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The Pentax SMC FA 77mm f/1.8 Limited is basically free of distortion (0.1%).
The chart above has a real-world size of about 120x80cm.
As a full format lens the 77mm f/1.8 enjoys a sweet spot advantage on the K10D.
However, originating in its very large max. aperture it still produces a moderate degree of
vignetting at f/1.8 (0.71EV). This may be visible in very critical scenes. The problem is
negligible from f/2.8 onwards.
The FA 77mm f/1.8 Limited produced very good to excellent resolution figures
in the MTF lab. At f/1.8 the center performance is already very fine but the borders
quality isn't there yet. This may sound unimpressive but for portraits, surely the typical
application for the lens, this characteristic isn't really much of an issue because the
subject tends to be close to the center here anyway. Stopping-down to f/2.8 lifts the center
resolution to excellent levels whereas the borders improve to good figures.
The performance peak is reached at f/5.6 with an outstanding center and excellent
Below is a simplified summary of the formal findings. The chart shows line widths
per picture height (LW/PH) which can be taken as a measure for sharpness.
If you want to know more about the MTF50 figures you may check out the corresponding
Chromatic Aberrations (CAs)
Lateral chromatic aberrations (color shadows at harsh contrast transitions) are
very low for a lens designed in the film era. At around 0.2px on the average at
the image borders the issue isn't field relevant.
Some users were surprised to read this. Here's a 100% sample crop which illustrates the
a typical border situation where lateral CAs tend to occur in high contrast situations.
However, lateral CAs are only one aspect - purple fringing is another one. At large
aperture setting this kind of CA can indeed be a problem. Knowing how to force the
problem I was able to produce the following 100% crops:
Please don't let you put off by this problem though. This happens only in extreme contrast
situations when the bright portion is at or beyond the contrast latitude capabilities of the
Longitudinal Chromatic Aberrations (LoCA)
Typical for most large to ultra-large aperture lenses the SMC-FA 77mm f/1.8 Limited
suffers also a little bit from reddish to greenish out-of-focus contrast transitions
(just in front and behind the focus field).