Macro is the Greek word for ‘big’ and has come to mean any lens that can focus down to a close-up of a subject.
In optics the term has a very specific meaning. In general photography, we image the subject much smaller than its actual size. You can fit a cathedral on the APS-C format using the right lens from the right distance. The closer the subject to the camera, the larger its image. Eventually the image is the same size as the subject. This is called life-size or 1:1. The repro scale states the relative sizes of image and subject, so a scale of 1:2 means each unit of dimension in the image, say, 1cm, covers twice that unit in the subject: 2cm. Therefore the image is half life-size. Sometimes the scale is given as a magnification, so 1:2 becomes 0.5x, although it is actually a reduction.
Macro and micro
All pictures taken at distances from infinity to 1:1 produce images smaller than life size. In optics they are thus termed micro (‘small’ in Greek).
Again, in optical terms, macro begins to apply when the image is larger than the subject. On the repro scale, 2:1 means the image of the subject is twice its size (2:1 or 2x magnification).
The macro region extends from 1:1 to about 10:1 or 10x magnification, and is termed macro photography.
Micrography vs microphotography
Achieving greater magnification needs the camera to photograph through a microscope, which is the realm of photo micrography. There is one other term to confuse the issue: microphotography. As we’ve seen, most exposures are of subjects imaged smaller than life-size so, strictly, they come under ‘microphotography’. Yet the term is used solely for the reduction of documents, drawings and similar material to a fraction of their original size.
Microphotography was used for archiving on microfilm, but its main use is in the photofabrication of printed circuits, large-scale integration and transistors. The special lenses, corrected for a narrow spectral brand, can give 1:2000-4000 reductions, which is 1 metre reduced to 0.5-0.25mm.
Macro lenses
In the strictest sense of the term, there are few true macro lenses on sale to photographers. Most are the special monofocal lenses that can focus to 1:1.
The one firm that correctly terms its 1:1 focusing lenses is Nikon with its Micro Nikkors.
Many zooms have the ‘macro’ designation but they do not focus to anywhere near life-size, or 1:1, but justify the claim by being able to focus to a shorter distance than would normally be expected for the focal length span.
Early closer focusing zoom lenses only did so at the maximum focal length, which simplified the corrections. The longer a zoom lens’s focal length, the further away is its minimum focus distance. This is because it becomes more difficult to maintain corrections.
Also, the increase in overall length of the lens extension or internal group movement brings unacceptable growth in size and weight. Repro scale and magnification always relate the image size to that of the subject. However, by enlarging the image, one can obtain a larger-than-life print.
Lens construction
1866 Rapid Rectilinear
Thomas Dallmeyer realised a symmetrical construction gave good curvilinear and spherical aberration corrections, hence his Rapid Rectilinear of 1866.
1920 Plasmat
Paul Rudolph’s 1920 Plasmat – the final evolution of the perfectly symmetrical design. His 1892 Protar was the first to use the new Jena glasses.
Modern 1:1 focusing lens
A modern 1:1 focusing lens. An SLR requires a retrofocus design, so the front group needs a broad front element to collect the light to pipe down the long barrel. However, its symmetrical design ancestry is visible.
Macro photography: the challenges
The difficulty of maintaining corrections with a ‘macro’ zoom lens in close-up invariably means that it is really only suitable for imaging natural subjects, so it is unsuitable for those with linear, straight-line features including document copying.
Even at more usual subject distances, as when imaging buildings, a zoom lens has to be of a high calibre if horizontals and verticals are not to look bowed. This error is termed curvilinear distortion.
If the lines curve inward to the frame centre it is popularly called ‘cushion’, and if outward it is called ‘barrel’ distortion.
When taking photographs of flowers, small creatures and so on with interest mainly in the frame centre, quite a measure of field bowing may be allowable.
Yet for accurate imaging and copying marked drawings, errors are unacceptable. That is why lenses designed specially for close-up work to a 1:1 repro scale are necessary.
Nikon may use the term rightly, but it has to be recognised that optical firms generally call this type of optic a ‘macro lens’. A top-quality 1:1 focusing lens will be a monofocal (of fixed focal length) optic and not a zoom lens.
The two main problems are curvilinear distortion (described above), and spherical aberration. The latter results in the image field being bowl shaped and not flat. It leads to an increasing loss of focus away from the image centre and with it a variation in magnification. It derives from the curved, spherical shape of the surfaces of the elements. It improves on stopping down, since a less deeply curved section of the lens components is being used.
Correcting spherical aberration
Spherical aberration can be corrected by moving one or more groups of elements in the lens as focused distance changes. Such a feature is often called a ‘floating’ element. Its incorporation in a zoom lens that already has moving groups is usually impractical.
The lower spherical aberration in zoom lenses nowadays is derived from the use of aspheric surfaced elements, those whose curves do not form part of a sphere. Their use has also made lower levels of curvilinear distortion possible in zoom lenses compared with earlier designs. Parallel progress in monofocal lenses has maintained the performance difference between the two in the best designs.
Image: Depth of field becomes very shallow as the subject gets closer and, as this shot of a dictionary page shows, even at f/11 it is limited to a few millimetres
Macro monofocals
A macro zoom lens may not work as close in or as well optically as a specially designed macro monofocal optic, but it does extend the choice of subjects in a worthwhile way.
We are all cost conscious and a zoom lens may be an economic necessity. If experiments with a macro zoom lens whet an appetite for more, it is time to look into the special macro monofocal lenses available.
Classically, close-focusing lenses have been based on a symmetrical design. This means that the rear group of elements behind the aperture diaphragm are identical to the one in front of it, only reversed or turned round.
In this way the residual errors in the front group are cancelled out by the opposite sign of the rear group. The effect is most efficient when imaging at 1:1 and decreases as focus distance increases.
Also, it does not work so well with wide apertures, which is why most early close-focusing lenses were of f/3.5-4 aperture. These are still found, though f/2.8 is the more normal maximum. Simple, straight symmetry has been extensively elaborated with modern optical technology to provide top-grade corrections, especially in colour, together with the wider aperture.
In the second part of his article, Geoffrey Crawley be looking at the special equipment available for close-up work and how it is used.
