Aperture question, re: sharpness

Started Feb 26, 2014 | Questions thread
Jack Hass Regular Member • Posts: 392
Re: Aperture question, re: sharpness

LMNCT wrote:

The sharpest aperture on a lens is most likely 2 or 3 stops down from the maximum. Or, on a lens with an maximum aperture of f 1.4, that might mean that f 2 is actually sharper. Some lenses are made to be used wide open and will exhibit their best sharpness at that point. Closing any lens down as far as possible will not deliver best quality.

Not exactly. This all depends on what the sensor format is. For MFT, most will suggest anywhere between F2.8 - F4 will be the sharpest, since this isn't extremely fast yet is likely at the early stages of diffraction. Faster lenses have more glass, so a F1.4 will have more of a FF like profile to the elements, ie more flaws as you move outward from the center. Faster glass is bigger glass, which is harder to make as clean as smaller glass. This is why the center is better than the edges, usually.

This all means that the smaller the aperture the cleaner the glass for any particular lens, but there is a sweet spot between the weak wide open and weak diffracted. This is where i got the F2.8 - F4. Since diffraction affects FF less for example (or it affects FF at later apertures), the sweet spot will likely be at smaller apertures, something like F5.6 - F8. The rule of thumb is, the sharpest stop on the lens is the smallest aperture before diffraction begins to override the benefit of stopping down.

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