Some reasons were already mentioned. One was to manage contrast and to adopt to high contrast scenes.
Originally Posted by Ektagraphic
There is another method to adopt to higher contrast scenes, with which you can increase the dynamic range by one stop.
This method is used for decades by photographers working with the zone system.
Mostly used with BW film, but it also works well with color film.
It is based on the physical fact that when you double the light intensity for zone I, it doesn't effect the highlight zones in a visible way.
You measure your exposure, and the value for light intensity for zone I is 1.
Then the value for highlight zone VIII is 128 (128x or seven stops more light intensity).
If you double the light intensity for zone I, the value for light intensity will be 1 + 1 = 2.
Then the light intensity for highlight zone VIII will be 128 + 1 = 129.
The difference between 128 and 129 is less than 1%, and that is not visible in the picture.
Therefore you can increase shadow detail by giving extra exposure to the shadows without affecting the highlights.
A trick successfully used for decades by old school photographers knowing the zone system.
It works best with using a tripod and multiple exposure setting: Measure the right exposure, then stop down for the first exposure increasing the shadow detail.
Stopping down will be 4 stops (for zone I) with BW film in general, and because of the steeper characteristic curve of color slide film in the 3 1/3, 3 1/2 or 3 2/3 stops range with slide film. It depends on the film type how much you have to stop down for best results. With more contrastier films with less dynamic range like Velvia 50 stop down less, with films with higher dynamic range like Sensia 100 or Astia 100F stop down a bit more for best results.
Make the first shot with this stopped down value.
Then you make on the same negative / positive a second shot with normal (in case of slide film highlight oriented) exposure.
Your shadow detail will be increased by the first extra exposure, but it will not visible affect the highlights, which are determined by the second, normal exposure.
If the contrast of the scene is one stop more than the dynamic range of your film, you gain one stop more and can adopt dynamic range of the film to scene contrast.
If the scene contrast is two stops more, you loose only one zone in the shadows with this technique instead of two zones.
In this last case with two stops more scene contrast you can combine this technique with pulling one stop to gain the needed two stops.
I always get excellent results with this zone system technique.
Another recommended option for managing high contrast scenes, which is often overlooked by photographers, is modern fill-in flash technology. With fill-in flash you can also work with zone system technique. It is possible to set the shadows exactly in the zones you want them by reducing the power of the flash.
My F 80 / F90X / F6 / SB-800 combinations are doing little wonders in this respect. Precise light adjustment, combining both highlight and shadow detail in high contrast scenes. You don't see it in the picture that fill-in flash was used. Very natural results.
Last edited by Henning Serger; 09-07-2011 at 02:45 PM. Click to view previous post history.