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Chapter 8 Pre-exposure Introduction to Pre-exposure Return to chapter 7 Next page |
Pre-exposure is among the most powerful of all techniques
that I have used in available light night photography. Beyond night
photography, it is a handy tool to use for some high contrast
subjects. The technique used to create the pre-exposure is the same technique
used for efficient film testing. Pre-exposing film is also known as
'flashing' the film.
The concept is pretty neat. Most of us have all had the experience of under-exposing film so severely that no image formed at all. In the process of creating the blunder, we made an exposure, some photons struck the surface of the film but no image formed. It turns out that the film needs a certain amount of light before it produces density. There is a threshold of exposure that must be achieved before the film registers useful density.
Here are some pretend numbers to help describe the effect. Let's say it takes 1 million photons to register even the faintest useable density on a given film. So for this film, Zone 1 needs 1 million photons. Zone 10 requires about 1000 times as much light, or in this case, about 1 billion photons under the condition of 'Normal' film development. Again, we are only talking pretend numbers here. So now our full range negative has been exposed to between 1 million and 1 billion photon, depending on the subject area.
If we screw up and underexpose by 10 f-stops, we get no image at all. If we make an exposure and the brightest part of the image is not allowed to reflect 1 million photons to the film, the film will be clear after development. However, any part of the film that happens to have more than the minimum necessary exposure will show density on the negative after film development.
The general idea behind pre-exposure is to carefully expose the entire piece of film with the minimum light required to register density. In the pretend example, if the film is exposed to an even 1 million photons , then the film will register density or detail everywhere that light strikes it during the main exposure.
Pre-exposure requires two exposures for each piece of film:
The pre-exposure 'fills up' the film with an amount of light that is almost enough to register density on the film.
The main exposure adds light to the pre-exposure. Since all areas of the film are almost 'triggered', even small amounts of additional light registers as useful density on the film.