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FotoTV.Classic: Modulation Transfer Function

MTF Charts Explained by Ralph Lambrecht

In this film, photographer Ralph Lambrecht explains Modulation Transfer Function, MTF, in the simplest possible terms, supported with numerous visual graphs and examples. He also gives us some rules of thumb for reading charts and graphs.

The modulation transfer function is, as the name suggests, a measure of the transfer of modulation (or contrast) from the subject to the image. In other words, it measures how faithfully the lens reproduces (or transfers) detail from the object to the image produced by the lens.

MTF is the spatial frequency response of an imaging system or a component; it is the contrast at a given spatial frequency relative to low frequencies. The essential meaning of MTF is rather simple. Suppose you have a pattern consisting of a pure tone (a sine wave). At frequencies where the MTF of an imaging system or a component (film, lens, etc.) is 100%, the pattern is not attenuated— it retains full contrast, and at the frequency where MTF is 50%, the contrast half its original value, and so on. MTF is usually normalized to 100% at very low frequencies. But it can go above 100% with very interesting results.

MTF can also be defined as is the magnitude of the Fourier transform of the point or line spread function— the response of an imaging system to an infinitesimal point or line of light. This definition is technically accurate and equivalent to the sine pattern contrast definition, but can't be visualized as easily unless you're an engineer or physicist. The modulation transfer function is useful for characterizing not only traditional optical systems, but also photonic systems such as analogue and digital video cameras, image intensifiers, and film scanners, telescopes and MTF even applies to the human eye.

MTF is the most widely used scientific method of describing lens performance. An ideal lens would perfectly transmit 100% of the light that passes though it. But, no lens is perfect, and therefore there are losses. When these losses are measured in terms of contrast this is called the modulation of contrast. In other words, how much contrast is lost — modulation simply being another word for variance. That being said, a photograph's detail is an integral part of its appeal. Many photographers spend a great deal of time, energy and money acquiring equipment to make sharp images, therefore Lambrecht suggests when purchasing new lenses, comparing MTF graphs can be very helpful.

 

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