V1.02
Thom Hogan’s Complete Guide to the Nikon D300
Page 377
Let’s look at one of Nikon’s published
MTF
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tables to see why the 1.5x change
of view is important. First things first: the
vertical axis is an indication of contrast on a
high frequency test chart (red being a
frequency of 10 lines per millimeter and
blue being 30 lines per mm). The solid and
dotted lines indicate differences in the
orientation of the line pairs being measured
(sagittal and meridional orientations, thus
the S and M in the legend). The higher a
point is on the vertical axis, the better the
“resolving power.”
The horizontal axis of the chart shows the
distance from the center of the image area
in millimeters. The charts show the full
distance to the edges of a 35mm film
camera—for the D300 we need only go to
about 12 or 13 on the horizontal axis, as
the smaller sensor means we don’t get as
far from the center.
Note that the lens being tested holds 30
lpm MTF values to about 0.5 up through
10mm from center. A value of 0.5 would
be considered “acceptable.” But look what
happens on the full 35mm frame at 20mm
from the center (the 35mm frame edge):
we’re at 0.2, which would be considered a
poor showing. In other words, the corners
of the image using this lens on a D300
would look better than the corners using
this lens on a film body.
The Autofocus System
The D300 uses a new autofocus system, introduced with the
D300 and D3 together. While arguably state-of-the-art,
Nikon’s documentation of the autofocus system leaves a lot of
107
Modulation Transfer Function
. That’s the name for a fancy test that measures the
ability to resolve small alternations of black and white high-contrast lines (i.e. a test
chart).
