ABSTRACT
For example, you might be using a rollfilm camera with an 80 mm lens to photograph a small 10 cm high product 5 cm high on film. The hand-held meter reads 1⁄2 sec at f /16. Following formula (i) above, magnification is 0.5, so exposure needs multiplying by 21⁄4 times. In practice, this means changing to 1 sec at f /16. *Warning note on telephoto and invertedtelephoto lens designs. The above formulae are sufficiently accurate for most large-format camera lenses, enlarging lenses, and normal focal length lenses for rollfilm and 35 mm cameras. However, expect some discrepancy if using any formula containing V or U for lenses of either telephoto or inverted-telephoto construction. This is because it is difficult to know from where to make simple measurements with a ruler alongside such a lens. In these circumstances you can still calculate close-up exposure increase accurately using the formula based on M rather than V or U.
