Tuesday, January 4, 2011
DPP Solutions: MTF Curves
How to read and interpret MTF data
Some manufacturers also show lines for wide open and a middle aperture (often ƒ/8). These show the effects on performance of stopping the lens down. Lens performance generally increases as you stop them down from maximum aperture, since doing so reduces the effects of various aberrations; then performance decreases as you continue to stop down and diffraction adversely affects the image.
For zoom lenses, you’ll want to see MTF curves for wide, middle and long focal lengths, of course. Bear in mind that, even for fixed-focal-length “prime” lenses, telephotos generally have “better” MTF charts than wide-angles—wide-angles present more design challenges.
Another type of MTF chart you sometimes see also plots contrast along the vertical axis, but plots spatial frequency (lp/mm) rather than distance from the image center across the horizontal axis. Again, the curve line will descend as it extends to the right, indicating that the more closely spaced the target lines, the less efficiently the lens can reproduce them. The higher up the line, the higher the contrast; and the less the line curves down to the right (i.e., the flatter the line), the better the lens reproduces fine details.
All that said, MTF curves are just another tool to help you evaluate lens performance. They don’t tell you about color fringing or the effects of focus change or the effects of the color of the “white” light used for the measurement. And MTF curves that are calculated rather than actually measured don’t take into consideration production tolerances.
Also, remember that the lens is just one link in the image chain, which includes the image sensor, monitor or print, and the viewer’s eye. By all means, check the MTF curves as part of your research when deciding which lens to buy; but also consider reviews from photographer friends who use the lens and published reviews from sources you trust, and best of all, try the lens yourself. You often can rent pro lenses from major camera stores.
If you would like to get into this topic in more depth, an online search for “Zeiss MTF Nasse” will lead you to a very thorough, two-part illustrated discussion of MTF curves and their ramifications (H. H. Nasse is the author).
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