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Focus Shift and Spherical Aberration
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, but it an also affect f/2, f/2.8, or even f/4 lenses, sometimes in extreme ways.
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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Diglloyd DAP is DSLR-oriented, but also contains workflow and other topics. Much of the focus is on Canon and Nikon but also Pentax and Pentax medium format.
Special emphasis is placed on lens evaluation, focusing on Canon and Nikon and Sigma lenses, but with a few others like Rokinon/Samyang.
- Make better images by learning how to get the best results right away.
- Save money by choosing the right lens for your needs the first time, particularly some of the new Sigma Art lenses vs Nikon and Canon.
- Workflow discusses image organization, raw conversion and post processing. Many examples show processing parameters for direct insight into how the image was converted.
- Jaw-dropping image quality found nowhere else utilizing Retina-grade images up to full camera resolution, plus large crops [past 2 years or so].
- Real world examples with insights found nowhere else. Make sharper images just by understanding lens behavior you won’t read about elsewhere.
- Aperture series from wide open through stopped down, showing the full range of lens performance and bokeh.
- Optical quality analysis of field curvature, focus shift, sharpness, flare, distortion, and performance in the field.
Want a preview? Click on any page below to see an excerpt as well as extensive blog coverage, for example on Nikon or on Canon or on Pentax.
