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[OM] Small CCD Limitations

Subject: [OM] Small CCD Limitations
From: Winsor Crosby <wincros@xxxxxxxxxxxxx>
Date: Tue, 29 Oct 2002 10:08:04 -0800
I was looking at Norman Koren's site, http://www.normankoren.com/Tutorials/MTF7.html

in which he discusses digital v. film argument in some detail. (He says digital has just won the battle with the latest round of cameras) One thing that he points out which I had not considered before is diffractive resolution loss with physically small sensors, and lenses. We are used to the beginning of softening due to diffraction with 35mm lenses at F16. Since diffraction is a characteristic of the physical size of the aperture which becomes smaller for the same F stop in smaller formats it will be greatly increased in the small digicams. Expect softening in the new Oly to start about F8?

Small sensors run into problems with lens diffraction, which limits image resolution at small apertures-- starting around f/16 for the 35mm format (43.3 mm diagonal). At large apertures-- f/4 and above-- resolution is limited by aberrations. There is a resolution "sweet spot" between the two limits, typically between f/5.6 and f/11 for good 35mm lenses. The aperture at which a lens becomed diffraction-limited is proportional to the format size: a 22 mm diagonal sensor becomes diffraction-limited at f/8 and an 11 mm diagonal sensor becomes diffraction-limited at f/4-- the same aperture where it becomes aberration-limited. There is no "sweet spot;" the total image resolution at optimum aperture is considerably less than for larger formats. Of course cameras with small sensors can be made very compact, which is attractive to consumers.

Winsor Crosby
Long Beach, California

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