The optical transfer function (OTF) of an imaging system (camera, video system, microscope etc.) is the true measure of resolution (image sharpness) that the system is capable of. The common practice of defining resolution in terms of pixel count is not meaningful, as it is the overall OTF of the complete system, including lens and anti-aliasing filter as well as other factors, that defines true performance. The optical transfer function is roughly the equivalent of phase and frequency response in an audio system, and can be represented by a 3D graph of light amplitude (brightness, i.e. intensity) versus phase and spatial frequency (cycles per picture width).
In the most common applications (cameras and video systems) it is the modulation transfer function (MTF), the magnitude component of the OTF, that is most relevant, although the phase component can have a secondary effect. While optical resolution, as commonly used with reference to camera systems, describes only the number of pixels in an image, and hence the potential to show fine detail, the transfer function describes the ability of adjacent pixels to change from black to white in response to patterns of varying spatial frequency, and hence the actual capability to show fine detail, whether with full or reduced contrast. An image reproduced with an optical transfer function that 'rolls off' at high spatial frequencies will appear 'blurred' in everyday language.
Read more about Optical Transfer Function: Example, Factors Affecting MTF in Typical Camera Systems, Oversampling and Downconversion To Maintain MTF, Trend To Large-format DSLRs and Improved MTF Potential, Measuring Modulation Transfer Function, More Advanced Details
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