Coastal Optics 60mm f/4 UV-VIS-IR APO Macro — optical performance
This page discusses overall optical performance.
MTF (modulation transfer function)
The Coastal 60/4 is diffraction-limited by f/5.6 across the UV-VIS-IR spectral band, except at infinity, where modest field curvature requires f/8 to sharpen the edges and corners.
A few exotic lenses approach this level of performance, such as the Leica 300mm f/4 APO or the 90/2 APO Summicron-R or the 180/2.8 APO-Elmarit-R, but none of them can do so outside the visible light range.
MTF at infinity, visible light (400 - 700nm), f/4, f/5.6, f/8
Wide open at f/4, MTF is excellent, indeed outstanding in the center, dropping off gracefully into the extreme corners. Conventional lenses stopped down 2-3 stops might show somewhat better performance in the corners in most of the visible light range, but most will also show chromatic errors that are not seen with the Coastal 60/4. One stop down at f/5.6 the 60/4 offers world-class performance; stopping down just 1/3 or 2/3 stop from f/4 yields most of this improvement.
More important for many subjects will be depth of field, which makes f/8 the optimal aperture for overall results in practice. While a graph for f/11 is not available, diffraction can be expected to reduce performance noticeably.
MTF at infinity for UV & IR, at 1:2 for visible light
Performance in the UV band is simply outstanding, and still outstanding in the infrared band, though slightly lower due to diffraction, which degrades performance with longer wavelengths.
Chromatic aberrations and distortion
This level of chromatic correction is extremely difficult to achieve, which explains why the Coastal 60/4 uses so many fluorite elements, which are expensive (and even the supply of calcium fluoride glass is limited).
Distortion is extremely well controlled for a 60mm lens.
Relative illumination and spectral transmission
See the Spectral Transmission page for more on spectral graphs. Vignetting is minimal, an excellent performance by f/5.6, the optimal aperture.
Diffraction limit
The Coastal Optics 60/4 UV-VIS-IR APO macro is diffraction-limited by about f/5.6 - f/8, depending on focus distance. Some improvement in contrast is seen by stopping down from f/4 to f/5.6.
For critically-sharp images (excluding depth of field requirements), f/5.6 should be the aperture of choice at closer distances, and f/8 near infinity (Nikon D3). On higher resolution cameras such as the 21.1MP Canon EOS 1Ds Mark III, a small but observable drop in image contrast begins by f/8, due to diffraction.
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