Choosing a lens
Each manufacturer sets its own minimum quality image acceptance standards, but every
lens – even though one of the hundreds of the same design – is also an individual product. Glasses, shaping, spacing and centering can differ in minute ways as lenses come off the production line. So, although quality control should ensure that no lens giving less than a minimum standard gets through, some lenses may in fact be considerably better. You will also find two manufacturers offering lenses of similar specification at quite different prices. One significant reason may be that the cheaper manufacturer sets slightly lower standards and operates less-strict quality control. The result is more of a ‘lucky dip’ from which you may (or may not) draw a good lens.
Understanding some of the lens designer’s problems shows you the importance of using a lens only within its intended performance range. A telephoto lens fitted with an extension tube may focus close enough for 1:1 copying, but your results will probably be of poor quality. (This is not the same as a lens, such as a zoom, designed to offer ‘macro mode’ which shifts internal floating elements to correct for working at one close distance.) Again, some true macro lenses give indifferent results when focused for distant landscapes.
Most lenses give their best image quality stopped down to about the middle of their f-number range. The wider you open up, the less certain aberrations are corrected; but too much stopping down begins to lose quality because of diffraction (Figure 3.19). So the upper and lower ends of the f-number scale are limited by lens design and what the manufacturers regard as the limits of acceptable image quality. Sometimes two lenses of the same brand and with identical focal lengths are radically different in structure and price, because one opens up an additional stop or covers a larger area (allowing camera movements) without worsening quality.
Lenses for specialized purposes have their aberration corrections biased in a particular direction. For example, a lens for an aerial survey camera must give optimum quality for distant subjects and avoid any distortion of shape because results are used for measurement purposes. A lens for enlarging or copying has its corrections balanced to give best quality when subjects are close. A shift lens, or any lens for a camera offering shift movements, needs generous covering power (and to achieve this will probably not have a very wide maximum aperture). A lens for 35 mm press photography or surveillance work may only just cover its format, because all the design emphasis is directed towards widest possible aperture.
New lenses designed for digital backs outputting large files give higher resolution than is needed for silver halide film. (Circles of confusion should not exceed in diameter the ‘pitch’ or distance between each of the millions of elements in a CCD matrix). And a lens designed for the curtailed resolution requirements of video cannot be expected to perform well while taking silver halide photographs.
When choosing a lens decide too the handiest size and weight for your expected shooting conditions. This is especially important with long focal length lenses to be used on difficult locations. Do not go for maximum apertures wider than you will ever need – it only increases bulk and cost. Lens condition is especially important if you are buying secondhand. Elements with scratches (perhaps from excessive cleaning?) will more seriously affect image quality than a speck of black or even a bubble in the glass. A lens which has been dropped may show no obvious external sign of damage yet have internal elements miscentred, giving general deterioration of image quality. So if you are buying an expensive secondhand lens always try it out first under your own typical working conditions or invest in a professional test (available from most major camera repairers).