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3 Choosing lenses > Checking lens image quality

Checking lens image quality

Lens quality is difficult to pin down by something simple such as a number. Sharpness is subjective – clarity judged by human vision – and has to include image contrast as well as how much fine detail is actually present. Look carefully at Figure 3.14, for example. You can do simple practical tests yourself to get an idea of the quality of a lens. This is helpful if you are trying out a secondhand lens before buying. On the other hand, manufacturers do much more sophisticated tests and publish typical performance as figures or graphs for each of their lens range.

image

Figure 3.10 Coma. Each circular zone of the lens focuses oblique rays from an off-axis point at a different position relative to lens and lens axis. The patch of light formed (drawn as it appears at point of focus C) is pulled out like a comet. This is an oblique form of spherical aberration (Figure 3.5).


  

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