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Forum: Pentax SLR Lens Discussion
11-03-2015, 06:52 AM
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Saturation point on this topic for me.
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Forum: Pentax SLR Lens Discussion
11-02-2015, 02:32 PM
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Lenses sharp and with little CA @ f2.8~4 are generally those that have a maximum aperture one or two stops wider (i.e. F1.4~2). Few lenses perform best at their maximum aperture*, so any f2.8 will not be best @ f2.8, nor an f4 @ f4.
* FYI: A perfect lens, in theory, preforms best at its maximum aperture and will show IQ degradation from diffraction even one stop down. Some APO enlarging lenses are near perfect, but** only at specific, rather short lens-to-subject distances so they are not really suitable for general photography. Some macro lenses will reach best IQ only one stop down. In the era of view cameras, some lenses for 4X5 and larger formats were specifically designed to operate best @ f64 (usually called "process lenses"), but such a lens design is impractical and would not give equivalent IQ for small-format cameras.
**The "but" here I should clarify. Enlarging lenses only have their best IQ at particular, short lens-to-subject distances. All lenses have an optimum L-2-S distance for best IQ, otherwise any random lens would be suitable for macro.
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Forum: Pentax SLR Lens Discussion
11-02-2015, 01:40 PM
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Fairly consistently maximum sharpness is at f5.6 to 8 on a huge variety of SFL and zoom lenses. CA and aberrations are also commonly best around f5.6~8, but not always.
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