the other day i saw great light in our sitting room. The only trouble was that the range of light was so great that i would have either blown out whites or no detail in my blacks. It had been a while since i did any pre-exposure, and i could not find my cheat sheet, so i ran some tests. Here are the results:

more detail on the process if you click on the image

ironically, the shot i screwed up, and so didn’t mark down the details, is probably the best. You will notice that the greater the pre-exposure, the lighter the couch. This could be fixed in printing, but not fixing it serves better for the purpose of comparison.

The theory is that for each ‘zone’ on your negative/print, with 1 being the darkest and 10 being the lightest, you need twice as much light to attain that zone as you did for the one previous. So, if you need 1 unit of light for zone 1, you need 2 units for zone 2 and 4 units for zone 3. If you pre-expose for zone 3, in this example 4 units of light, and then expose that same film as normal, your low zones are affected greatly - zone 3 for example has received twice as much light, 8 units, which brings it into zone 4. The higher zones, are not affected as much - zone 7 started out with 64 units of light and the pre-exposure only brought it up to 68 units, not very noticeable. This shortens the range of zones and allows you to capture detail at both the high and low ends of the scale.