Local Contrast and Perceived Tonal Values

dsc_9009srgb.jpgI have talked a bit about this shot in a couple of previous articles but never explained why I set up such a ridiculous shot for my workshop in the first place. To follow along and eliminate any ambiguity or to confirm what the words in this installment are saying, feel free to load the linked image into your favorite software that can display luminance values of specific areas of the image.

First off the lighting setup: Large box (3’x4’) at 45° to camera right and high enough to cast shadow under the eyebrows. Exposure metered at the face is f11. The light that is actually included in the shot is as you can verify behind and at about head level pointed strait ahead. It has a very narrow grid on it and a full CTO gel. Exposure value is again at f11. The last light is a 1x3 strip box located to camera left and about 45° behind the model and slightly above. Exposure value is about 2/3 to 1 full stop below f11, Yep below, like f8 to f8 1/3.

A couple of quick questions, no cheating with your cursor or eyedropper tools yet. What is the brightest area in the photo? The darkest area? Ok that’s easy enough, the light source in the back and the black background will values of 255 and 0-3 respectively. So I we don’t get confused all of the left right references are from the camera’s perspective not the model’s. Now here are a couple of harder questions. What’s brighter the highlights on the right side of the hair or the left side? Most people would have the first impression that the left side is brighter. How about the highlights on the right side of the model’s cheek or the left side? Again most people would guess they are about the same. The answers - the highlights on the left side of the model’s hair are the same values as on the right at best and in most cases are darker. As for the highlights on the right side of the cheek versus the left side, the values are miles apart, like a couple of stops. Don’t believe me check out the luminance values yourself. The cheek highlights are in the 180’s on the right and in the 90’s on the right.

How is this possible? Why do the values look similar or in some cases even reverse of what the initial perception of brightness is? The simple answer is context. say an exposure value of f11 in a sea of tones that are f8 to f16 is not going to stand out very much in terms of contrast. An exposure value of f8 stands out like crazy in the context of black or very low exposure values. Here is another image that may surprise you if you measure the luminance values. When put in terms of exposure values it seems like a simple concept, pretty much obvious. What is not obvious when first learning how to use lighting gear is why your images look flat. The answer is usually dumping way too much light on the scene or subject and eliminating shadows and local contrast. A little bit of light goes a long way in the shadow areas of an image. If you don’t have a bunch of lighting gear, let’s say two lights and two umbrellas. If you are using one for a main and one for a fill there is a good chance they are too far away from your subject and too close to the walls, ceiling, etc. What you get with this is pretty much very similar light values everywhere because umbrellas through light all over the place. Instead of using the second light for fill try using it as a rim light instead on the shadow side and slightly behind your subject. Try it with the umbrella, if you still have too much light coming from all over the place try a plain old dish on it and ditch the umbrella. Check out the image to the left. She seems pretty bright in the image and well separated from the background. Lighting set up is a large 3x4 box slightly to camera right, 1x3 strip light about 45° behind and to camera left. If you look you can see the rim lighting starts where the main light would be falling off into almost complete shadow. It’s a little bit brighter than the subjects skin that is lit by the main light but not much. If you measure the skin values on most of the model they are pretty much in the mid-tones. She looks bright in the image for a couple of reasons. One the rim light and the separation that goes along with it. Two the specular highlights on her skin. Last but not least and maybe a surprise. She is way brighter than the background. Check out the luminance values yourself, the brightest background values are between 40 and 50. By comparison her skin is in the 120 to 140ish range. Hey the background doesn’t look that dark does it? Why? Again due to context. The low values and I am talking about the red not the black, are in the teens, of course the blacks are actually black. To get the background to pop like this but still leave the model looking bright I used a red gel and a grid on a light and brought the background up while leaving it fall off to black towards the top.

Hopefully this will give you some helpful ideas on using your lighting gear a little more effectively. As is the case with most photographic things. A little bit goes a long way. Feel free to hit me back with questions or comments.

RB

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