TECH SUPPORT Diffraction Reaction Q. Years ago, the majority of the lens tests published in photo magazines would report the aperture by which the lens being tested became “diffraction- limited.” As the diaphragm was closed down, image degradation due to diffraction increased, and at some point, image degradation was due mostly to diffraction, and therefore the image quality had become diffraction- limited. I have noticed that lens tests no longer report the aperture by which the tested lens became diffraction-limited. Why is that?
A. You can easily discern diffraction effects on image quality in the SQF charts in our lens tests. SQF scores typically peak at mid apertures, then decline as diffraction takes its toll. If you turn an SQF chart sideways (counter-clockwise) the chart will form a classic bell curve, with the peak near the center, and dropping off to either side. As this degradation occurs along a continuum, it makes little sense to state one single aperture at which image quality is diffraction-limited. Better to use our SQF scores to determine optimal apertures based on the kind of photography you are doing, the depth of field you need, and the intended degree of enlargement. |
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