Friday, May 25, 2007
Lens Design And Technology In The Age Of D-SLRs
Lens quality has grown exponentially in recent years. The optics being produced for professionals today incorporate some high-tech miracles to get the job done.
If you're technically inclined, you've probably heard of or are familiar with MTF (Modular Transfer Function) Data. The tests are extremely accurate, but it's debatable as to their usefulness beyond a certain metric. With modern lenses, the capabilities of the optics often surpass the sensor's ability to record the image data. The test is performed by projecting a pattern of lines through the lens and measuring the resulting contrast. If the projected image is precisely the same as the test image, the contrast is 100%. If there's a complete loss of contrast seen by no distinct lines, the value is 0. Generally, values above 80% are where the best lenses reside, while values under 40% usually aren't suitable for professionals.
The test is done in two directions, radially and tangentially—lines from center toward edges and concentric circles. If the projected pattern varies from the test pattern, the lens has astigmatism. The closer the lines are in the test (higher frequency), the lower the MTF values will be.
Contrast: The 5 line pair per mm (lp/mm) and 10 lp/mm will show the contrast inherent in a lens. Small differences can be seen easily. Think of this as being a measure of the general sharpness look. Professional lenses should perform over 95% at 5 lp/mm for both directions. It's possible for a lens with mediocre sharpness but good contrast to seem sharper-looking than a lens with higher sharpness and lower contrast. The two work interconnectedly.
Sharpness: Measuring values at 10 to 40 lp/mm will show the lens' sharpness. The 40 lp/mm readings will indicate how well a lens can render very fine elements in the frame (think of eyelashes on a model). A pro lens should have an MTF value over 50% at 20 lp/mm.
As useful as MTF data are, the test isn't the end-all, be-all in determining lens quality. A variety of distortions and color issues will never show up in MTF tests. Still, MTF gives us a good quantitative way to get a sense of the overall capabilities of a lens.
A well-known photography curmudgeon, Fred Picker, once chided optical bench tests as being of almost no use. He asked rhetorically how many of us make a living taking pictures of optical benches? He made an interesting point. These tests are useful, but they shouldn't be overemphasized when evaluating any lens.
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