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Monday, May 25, 2009

Lens MTF Demystified

As we look at the chart keep in mind that the higher up the chart a line is, the higher the contrast, and that 0 to 20 across the bottom scale represents from the center of the lens to the edge. Ignore the dotted vs. dashed lines for the moment.

Here are some rules of thumb for reading a chart...

— the higher up the chart the 10 LP/mm line is (the thick lines), the higher the contrast reproduction capability of the lens will be.

— the higher up the chart the 30 LP/mm line is (the thin lines), the higher the resolving power and thus subjective sharpness of the lens will be.

— keep in mind that the black lines show the lens wide open while the blue lines show the lens stopped down to f/8, so the closer these sets of lines are to each other the better the performance of the lens when used wide open. The very best lenses will have the black and the blue lines close together.

— generally speaking a lens whose thick lines (10 LP/mm) are above .8 on the chart should be regarded as having excellent image quality. Above .6 is regarded as "satisfactory". Below .6 is, well, below.

One of the areas of image quality that MTF can help determine is bokeh. This is a Japanese word used to describe the manner in which a lens reproduces the out of focus areas of an image. Some lenses are harsh in this regard, while others produce a much more pleasing out of focus image. This is where the meridonial and sagital lines come in, and as you'll recall are represented by the dotted and dashed lines. The closer these lines are to each other the more pleasing the bokeh of the lens. Fascinating, huh?