Pre-exposing film?

[Jehu]

I just read a suggestion for night photography. It sounds interesting and I think I'll be trying it this weekend.

The suggestion:

Put a gray card in front of the camera (filling the lens) and focus to infinity
shoot it at minus 3.5 stops.
Use the pre-exposed film for a night shot such as star trails.

This supposedly will help with shadow detail and reciprocity failure. I'm looking forward to trying it.

[rdenney]

I just read a suggestion for night photography. It sounds interesting and I think I'll be trying it this weekend.

The suggestion:

Put a gray card in front of the camera (filling the lens) and focus to infinity
shoot it at minus 3.5 stops.
Use the pre-exposed film for a night shot such as star trails.

This supposedly will help with shadow detail and reciprocity failure. I'm looking forward to trying it.

It's called flashing the negative, and what it does is establish a minimum exposure sufficient to consume the flat toe of the response curve. That way, the image exposure will add to that exposure and will be up in the steeper part of the response curve, providing more tonal separation.

Adams demonstrated it (with very little explanation--you had to know about it to realize that's what it was doing) in the FilmAmerica biopic made in 1980.

I've never experimented with it in much detail, but I can think of a few of my pictures that might have benefitted from it.

Rick "remembering a couple of quick experiments from back in college" Denney

[Greg Lockrey]

It's called "flashing". The object is enhance your zones 1-3 and bump them up to about 2-4. It basically doesn't effect the higher zones due to the shape of the density curve of film. This works best if you use the Zone system or equivlant. I used to set up my enlarger to project the right amout of light onto the negative prior to using. Using the enlarger as a source allows you to calibrate the number of tops you wish to flash. The "out of focus on the grey card" works as well, but you do have to measure the ambient light each time.



(You got your explanation in before I did, Rick )

[Donald Miller]

The reason that this works is best illustrated by assigning numerical values to steps of exposure. Beginning with the number 1 at zone one and number 2 to zone two when we get to zone three we have the number 4 and at zone four we have the number 8. Zone five would have the number 16. When we finally arrive at zone eight we will have the number 128. This is due to the actual doubling of light as stops of exposure increase. Thus if we add a zone three pre-exposure we have added 4 units of light. When this is coupled with a second exposure we will have elevated the zone one film exposure to zone three and the net effect at zone eight will be miniscule (4 units added to a value of 128)

I have used this quite often and the effects are notable. This is best used to compress a tonal scale where the SBR of the subject/scene exceed the latitude of the film.

[AFSmithphoto]

There's a pretty good explination of how it works in "the negative."

If I remember right its a far more eloquent version of the following:

Considering that each stop represents a doubling of light, you can abstractly chart the light required to record each succesive stop at medium gray like this:


1 2 4 8 16 32 65

If you set your lens and shutter speed to a combination that puts "4" at zone V than "16" will be at zone VI and "32" at VII etc . . .

Now suppose you add a base exposure to the film, equivelent to the amount of light required to put "1" at zone V. That amount of exposure will be added to EVERY subject, no matter what exposure they fall at, but here's the magic; if you add 1 to each abstract stop you get

2 3 5 9 17 33 65

Now if you set your lens and shutter speed to put "4" at zone V it will be only mildly brighter (25%, or less than a third of a stop) and higher values will see an even less perceptable increase. However, what reads as "1" on the meter will be at zone III instead of zone II, which is a dramatic increase.

[Drew Wiley]

I was never fond of flashing, either in the darkroom or in the field. You give up too
much in the shadows, and you have to be careful which film you apply this to. It
especially muddies the low values with color film. But it can be fun experimenting.

[Greg Lockrey]

I was never fond of flashing, either in the darkroom or in the field. You give up too
much in the shadows, and you have to be careful which film you apply this to. It
especially muddies the low values with color film. But it can be fun experimenting.

I don't understand about what you mean by giving up too much in the shadows? The purpose was to move up those zones a stop or two to enhance the detail that would have otherwise been lost. Yes, if it's overdone, then you make mud.

[Nathan Potter]

I think this is a bit tricky to get a grip on. At first I could see a bit of advantage in the flashing technique. But after a few trials with B&W and color I was disappointed like Drew alludes to above. Then I started to think about it more critically. The first thought is why am I doing this rather than exposing the film for where zone II and III have decent discrimination of shadow values in the first place. So place zone II and III where the slope (gamma) of the sensitometric curve is steep; and if the brightness range is great employ a variable development technique to better fit the SB to the film. Or even better use a technique to linearize the toe of the curve so that the rate of change of the slope within the toe (the double derivative, d2D/dE2) approximates the gamma (dD/dE) at midslope. With such a technique, say using a double development the best discrimination of shadow values can be obtained. I have found Diafine very useful for this but takes a bit of calibration to work out the best shadow value discrimination.

Nate Potter, Austin TX.

[Jim Noel]

In the days of the SPeed Graphic as the major press camera this was a very common practice. news photographers often laid out a supply of film and flashed it with a light source of known value. Then when photographing at night the film was already at the threshold and minimal exposure would insure a good image.

[percepts]

The technique of pre (or post) flashing film is for the purpose of boosting shadow exposure a little. Over doing it will muddy the shadows as has already been said. It will also a very small effect on the highlights.

More importantly is the question of when and why you would want to use the technique. On person has already said it is used to compress the scale which I agree with. Normally you would use it when you need to reduce exposure to contain the high values and the the preflash is used to bolster the resulting loss of shadow detail.

But in the case of star trails you are not looking at reducing exposure to contain high values. You are looking at extending exposure to bolster the high values. Unless you happen to live many many miles from an urban center, you will have a degree of light polution and combining that with a long exposure means the sky will be less than black. Using a preflash will make the sky even lighter, maybe pushing it upto zone 2, 3 or even higher. I would ask if that is what you really want for your star trails photograph.