Zeiss has a pretty good paper on how to read MTF charts. It is rather detailed and extensive, but if you are interested in fully understanding how an MTF represents a lenses quality (and how accurate the MTF may be), it is an excellent read.
How to Read MTF Curves
A very interesting little facet of the article denotes three important properties of MTF, which lead to an intriguing conclusion about the maximum contrast range of a camera lens. I found this to be interesting not only in the context of understanding lens MTF, but also as an important factor in the camera film/sensor dynamic range argument.
We can then recognize three important
properties of MTF here which we should
remember when reading MTF curves:
- Small differences in higher MTF values are particularly significant at
high object contrast levels.
- On the other hand, weak tonal value variations of less than one
aperture stop do not require high MTF
values. Differences above 70-80% are
then hardly relevant.
- With very low MTF values, it practically does not matter how high
the object contrast is; the image
contrast is always low.
Incidentally, this is why the
datasheets of films always also gave
the resolving power for the low
contrast of 1:1.6. The resolution
figures for the contrast of 1:1000 can
only be measured using contact
exposure. For the finest structures
(i.e. very high spatial frequencies),
no lens in the world is capable of
producing a contrast of ten aperture
stops. Estimating the amount of
information of film images based on
this higher resolution value is thus
Another excellent resource for how to read MTF charts can be found in the last parts of Canon's book "Lens Works". The part "Optical Terminology and MTF Characteristics" provides an EXCELLENT overview of lens types and capabilities, and provides a wonderful, visual review of MTF charts for many Canon lenses.