Pre-flashing film

[alanrockwood]

I understand that pre-flashing film can sometimes result in an increase in film speed. By "pre-flashing" I mean giving the film a little bit of exposure before taking a picture, with that extra bit of exposure spread evenly over the film. It is also my understanding that what it does is give a longer toe to the film, leaving the high-exposure regions relatively unaffected. Basically the shadows would be low contrast, but would have at least some contrast, whereas without pre-flashing the contrast would hold up pretty well in the low-zones until relatively suddenly there is no contrast.

I would be interested in a general discussion of pre-flashing of film and post-flashing of film as well. However, what I am especially interested in is whether the pre-flashing wears off. I read somewhere that you have to develop the film soon after the pre-flashing or the effect wears off. However, what I read was not entirely clear on this point, and especially it was not clear on whether this temporary effect would apply the same way to pre-flashing and post flashing.

Any thoughts?

 

[grain elevator]

I thought one had to expose the image soon after pre-flashing, not develop. How you describe what happens to contrast sounds right, that means it raises the toe, not extends it.

 

[Ian Grant]

Astronomers used to pre-flash film to get a speed increase, and it was also used to control contrast with slide duplicators although the additional flashing was alongside the actual exposure.

LA Mannheim wrote in the Focal Encyclopedia of Photography that the Flashing or Latensification exposure is best done in a darkroom with a very dark green Panchromatic safe-light filter and 15watt bulb at a distance of around 15ft and for between 30mins to and 2 hours. He indicates there will be an increase in base fog but overall the improvements are greater. He's suggesting an Ilford 908, Wratten 10/10H safe-light filter.

Ian

 

[RalphLambrecht]

I understand that pre-flashing film can sometimes result in an increase in film speed. By "pre-flashing" I mean giving the film a little bit of exposure before taking a picture, with that extra bit of exposure spread evenly over the film. It is also my understanding that what it does is give a longer toe to the film, leaving the high-exposure regions relatively unaffected. Basically the shadows would be low contrast, but would have at least some contrast, whereas without pre-flashing the contrast would hold up pretty well in the low-zones until relatively suddenly there is no contrast.

I would be interested in a general discussion of pre-flashing of film and post-flashing of film as well. However, what I am especially interested in is whether the pre-flashing wears off. I read somewhere that you have to develop the film soon after the pre-flashing or the effect wears off. However, what I read was not entirely clear on this point, and especially it was not clear on whether this temporary effect would apply the same way to pre-flashing and post flashing.

Any thoughts?

Igave up on preflashing because it really kills mid-zone contrast and does very little for the shadows.

 

[Andrew O'Neill]

Igave up on preflashing because it really kills mid-zone contrast and does very little for the shadows.

Not in my experience, if done right. It does a lot for the shadows. Zone I to II pre-exposure is ample. Too much can be bad, especially when coupled with reduced development.

 

[Ian Grant]

Not in my experience, if done right. It does a lot for the shadows. Zone I to II pre-exposure is ample. Too much can be bad, especially when coupled with reduced development.

It would be interesting to hear how you go about it, and if you have any examples it would be good to see them. It's not something I've felt a need to do.

Ian

 

[Doremus Scudder]

Pre-flashing film doesn't really give you and overall speed increase; it's really just a bit of extra overall exposure and effects the shadow values mostly. The effect is very similar to overall flare from an uncoated lens or internal reflections. There were (still are) many photographers that love the rendering of tones given by uncoated lenses; that extra density in the shadows, even though the contrast in the lower values is somewhat reduced. Using pre- (or post-) flashing can accomplish much the same.

I used to flash Kodachrome a lot to hold the high values (transparency materials reverse the affected tones), but use it only rarely with black-and-white negative materials.

Keep in mind that pre-flashing, while getting you more shadow density and detail isn't speeding up the rest of your film appreciably. That means that mid-tones and highlights will be rendered "as usual." Pre-flashing your film and then shooting it as if it had a higher E.I. can be disappointing, especially if you develop normally, since you're essentially underexposing the higher values. Pre-flashing in conjunction with an increase in development time, on the other hand, can give gratifying results.

A good way to think about it is arithmetically. Let's say that Zone I is 1 exposure unit (just an arbitrary value for this thought exercise). Then, Zone II is 2 exposure units, Zone III is 4 exposure units, Zone V is 8 exposure units, Zone VI is 16 exposure units, Zone VII is 32 exposure units and Zone VIII is 64 exposure units.

Now, we pre-flash our film with an overall Zone I exposure and then record a scene with luminosities through Zone VIII.

Now, Zone I will have 2 exposure units (the Zone I exposure plus the pre-flash). Zone II will have 3 exposure units, Zone III will have 5 exposure units, ... Zone VII will have 33 exposure units and Zone VIII will have 65 exposure units.

