1.6 stops (1/3). I thought the idea was that the pre-exposure should be small enough that it wouldn't give any fog. For slide film that means something like -3 stops (maybe -2.5 stops). At -2 stops (1/4 exposure) or -1.6 stops (1/3 exposure), you'd certainly raise the overall fog level wouldn't you?

-- Bob Atkins (bobatkins@hotmail.com), August 19, 1999.

Bob;

The experimenting I did with kodalith, I used 1/3 of the exposure, -1.6 stops. This indeed produce a general fog, but I just printed thorough it and the contrast was significantly reduced. Incidently, kodalith is a major pain to use for pictoral photography (keeping the contrast down and an E.I of 3), but the grain(lack of and resolution) in 35mm was fantastic. Agfa copex is similar. I used the rest of the 100 foot roll to do lens tests and settled on Tech Pan.

-- Gene Crumpler (nikonguy@worldnet.att.net), August 19, 1999.

Thanks. From what I understand from Adam's, the idea (for "normal" films!) is not to pre-flash the film to the point where you see an overall reduction in contrast (fog), but to give it JUST enough pre-exposure that the shadows come up to the point where you can see detail instead of just black. For normal film I guess this is maybe -4 stops for negatives (B&W anyway), -3 stops for slides. The pre-flash just raises the film to the point JUST below fog, then shadow exposure takes it above that (thus giving some detail), while higher exposure levels for mid-tones and highlights are insignificantly affected.

I can see why Kodalith might need a bit more severe treatment though!

-- Bob Atkins (bobatkins@hotmail.com), August 19, 1999.

The whole idea is pretty clear from the H&D curve for the film. The goal is to do a pre-exposure to move the real exposure over to the right so that it will be on the linear part of the curve, instead of on the toe. If the real exposure is completely on the linear range, then, if you match the paper right, you will be able to get pretty good separation of tones, even in the shadow regions of the image. There were threads on this in the Original Q&A forum. I remember the argument was given there that linear superposition should work, and, hence post exposure flash should be just as effective as pre- exposure. I disagreed with this argument based on personal experience, but I was never able to successfully convince the post exposure believers.

-- Glen Johnson (gjohnson@engr.udayton.edu), August 20, 1999.

There was a guy back in the mid 70's who modified a Nikon body to give additional light DURING the main exposure. He put tiny "grain of wheat" lamps in the mirror box. The claim was that much less exposure was necessary so there was no fogging or loss of contrast but a 2 stop speed increase. This could be done (or is being done) today with LED's.

-- Tim Brown (brownt@ase.com), August 20, 1999.

It depends why you want to fog or flash the film. The method I have used most in the past is to flash film AFTER exposure to affect a contrast decrease. I have done this with both colour negative and transparancy films.

I work for a large cultural organization where most of the work I do is copying; when copying an original drawing or painting in B&W, contrast changes are affected through the use of filters and/or by manipulation of exposure and development.

For colour contrast changes, I try to match the film to the subject. By that I mean that an inherently contrasty subject like an oil painting requires a lowering of contrast. To this end I will expose a sheet of Kodak EPN film, 4x5, and flash it after the main exposure.

My method in the past has been to give the film a second exposure of a plain white card through a 2.0 neutral density filter. This is fine for small subjects; when your original is 6x9 feet, it is much easier to make a permanant setup where a camera is dedicated to flashing and film which has been exposed may be easily given a second exposure.

I have tried pre-flashing film in a sensitometric laboratory when I was studying for my undergraduate degree in Photographic Technology some years ago. I couldn't get it to work properly; my problem was exposing the film to a VERY small ammount of light. I wasn't able to find the exact level at which the film was flashed for a speed increase, as opposed to fogged and rendered useless!

-- Terrence Brennan (tbrennan13@netscape.net), September 10, 1999.

Well, better late than never, but I just now wandered into this thread almost two years since the last post, but figure someone else might come along some day, so I'll make a contribution. I have to vigorously agree with Gene's comment that a grey card exposed at 1/3 or even 1/4 the "correct" exposure would be too much pre-exposure. I have created a MS-Excel spreadsheet with charts that graphically illustrate how much each Zone is lifted with various amounts of pre-exposoure. (Write me and I'll send it to you.) The midtones (and highlights) remain essentially unaffected at pre-exposures of -3 stops and lower, but even at -2.5 stops, they too are lifted a bit, along with the shadows.

