Optical transfer function: Difference between revisions

Revision as of 12:08, 20 August 2019

Revision as of 12:10, 20 August 2019

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Another factor in digital cameras and camcorders is lens resolution. A lens may be said to 'resolve' 1920 horizontal lines, but this does not mean that it does so with full modulation from black to white. The 'Modulation Transfer Function' (just a term for the magnitude of the optical transfer function with phase ignored) gives the true measure of lens performance, and is represented by a graph of amplitude against spatial frequency.

Lens aperture diffraction also limits MTF. Whilst reducing the aperture of a lens usually reduces aberrations and hence improves the flatness of the MTF, there is an optimum aperture for any lens and image sensor size beyond which smaller apertures reduce resolution because of diffraction, which spreads light across the image sensor. This was hardly a problem in the days of plate cameras and even 35&nbsp;mm film, but has become an insurmountable limitation with the very small format sensors used in some digital cameras and especially video cameras. First generation HD consumer camcorders used 1/4-inch sensors, for which apertures smaller than about f4 begin to limit resolution. Even professional video cameras mostly use 2/3&nbsp;inch sensors, prohibiting the use of apertures around f16 that would have been considered normal for film formats. Certain cameras (such as the [[Pentax K10D]]) feature an "MTF autoexposure" mode, where the choice of aperture is optimized for maximum sharpness. Typically this means somewhere in the middle of the aperture range.<ref>{{cite web|url=http://www.b2bvideosource.com/mm5/merchant.mvc?Screen=CAMERA_TERMINOLOGY&Store_Code=BVS|title=B2BVideoSource.com: Camera Terminology|website=www.B2BVideoSource.com|access-date=2 January 2018}}</ref>

=== Trend to large-format DSLRs and improved MTF potential ===