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- A varifocal lens changes its focus when the focal length is changed and so must be refocused.
- A parfocal zoom lense maintains focus when the focal length is changed.
True parfocal zoom lenses are relatively uncommon. High-end cine lenses are often true parfocal zooms out of functional necessity, but many if not nearly all “zoom” lenses are actually varifocal lenses.
The difference makes a difference: accurate focus with a long telephoto lens is sometimes a tricky business. When a small change in focal length is made (say to frame a subject better), a varifocal lens demands that the lens be refocused.
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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.
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- 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.