Pentax SMC-FA 77mm f/1.8 Limited - Review / Test Report - Analysis
Lens Reviews - Pentax

Distortion

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.

Vignetting

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.

MTF (resolution)

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 borders.

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 Imatest Explanations

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 sensor.

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).




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