As can be noted from the above formula, if the subject luminance is fixed and the magnification changes, the film plane brightness (illumination) varies in the inverse proportion (M + 1)2. With a large subject distance, M is small and this effect becomes nearly absent, but as the subject distance decreases, M becomes larger. In an actual photographic example, with a magnification M of 1, film plane brightness becomes (M + 1)2 or 4. This is 1/4th that of a subject distance at infinity and exposure must be increased 4 times. When the measured exposure value is 1/60 sec shutter speed, this must be compensated to 1/15 sec (1/60 x 4 = 1/15). In closeup photography, (M + 1)2 is referred to as the compensation factor.
Measuring Method
Hold meter as close as possible to the subject and point Lumisphere toward the camera. If light source is very close to the subject, temporarily remove the subject and hold the Lumisphere in the subject position. The light then striking the Lumisphere becomes exactly the same as that illuminating the subject. To obtain exposure compensation for closeup:
1.Determine exposure by normal incident light method. 2. Measure subject and image sizes (groundglass).
3.If the image is larger than the subject, it is enlarged, while a smaller image is compressed. Divide the larger value by the smaller value.
4.Obtain compensation factor from Table 1.
5.Apply compensation factor to measured exposure value to produce compensated exposure value.
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