photomacrography

Photomacrography is the making of an image that is larger than the object, using conventional camera techniques. It is often miscalled ‘macrophotography’, which in fact describes techniques for making giant prints, quite another thing. Photomacrography is formally described as photography over a range of magnifications from ×1 to ×20. Many modern zoom lenses claiming a ‘macro’ facility in fact operate at a maximum magnification of only ×0.5 or less. A true macro lens is fully corrected for ultra-close-up work, whereas normal lenses used in this manner may show noticeable aberrations. Ordinary camera lenses can be used (at a small aperture) for magnifications at around ×1 to ×2, using extension tubes or a bellows extension, but at higher magnifications the reversal of the usual conjugate distances requires the lens to be reversed in its mount. There is little depth of field, and it is usual to focus by adjusting the camera back rather than the lens, or with cameras with fixed backs by mounting the camera on a sliding platform.

The most important effect of high magnification is the increase in exposure that is required. The effective f-number is increased, as this is no longer simply focal length ÷ lens diameter, but extension ÷ lens diameter. The increase in f-number is equal to the marked f-number × (1 +m) where m is the magnification, and the required increase in exposure because of this is (1 +m)². So a magnification of × 1 (double extension) requires four times the indicated exposure, and a magnification of ×2 (triple extension) requires nine times the indicated exposure. Through-the-lens (TTL) metering takes this increase into account, but high magnifications send the required exposure soaring, and this is aggravated by the necessity for using a small aperture in order to secure adequate depth of field. Meters do not take account of the reciprocity problems that go with very long exposures, such as underexposure, unduly high contrast, and incorrect colour balance (though film manufacturers provide lists of correction filters to use for long exposures). It is therefore advisable to bracket exposures upwards by several stops when shooting macro.

Because of the long exposures certain other precautions are necessary. The camera must be on a rigid support, and subjects such as flowers must be fully protected from any wind. Even in the studio, flowers and leaves can move towards the light during a long exposure, and because of this and the risk of overheating it is preferable to use flash (which also takes care of reciprocity problems).

Most beginners automatically choose their shortest focal length lens for macro work, as it produces the largest image with a given set of extension tubes. This is a mistake. Such a lens will be covering a much larger area than the format for which it is designed, and will thus not be operating at its best. In addition it is very close to the subject matter, which can result in perspective distortion. A longer focus lens (say, 100 mm for a 35 mm format) gives better proportions, puts the background well out of focus, and keeps the camera a reasonable distance from the subject, leaving plenty of room for flash illumination. Depth of field depends only on magnification and f-number, and is independent of the focal length of the lens. It should be emphasized that, in photomacrography, image definition is not improved by using a small aperture. The depth of field will certainly be increased, but the resolution, even at apertures as large as (a nominal) f/4, will be governed by the laws of diffraction.

— Graham Saxby

Bibliography

• Meehan, J., The Art of Close-up Photography (1994)