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Karl Lee

Determining the "Sweet Spot" of a Lens in terms of Aperture and Sharpness

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I've read that in many cases, cine lenses are considered to be at their "sharpest" at a certain aperture or within a particular aperture range. Is there a particular aperture or aperture range at which most lenses tend to perform the best in terms of sharpness, or does it really vary from one lens to another and depend on a number of different factors (focal length, prime or zoom, etc.)?

 

I, for example, have a Canon 11.5 -138 S16 zoom, so I'd be curious if there's a particular "sweet spot" in terms of a specific aperture or aperture range.

 

Thanks!

 

 

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Generally the sweet spot is 2-stops closed down from the widest aperture on the lens (so for a f/2.8 zoom, that would be f/5.6.)

 

But in reality a zoom can vary in sharpness throughout its range and the best thing would be to look at the lens on a lens projector.

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Yes 2 to 2 1/2 stops down from wide open is a good general guideline, most lenses will be at their best around there but not yet have reached a point where diffraction begins to have an effect. It depends on the lens of course, a very well corrected lens may peak only one stop down, a fast lens with coma and astigmatism may need to be stopped down 3 or more stops before the aberrations are minimised. Often the centre improves before the corners do, so it can be a question of degrees, and as David mentioned, the performance of a zoom can vary with the focal length.

 

The sweet spot is really a balancing act between the aberrations present when a lens is wide open, and the gradual loss of sharpness caused by diffraction as the aperture gets smaller. For 16mm format, where a circle of confusion figure of 0.015mm might be desired, stopping down past f/11 will create an Airy Disk (the blur circle of a point source) too large to contain within that CoC, so for critical applications f/11 is about where the effect of diffraction begins to become noticeable.

 

We have an Angenieux 11.5-138 in our rental fleet. At the wide end it conforms to the 2 stops down guideline, at around a T4/5.6 split the aberrations have mostly receded. Towards the long end it probably needs 3 stops, so maybe T5.6/8.

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You mean rear-mounted focal extenders?

 

Well, they tend to amplify the aberrations in the lens they are attached to (at least the central portion being magnified), plus add some of their own, so for best results you'd want to stop down at least the 2 stops required for the original lens, and maybe more. Some extenders are better than others.

 

I'm reasonably sure the effects of diffraction are determined by the new effective f stop of the lens/extender combination ie with a 2x converter f/5.6 on the iris scale is now f/11, which means there may be only a very small window where the aberrations are reduced before diffraction begins to kick in.

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