For in-depth coverage of focus shift and many other topics, see Making Sharp Images.
Focus shift is a little-discussed optical phenomenon which can lead to blurred images and erroneous conclusions about lens sharpness. It is of particular concern when shooting a “fast” f/1.2 or f/1.4 lens stopped down to the f/2 - f/4 range. While slower lenses (eg f/2) can be affected this is rare.
Observe the crop below. Precise focus was achieved at f/1.2 on the eye at right. However, the image was taken at f/2.8. Focus has shifted to the eye at left, blurring the desired eye. If you’ve just promised a customer a fine portrait, you’re going to be exceedingly unhappy with such results. In fact, the right eye is barely “rescued” until f/8, so you’re going to have to understand and deal with focus shift with this lens!
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