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Re: filmscanners: Sharpness of color chrome vs color negative.

Laurie asks:

> That's nice; but pardon my ignorance, what is an MTF spec?

MTF is the modulation transfer function: it is a measure of how well small
details are recorded or focused, and is expressed as a percentage for a given
resolution (in line pairs per millimetre, usually) and contrast ratio (1.6:1,
1000:1, etc.).

A MTF of 50% at 50 lp/mm means that about fifty percent of the contrast between
black lines and the white spaces between them is preserved when their spacing is
50 line pairs (one black, one white) per millimetre.  The MTF can never be
higher than 100% for a lens, since at 100% the lens is transmitting 100% of the
contrast in the target--perfect resolution.  The MTF _can_ be higher than 100%
for film, though, since some films will exaggerate contrast at some resolutions,
causing the contrast to appear higher on film than it was in real life (this is
called accutance).

When you look at an MTF chart for film, the horizontal scale is usually lp/mm,
and the vertical scale is modulation transfer in percent.  The curve on the
graph represents the percent of modulation transfer at a given resolution.  It
usually starts just above 100% for low resolutions, and gradually dips downwards
for higher resolutions, eventually dropping below 30%, at which point most
graphs stop.  The 50% point is often considered the nominal resolving power of
the film.  For film, a MTF chart shows the curve for a specific contrast ratio
in the target, usually 1.6:1 or 1000:1.  Higher contrasts produce higher
resolution figures, since they tend to show up better on film.

When you look at an MTF chart for a lens, the horizontal scale is usually the
distance from the center of the frame, and the vertical scale is again the
percent of modulation transfer.  There are usually four curves, representing
resolutions of 5 lp/mm, 10 lp/mm, 20 lp/mm, and 40 lp/mm.  The curves show how
much of the resolution is retained as you move outward from the center of the
image.  The MTF is never higher than 100% for a lens, since a lens cannot show
more contrast than what exists in the original scene.  A good lens will show
relatively flat curves that are quite high on the MTF scale all the way out to
the edges of the frame.  A poor lens will show a sharp drop in resolution as you
move outward, and even in the center, the curves will start quite low,
especially for the 40 lp/mm curve.  The exact MTF for a lens depends on the
aperture setting, focus setting, and focal length (for zooms); MTF is usually
best for some intermediate aperture, like f/5.6.


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