If you had the following situation, what would you do? The bride is a beautiful fair-skinned girl from Scandinavia and the groom is a very handsome man from central Africa with beautiful black skin tone. I would pray alot because our EXPOSURE meters and film ASAs are based on an 18% gray card, which is somewhat near the reflectance of the Caucasian skin tone found in north America. If the rest of the picture consists of normal color and brightness values, and if you used an average meter reading, then you would over-expose the bride while the groom would be under-exposed. This is an extreme example, but serves as a good lead in to effective film speed or exposing for the subject.
This topic is also covered in another way under the topic "picturing black dogs."
Problems arise because we get involved in composing pictures and forget to think about the components of the camera and the way cameras are designed to behave. The film speed we all take for granted is based on a setting that has the reflectance of an 18% gray card. The fair- skinned bride and her white dress are far brighter (more reflective) than an 18% gray card. Thus the skin and dress area of the film will be over-exposed. On the other side, the groom and his black tux will reflect very little light and will be under-exposed in that area of the film. In the extreme example discussed above one can only hope that the dynamic range of the film will carry the over-exposure of the bride and limited under-exposure of the groom. In every picture we have highlights and shadows that are in fact over and under exposure. The goal is to work within a acceptable range that the film can handle. The trick is to look at the whole picture and meter for the reflectance of the subject or adjust your film speed to allow for the deviation so that the subject is near the middle of the working range of the film.
The good news is that todays' films have a very large working range (dynamic range) and often bale us out. The color film of 10 or 15 years ago had a very limited working range. The Fuji film that I like for my dog work, Fuji NPS 160, has an over-exposure tolerance of 3 F-stops and an under-exposure tolerance of 2 F-stops before color shift occurs. It is best to work in the middle of the exposure curve and expose the film for the subject. How do you do this?
If you have a spot meter you are all set. Take an average reading of the subject and expose accordingly. Many newer 35 mm cameras have a spot meter mode. Be sure to use it for extreme subjects. If you don't have a spot meter, then try moving in closer till the critical subject area fills the view finder and then take a reading. You could do this for each picture by holding the exposure lock (usually release button half way down), but that is time consuming and looks awkward. Instead move back and take a full scene reading, then reset your ASA override button +/ till the overall reading is the same as the close-up reading was. Now you are exposing for the subject, not the background.
If you use manual cameras, like I do, then the answer is simple. For black dogs, I will over- expose the film by one (1) to (11/2) F-stops by simply setting the lens that much below the metered value. For very white dogs, I will cut the exposure by 1/2 F-stop by setting the lens that much over the metered value. On our 35 mm I set the film compensation setting to these values.
If the film you are now using isn't doing the job for you, talk to the technical department of the film manufacturer for tips, or try other films till you find one that suits the subject. Different films have difference tolerance. A friend of mine photographs horse show jumper classes. He uses Fuji 400 Super G film because 400 Pro film is designed to give pleasing skin tones but doesn't give correct colour for horse coat texture. Super G is listed as an amateur film. It is designed to give a true color that is more saturated. That's why it makes the horse look good. The message is: choose a film to suit the subject and then expose the film according to the subject, not according to general a meter reading.