Diffraction Limit Discussion Continuation

Started Feb 21, 2014 | Discussions thread
Jonny Boyd
Forum MemberPosts: 89
Re: Duck, duck, goose.
In reply to Ulric, Feb 26, 2014

Ulric wrote:

Jonny Boyd wrote:

Ulric wrote:So your single-sentence definition is something like "Resolving power decreases visibly when the lens is stopped down one stop"?


'The diffraction limit for a system occurs at the aperture at which any further reduction in settable aperture size will cause a visible decrease in image resolution.'

'for a system' clarifies that the sensor resolution and viewing method are factors.

'settable aperture size' allows for the fact that different lenses have different limitations on how precisely the aperture is controlled.

So in your opinion, a system where the lens has half-stop aperture settings has a different diffraction limit than an otherwise identical one where the lens has full-stop settings? Many lenses (especially for video) have no stops, then what?

Well fairly obviously if the diffraction limit was at an intermediate f-stop (say f/6.3) then a lens which can't stop down to that exact aperture will in practice be limited at the next aperture instead. In reality f/4 on one lens may not be exactly the same as f/4 on another anyway. They might actually be f/3.998476265 and f/4.017465, but the manufacturers round the numbers to the closest stop. This is just an extension of that – an issue of precision. If you could precisely set a lens to any aperture setting, then it wouldn't be necessary to add the caveat.

I'm not sure what the mechanism is for stopless aperture lenses like cine lenses, but again there's likely to be a degree of granularity in how they're stopped down, so the mathematically derived limit may not in practice to be able to set beyond a certain level of precision. That's just the nature of working with physical systems rather than mathematical models.

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