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Field Position and Orientation
For an adequate representation of performance, MTF's
must be determined for several field positions, depending
on the angle covered by the lens and the nature of its
correction. In general, sufficient samples must be taken
to represent the full range of performance across the field,
in several focal planes. Preliminary observations will be
necessary to establish the positions required.
The measurements must be made for at least two orien
tations of the target. The convention adopted in the ISP
Resolving Power standard could be followed, thus the MTF
could be specified for radial and tangential orientation
of the target, (i.e. radial direction of lines, slit, or
edge, depending on technique).
The orientation of the lens should also be specified
by reference to some index mark, e.g. determination made
with mark upwards.
Expression of Results
Normalization
The exact meaning of the MTF values reported is not
always made clear. The MTF is sometimes taken to be the
ratio Mi t Mo where M 0 is the modulation of the object (a
sinusoidal target) and Mi is the modulation in the image.
More correctly, the results should be normalized to 100$
at zero frequency, i.e. the MTF should be defined as:
i ♦
MTF = lim
LO-sr o
This definition is emphasised because the MTF of
any lens reaches 1.0 by convergence, at zero frequency,
and is less than 1.0 at any finite frequency. For example,
the MTF of an f/5.6 camera lens could be 0.9^ at 5 cycles
per mm. If a laboratory assumed that 5 cycles per mm. was
low enough to be considered ’’zero frequency”, its results
would be higher than those found by another laboratory which
normalised correctly by extrapolation to zero frequency, or
whose apparatus included means for correct normalisation.
It should be noted that the MTF so defined does not express
the ’’contrast” of the image for large areas; this requires
measurement of veiling glare.
Co-Ordinates for Plotting
The choice of ordinate and abcissa scales is not a
| |w
M ± (CO)
M Q M
