3 LANGFORD'S ADVANCED PHOTOGRAPHY Buying lenses 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 E ach manufacturer sets its own minimum quality image acceptance standards, but every lens ­ even though one of hundreds of the same design ­ is also an individual product. Glasses, shaping, spacing and centring can differ in minute ways as lenses come off the production