Tuesday, January 4, 2011
DPP Solutions: MTF Curves
How to read and interpret MTF data
Radial (sagittal) targets have lines parallel to a diagonal line drawn from image corner to image corner. Tangential (meridional) targets have lines perpendicular to those of the radial targets.
Basically, MTF curves tell you how well a lens reproduces the contrast of a test target. The target is a series of bright and dark lines that gradually become finer and finer—starting from just a few bright/dark line pairs per millimeter and progressing to 100 line pairs or more per millimeter. An ideal lens would reproduce the test chart exactly as it is in real life. Such a lens doesn’t exist, and the modulation transfer curve—plotting the amount of the original target’s contrast that the lens can reproduce—shows us how close to “perfectly” a given lens reproduces the target’s contrast.
There are several types of MTF curves. The most common one used by camera-lens manufacturers plots both contrast along the vertical axis and distance from the center of the image frame across the horizontal axis. The vertical-axis scale runs from 0 at the bottom (no contrast at all) to 1.0 at the top (indicating the lens reproduces 100% of the target’s contrast). The horizontal scale runs from 0 (center of the image frame) to 20 (20mm out from the center) or so, depending on the image format. A chart for a “perfect” lens would show a straight line across the top, meaning the lens reproduces 100% of the test target’s contrast all across the frame. That doesn’t happen in real life, of course, so the line on the curve starts out high at the left (center of the image) and curves down as it moves to the right (farther from the image center). As a general rule, MTF lines of 0.8 (80% of the target’s contrast) indicate a very good lens; lines of around 0.6 (60% contrast) indicate a satisfactory lens (depending on your idea of “satisfactory”).
Of course, a single line just shows lens performance for a single spatial frequency (number of line pairs per millimeter), at a specific aperture (generally, wide open). So manufacturers include lines for two or three spatial frequencies, generally 10 lp/mm and 30 lp/mm, or 10, 20 and 40 lp/mm. The 10 lp/mm lines provide an indication of the lens’ contrast, while the 30 and 40 lp/mm lines provide an indication of the lens’ sharpness. Generally, thick lines are used for the 10 lp/mm curve and thin lines for the 30 lp/mm curve.
Lines also are shown for targets that are aligned parallel to the format’s diagonal (corner-to-corner) and for targets perpendicular to those. The former are called radial or sagittal lines; the latter are called tangential or meridional lines. These are shown as broken lines, again, thick ones representing 10 lp/mm readings and thin ones representing 30 lp/mm readings. The key here is that bokeh is better when the radial and tangential curves are similar.
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