I shoot medium format stereography with twin Mamiya 7 II's, which permit multiple exposure. My Provia 100 F chromes are the final product - I neither print them nor duplicate them - so I have to get everything right in the camera: exposure, color and contrast. I never pre-expose any higher than -2.5 stops and only use this with scenes that have very small areas of shadow relative to the entire scene - for example in the mottled light of a forest floor, but there can't be any large areas of shadow or it they will simply look milky.

Even a -3 stop pre-exposure can look just slightly milky if the majority of the scene is in shadow and you're just trying to raise that shadow while preventing the minority balance of the scene from being blown out, but most people won't notice it. Consider -3 stops to be the brightest pre-exposure you can make safely under most conditions (this is true for both chrome films and print films, color or b&w.) For narrow latitude chrome films, a -3.5 stop pre-exposure is useful only when the luminance ratio between highlight and shadow just barely greater than the film's latitude. A -4 stop pre-exposure is hardly worth doing for chrome films, but color print films and certainly b&w print films can enjoy elevation of the lowest zones right down to a -5 pre-exposure, but that's about it.

The most difficult thing with pre-exposure is... making the exposure. Get rid of the grey card. Take two 6-inch square pieces of acrylic diffusion material (like they make light boxes with) and glue them together, parallel, separated by a frame of 1/4-inch balsa wood. The perfect glue for this is "SuperGlue" or any cyanoacrylic cement used by hobbyists to glue balsa - it happens to work really well when bonding balsa to acrylic. Any good hobby store will have both the balsa and the glue - ask for a brand called Jet-Dry.

Anyway, how do you use this thing? Once you've metered your scene and determined that you need to elevate the shadows, hold the diffuser flush to the front of your lens then aim it directly at your main light (that means at the sun, if outdoors. Cover the lens first and keep it covered until you've swung away from view.) If you use a spotmeter or some other non-TTL meter, figure out how to achieve the same results - the idea is to both meter and make the pre-exposure directly into the main light. This guarantees that your pre-exposure will have a neutral color cast (try pre-exposing a grey card while standing next to a red brick wall if you'd like to see what I mean) and it greatly improves the probability that your pre-exposure will be accurately placed (grey cards are notorious for reflecting different amounts of light depending on what angle you hold them at and here, it's made worse by the need to pre-expose with the grey card at the same angle as we metered - assuming that angle was correct to begin with!)

So make a pre-exposure of the main light (of the sun) at -3 stops, for example, from that indicated by the meter, with the lens focused at infinity. Again -3 is the most pre-exposure you can give safely, without causing milky shadows. (Note, depending on the ISO rating of your film and what apertures and shutter speeds are available, you might have to use a neutral density filter to make the -3 stop exposure facing the sun. You might find you can't stop down enough at your highest shutter speed to make a -3 directly into the sun. (If you shoot outdoors in bright daylight with films as fast as 200 or higher, you might even want to sandwich a 6x6-inch acetate 0.6 ND filter between the two sheets of acrylic when building your diffuser. Buy a cheap acetate ND filter used for lighting, not an expensive, optically perfect 6x6-inch gelatin filter. But you can get by without doing this if you already own a ND filter for your lens.)

OK, now just swing the lens around to reframe your subject, cock the shutter without advancing the film and you're ready to take the main exposure - expose it normally, just as you would without having done a pre-exposure and feel free to do things like darkening the skies with split neutral density filter - now you've taken control of contrast right inside your camera! Using these tools together, you can get the luminance range of an entire scene compressed to fit the latitude of your film. Always take a second straight shot to see what effect the pre-exposure had in comparison.

Mike

-- Mike Davis (zilch0@primenet.com), May 01, 2001.

I remember an article in a magazine some years ago where the writer pre-flashed the WHOLE FILM before use.The film was hung in a dark room and exposed at a level of about 1/100 of "normal" exposure.This was acheived by using a dim incandescant source or a heavily filtered (ND) flash. The film is then rewound for use.

-- Mike Nicholson (pixelsaurus@yahoo.com), November 13, 2001.