How to read MTF chart ?
MTF charts (short for Modulation Transfer Function) provide
a graph analyzing a lens' ability to resolve sharp details in very fine
sets of parallel lines, and a lens's contrast or ability to provide a
sharp transfer between light and dark areas in sets of thicker parallel
lines. Fine repeating line sets are created parallel to a diagonal line
running from corner to corner of the 35mm frame, directly through the
exact center of the image area. These are called sagittal lines, sometimes
designated "S" on Canon's MTF charts. At a 90° angle to
these, additional sets of repeating lines are drawn, called Meridional
(or "M" line sets. Repeating extremely fine short parallel lines
spaced at 30 lines per millimeter measure the lens' ability to record
fine details, or its resolution. Even more important in the eyes of many
optical designers is the lens’ contrast capability, which is measured
with thicker sets of parallel repeating lines drawn at 10 lines per millimeter.
At first glance, it would appear that any good lens would record lines
running parallel to a diagonal drawn across the film with the same accuracy
as lines drawn perpendicular to them. However, in real-world testing,
this is often not the case. Especially in the Meridional direction, faithful
reproduction of fine line sets becomes increasingly difficult as you move
away from the center of the image toward one of the corners. And it's
a fact that almost all lenses produce sharper results in general near
the center of the frame than at the outer edges. MTF charts display the
lens' performance from center to corner. Running along the chart's horizontal
axis, labeled 0 to over 20, is the distance from the dead center ("0")
of a 35mm image along a diagonal line to the corner of the frame, which
is about 21.5mm away. On the chart's vertical axis is a scale representing
the degree of accuracy with which the fine and coarse line sets are reproduced,
in both the sagittal (parallel to the diagonal of the film format) and
meridonal directions. Solid lines on the MTF charts indicate the performance
of sagittal lines (parallel to the diagonal of the film), dashed lines
are for the perpendicular meridional test target lines. In theory, a perfect
lens would produce nothing but straight horizontal lines across the very
top of an MTF chart, indicating 100% accurate reproduction from the center
of the picture (toward the left of the chart) to its outermost corners
(at the right side of the chart). Of course, no such thing as a perfect
lens exists from any SLR manufacturer, so MTF charts typically show lines
that tend to curve downward as they move left to right (tracking the lens'
performance from center to corner of the frame). Canon's MTF charts give
results at two apertures: wide-open, and stopped down to f/8, with the
lens set to infinity focus. While MTF charts don't include many factors
that can be important when selecting a lens (size, cost, handling, closest
focusing distances, AF speed, linear distortion, evenness of illumination,
and of course features like Image Stabilization which may produce superior
real-world results), they can indicate to the knowledgeable reviewer some
of the optical characteristics they can expect from a particular lens. |