I'm learning to use a Gossen Digipro F2 light meter and am unclear about the benefits of "contrast measurement." The manual indicates that after "...aiming the meter at various areas of the subject... ...the entire measured contrast range is displayed on the analog aperture scale" from the lowest f-stop to the highest one. I've consulted several Gossen resources to no avail. Any help you could provide would be very much appreciated.
As example: In a portrait sitting, we generally deploy multiple light sources. A “key” often called “main” lamp is typically placed high to simulate afternoon sun. The placement of this lamp is an acquired skill. The shadows cast by the key light creates shadows that give an illusion of depth that can make our two-dimensional images come to life.
Good technique entails softening of the shadows cast by the key light. Otherwise, the shadows tend to go dark thus void of detail. To accomplish, we use a light meter to allow placing a second lamp so that it illuminates these shadows from the camera’s position. This lamp, called the “fill” is normally set subordinate to the key. Such a lash-up sets the contrast range of the portrait.
Should the fill be equal in brilliance to the key, the resulting image will be “flat”. The lighting ratio is 2:1. Say the main outputs 1000 units of light, 1000 play on the frontal portion of the face and near zero in the shadows cast by the key. The fill illuminates the shadows with 1000 units plus the frontal areas also receive 1000 units. That’s 2000 units on the frontal areas and 1000 units in the shadows thus the 2:1 ratio, this is flat lighting.
If the fill intensity is reduced 1 f-stop to 500 units, the frontal areas receive 1500 units while the shadows receive 500. This is 3:1 lighting, “normal” contrast. A flattering lighting.
Of the fill is reduced again by 2 f-stops to 250 units, the frontal area receives 1250 units and shadows 250 units. This is a 5:1 high contrast lighting.
If the fill is reduced to 3 f-stops subordinate to 125 units, the frontal area receives 1125 units and the shadows 125 units. This is 9:1 ratio, very high contrast (theatrical).
This is just one application, there are many other applications where knowing the scene lighting range allows for creative handling of the exposure setting and/or choice of developing/printing techniques and/or post-editing.
The German/English Manual for the F2 is here
The only possible benefit is that contrast measurement helps you make the picture you want to make.
It would be useful when you are looking for specific contrast ratios among parts of the scene. For example one stop between the key and fill sides of a head shot portrait...which is relatively straight forward to do without that feature of the F2.
On the other hand if you were working with four or five or more lit surfaces, for example a group portrait then having the entire range of measurements might be useful for adjusting the lighting if possible or deciding how to handle the variation of the lighting cannot be adjusted.
But if it doesn't help you think about how to make the picture you want, then it doesn't provide a benefit.
I don't think it is of much use in most normal situations; but in certain situations it can be handy.
For instance, say you are wanting to balance the flash exposure of a model indoors with the strong backlighting of a window they are in front of. Normally an incident meter would be used at the model's position pointing back towards the camera, but that won't work here... you need a second/separate reading for the backlighting.
If instead you read the backlighting and the available lighting in contrast mode, it will tell you the difference between the two in f-stops, and therefore how much flash power you need to add/adjust.
This isn't really much different from measuring multiple lights to set a contrast ratio; I just never found that to be very useful (if you know the setting of one light; you pretty much know the setting/ratio of the others in most situations).
The other advantage is that it keeps both measurements rather than having to remember/write down the previous one. So far, I haven't found the need for that in most situations either ;>)