Tim Tucker 2
No longer a newbie, moving up!
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Unfortunately it's not as simple as a *single exposure* and a *correction value*.
Film reacts to light, exposure is point intensities on the film over time exposed. These different intensities produce different densities on the film depending on the amount of light over time focused on the film. This is your recorded image. Below a certain level of light and there will be no reaction produced on the film because there is not enough light or energy falling on the film to produce a reaction.
There is another problem in that if the light intensity is very low over a long period of time then it slows the reaction, or rate of change on the film. Think of trying to heat a cauldron with a match, hold it close and you will get a hot spot, further away and the effect of the match will be dissipated and the further away you hold it the less effect it has, it is not a simple distance/time relationship.
Film is similar in that it requires a set amount of light energy over a set time to produce a predictable reaction or density in the film. Extend that, hold the match further away, and you have to hold it there a lot longer to produce the same reaction. There is no real *line* above which there is one exposure, and below a correction value but a gradual fading of the effect which increases as the time needed to capture a set level of light increases.
Now there is a further problem. The highlights, or lights, in your night scene will have a fairly high point intensity, akin to a match held close, and the shadows will be akin to holding the match further away. The highlights may focus enough light over time to produce a normal reaction in the film whereas in the shadow values the amount of light falling on the film over time is below the level needed to produce a consistent, or linear, reaction in the film. So where the shadow values need extra exposure to produce the correct density in the film the highlights don't and the extra exposure will produce a higher density, or overexposure. In effect the nature of the way film reacts *increases* the DR of the scene where exposure needed to capture the shadow values exceeds 1 sec.
If you increase *shutter speed* to correct this then you introduce an even longer time which again suffers the same effect in the shadows. It's a bit of a guessing game and varies with film emulsion but I use the Ansel method and within reasonable limits with common film produces predictable and usable negatives:
1 sec - 1 stop more aperture or 2 sec and -10% development time
10 sec - 2 stops more aperture or 50 sec and -20% development time
100 sec - 3 stops more aperture or 1200 sec and -30% development time
Film reacts to light, exposure is point intensities on the film over time exposed. These different intensities produce different densities on the film depending on the amount of light over time focused on the film. This is your recorded image. Below a certain level of light and there will be no reaction produced on the film because there is not enough light or energy falling on the film to produce a reaction.
There is another problem in that if the light intensity is very low over a long period of time then it slows the reaction, or rate of change on the film. Think of trying to heat a cauldron with a match, hold it close and you will get a hot spot, further away and the effect of the match will be dissipated and the further away you hold it the less effect it has, it is not a simple distance/time relationship.
Film is similar in that it requires a set amount of light energy over a set time to produce a predictable reaction or density in the film. Extend that, hold the match further away, and you have to hold it there a lot longer to produce the same reaction. There is no real *line* above which there is one exposure, and below a correction value but a gradual fading of the effect which increases as the time needed to capture a set level of light increases.
Now there is a further problem. The highlights, or lights, in your night scene will have a fairly high point intensity, akin to a match held close, and the shadows will be akin to holding the match further away. The highlights may focus enough light over time to produce a normal reaction in the film whereas in the shadow values the amount of light falling on the film over time is below the level needed to produce a consistent, or linear, reaction in the film. So where the shadow values need extra exposure to produce the correct density in the film the highlights don't and the extra exposure will produce a higher density, or overexposure. In effect the nature of the way film reacts *increases* the DR of the scene where exposure needed to capture the shadow values exceeds 1 sec.
If you increase *shutter speed* to correct this then you introduce an even longer time which again suffers the same effect in the shadows. It's a bit of a guessing game and varies with film emulsion but I use the Ansel method and within reasonable limits with common film produces predictable and usable negatives:
1 sec - 1 stop more aperture or 2 sec and -10% development time
10 sec - 2 stops more aperture or 50 sec and -20% development time
100 sec - 3 stops more aperture or 1200 sec and -30% development time