Looking at this, it's easy to see what's going to happen to the shadows in relation to the highlights as far as the added pre-flash exposure.

Hope this helps,

Doremus

 

[Mick Fagan]

Increasing Film Speed.

Before exposure is known as hypersensitising.

After exposure but before developing, is known as latent image intensification; or latentisification.

Hypersensitising was very useful when emulsions were considerably slower than they are now, but it is of little value with modern fast materials. These have already been hypersensitised during manufacture.

The effect of hypersensitising is usually fairly uniform over the range of colours to which orthochromatic films are sensitive, but it increases with the longer wave-lengths and is particularly pronounced in the infra-red region. Hypersensitising therefore affects pan films and plates more than others and, has special use in work with infra-red sensitive materials.

Latensification is similar to hypersensitising, except it takes place after exposure.

Post exposure fogging.

Fogging by light after exposure can also increase the effective film speed. While with hypersensitising a very short exposure to a comparatively strong light is given, fogging after the camera exposure yields the best results with long treatment by a very dim light. Light of a very weak intensity is much more effective in increasing an existing latent image than in starting a new one. Consequently the increase in the image density will be greater than the fog density, and the shadows will be strengthened, but overall contrast decreased.

The film is exposed to a very weak light for about 30-60 minutes. For supersensitive panchromatic materials a dark green (Wrattan Series 3) safe-light with a 10W bulb and a working distance of about 12 feet is best.

The speed increase obtainable by fogging after exposure is usually somewhat greater than pre-exposure treatment (about 100%, as compared with 50-75%).


Source:- Developing by K. Jacobson & R. Jacobson. 18th edition revised 1976 Updated 1978. Pages 256 – 260.



I myself have observed post and pre exposure in the graphic arts industry over a 15 year period from the late 70’s through to the early 90’s. These methods were used to enhance shadow detail when photocopying high contrast artwork using gallery cameras.

With my own 4x5” sheet film, a group of us did some experiments using FP4 film, (not FP4+). We had a dark green filter, 15W globe about 5m away and tried various times using 20” intervals. 20”, 40”, 60” and 80”. I’m not 100% sure, but I seem to recall around 20” seemed to work best for shadow and highlight detail with a film development increase of ½ a stop to maintain a semblance of normal contrast to allow printing with grade 4 paper.

Overall it was a very satisfying project, and taught us a bit more about photography and how much fun one can have just fiddling around. None of us ever kept on using any of these methods; we just bought faster film. It is however a great tool to have in the arsenal for shadow detail when you really need it.

Mick.

 

[Vaughn]

I find by carefully pre-exposing my film, I do not have to go out and photograph anymore.

 

[Raghu Kuvempunagar]

I used to flash Kodachrome a lot to hold the high values (transparency materials reverse the affected tones),

@Doremus Scudder: it's interesting that you say this about transparency material as any additional exposure only lightens the highlights and can't possibly help in holding the high values.

 

[alanrockwood]

I find by carefully pre-exposing my film, I do not have to go out and photograph anymore.

That would probably improve my photos.

 

[Pieter12]

Motion picture film would get post-flashed to bring up shadow detail.

 

[Doremus Scudder]

@Doremus Scudder: it's interesting that you say this about transparency material as any additional exposure only lightens the highlights and can't possibly help in holding the high values.

Yeah, I was being obtuse here... I based my exposure for transparency film on highlight readings, but this would often put the shadows into pure black. With pre-exposure I could boost the shadows and still use the optimum highlight exposure. Sorry for the confusion.

 

[DREW WILEY]

This question would have to be narrowed down to at least one of three specific categories, namely, whether you are talking about black and white film, color negative film, or color transparency film. In the last case, color chrome or transparency film, it is quite difficult to get a shadow boost without messing up the midtones because there's so little usable range to begin with - maybe only two stops in between. With respect to black and white film, why??? - it's generally so amenable to development tweaks, and with no risk of upsetting hues in the process, that I've never even bothered with flashing in any kind of black and white work.

What I do sometimes do, though rarely, is encounter a situation where the ideal warm/cool color temp balancing filter differs within the scene itself between direct sunlight and shade. Among current color neg films, Ektar is not artificially warmed with respect to this, which is a good thing for overall hue correctness, but a bad thing when deep shadows under open sun exhibit not only blue, but that annoying cyan contamination of blue characteristic of this particular film. So I'll pre-flash that sheet of film (or particular frame of roll film, if the camera allows double exposure) about two stops below midtone with a special flashing diffuser aimed at a portable gray disc. This contains an ND filter, a translucent diffusion sheet, and a warming red or magenta gel (cyan-cutter), all adding up to a net .60 density (hence two stops below middle gray so I don't have to change settings from the final subject exposure itself). How well does this work in relation to just overall light balancing filters? Well, I dunno quite yet. Haven't done the comparison RA4 prints yet.