“To me the face – the eyes, the expression of the mouth – is the thing that reflects character. It is the only part of the body that permits us to see the inner person!” – Philippe Halsman
Lighting Ratio describes the difference between the main light and fill light and this relative strength can be measured with a light or flash meter.
In camera meters measure the light that’s being reflected by the scene being photographed. Hand-held incident light , like my ancient but usable Luna Star F2 meters measure the amount of light falling on the subject. Spot metering measures a narrowly focused area in a scene but still measures reflected light. If you have time, please check out my post “Using a Handheld Meter for Portraits.”
If you take a meter reading for a subject of 1/500 sec at f/11 and want to use a slower shutter speed to allow for normal subject movement and choose 1/125 sec you will have to adjust the aperture (make it smaller) so the same (equivalent) amount of light falls on the sensor. By selecting either Aperture or Shutter priority exposure modes, your camera calculates the equivalent exposure for you, eliminating guesswork.
The Brightness Range method suggests taking two differentreadings from the scene you’re about to photograph. The first one (or second if you prefer) is a meter reading of the highlight area where detail is desired. A second reading it taken from the shadow area of the scene, again where you want the detail held. Your camera settings will be based on an average of the two readings and may be close enough, although you might try to bracket your exposure. Exposure for The above portrait was made with a Canon EOS 60D with EF 28-105mm lens with an exposure of 1/80 sec at f/5.6 and ISO 200.
Shea appears in my book “Available Light Glamour Photography” is available from Amazon with some of their associate vendors (non-Prime) selling new copies of the book for just $9.99 plus shipping, which is a heckuva deal for one of my favorite books.