Tony, you are close in your exposure rules of thumb, but no cigar today. Trees (i.e. green foliage) vary a lot from type to type, and even from month to month. Deciduous trees (e.g. maples) have light green colored leaves in the spring, while by August these same leaves have turned dark. Conifers have dark geen needles all of the time. With these variables, tree leaves are not good things to take your TTL meter levels from. I find that most trees are too dark for metering purposes.
Grass and weeds also change color due to rainfall amount, etc., and also are variable in the amount of light they reflect because of this change in color. Grass is less variable than tree leaves in terms of its reflectance of light. Caucasian skin is brighter than a gray card, and also varies from person to person. Just look at the difference in a Caucasian person's skin tone in February and in July after he has gotten tanned by the sun. Again, a variable, and, therefore, not reliable. Here are some tricks... The back of everyone's hand is a different shade of "fleshtone" because of the different races of people in the world, but nearly everyone in the world has the same shade of "fleshtone" on the palm of the hand ! What we all can do is meter off the open palm of our hand AND THEN OPEN UP THE EXPOSURE BY ONE F/STOP. This is really close for most applications. If you do not increase the exposure by the equivalent of one f/stop (either by aperture or shutter) the photo will be underexposed. Other things that are close to the reflectance of an 18% gray card are a dusty road, faded blue jeans and faded asphalt, but again there will be variables in these. But they are very close for determining exposure, especially if your meter malfunctions or if you just want to double-check your meter's accuracy in the field. Exposure Rule of Thumb--On a bright sunny day, your basic exposure will be the reciprocal of the film speed @ f/16. Kodachrome 64 will be 1/64 (or 1/60th) @ f/16. Then extrapolate an equivalent exposure for your needs (1/125 @ f/11, 1/250 @ f/8, 1/500 @ f/5.6, etc.). This works for all film since their speeds are directly proportional to each other. Of course, this will change at sunup, sundown, cloudy, hazy, etc. If you aim your TTL meter at a black Norfolk Southern diesel (close-up) and set the exposure, you will be 2 to 2-1/2 stops overexposed, and the black diesel will look like 18% gray on the slide, as this is how TTL meters are calibrated to perform. On the other hand, the opposite is true with a white Soo Line diesel--it will be 2 to 2-1/2 f/stops underexposed, and the white diesel will look like 18% gray also for the same metering reason. It's the nature of the beast with TTL metering, which is why I use a hand-held incident-light meter instead of a relected-light meter. I use this incident reading to determine the amount of light falling onto my subject, and then adjust my camera exposure accordingly (black diesel? white diesel?). I shoot a thousand slides a day, and I do not bracket exposure except in really weird lighting situations. And, yes, I do carry around a gray card in my camera bag, but I have not used it in years. (Actually, I cut them in half as the big cards get bent too easily.) John B. Corns --> SPORRS: Serious Photographers of Railroad Related Subjects
