Post Lighting

Adjusting lighting levels post process

(or how to make good use of just one flash)

A thought occured to me. Suppose you set up a camera on a tripod pointed at some subject. Suppose you then set the exposure to underexpose the ambient light a lot (or have no ambient light) and use only artificial light sources which can be turned off and suppose you then take a series of photographs with only one light at a time turned on and you adjust the brightness of each light so that the exposure is good in the resulting photograph (except in shadow areas of course).

You could use a strobe for this, which is probably the best bet. You can then take a number, perhaps lots, of photographs with the strobe in different positions/directions/degress of diffusion or whatever.

What this will then give you is a series of images each lit by one distinct light source.

Suppose you next load all those images into Photoshop or some other image manipulation program one on top of another and set the mode of each layer such that the resultant brightness of each pixel (in each channel) will be the sum of the brightnesses of that pixel in all layers (what is this called in Photoshop?) Any pixel which is in shadow from one light source but lit by another will thus be bright. Any pixel which is lit by many light sources will be very bright (overexposed really).

This will give you an image which is lit by every light source that you used. This isn't any complicated computational photography - just simple adding the brightnesses across all photographs.

NOW, the cunning part is that you can individually adjust the opacity of each layer and this will have the effect of individually controlling the brightness of each light source on the computer after the photograph has been taken.

When I have more time I will experiment with this and see what kind of results I can get. Obviously we must use a stationary subject.

Any comments? Has this been done much before? I can't believe I'm the first person to think of this!

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