Re: F/0.5 possible?

2 wrote:
"Don Stauffer" <stauffer@xxxxxxxxxxxx> wrote:


ASIDE - to perk interest - potential resolution goes DOWN as the aperture decreases (in physical size). So an f/0.5 has more potential resolution than the same stopped to, say, F5.6, but in Real Life lenses are a compromise and usually perform better stopped to the mid-aperture range.

--
2 in Minnesota



Not quite. This is only true for a true "diffraction limited" lens. I guarantee any lens faster than about f/2.8 is NOT a full diffraction limited lens. There is always a sweet spot with any lens. At that aperture the resolution is max. Any larger aperture, geometric aberrations build up and control resolution. Below that aperture (above that f/#) geometric aberrations are less than diffraction, and diffraction rules. At the optimum f/# the geometric blur circle and diffraction blur circle are equal.

Further complicating the problem is the fact that the blur circle due to diffraction is a different shape than that due to most geometric aberrations. The shape of most diffraction blurs gives indeed a blur look.

Diffraction blur "circle" on the other hand has some energy in central blur spike, but much scattered around at large distance from blur centroid. The result is that diffraction gives more of a "soft focus" effect than a common blur or unsharp edge.

This soft focus effect is somewhat pleasing in portrait work, and thus many portrait photographers shoot at higher f/#s than in other types of photography.

BTW, so called soft focus screens work because the screen makes many small apertures. One old standby for a soft focus is to hold a screen of a piece of a nylon stocking over the lens.

Also, REALLY REALLY fast lenses approaching f/0.5 have less diffraction than their diameter should indicate because transmittance falls off drastically for rays near outer limit of aperture, giving an "apodized aperture", which reduces effect of diffraction. However, the geometric aberrations really dominate in a big way for those kinds of aperture ratios.
.

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