How do I interpret an MTF Chart?

Question

As part of how Canon (and other lens makers) give technical information about their lenses, they supply an MTF (Modulation Transfer Function) chart. How do I read and interpret what the chart is telling me?

Here is a sample MTF Chart for the 16-35 f2.8 L II (one of my favorite lenses for walkabout photography). What do the various lines mean? What are the axes?

Answer 1

There is a really good tutorial that explains all the details at luminous-landscape.com.

If you don't want to read the whole article, this section covers the basics:

Here are some rules of thumb for reading a chart...

— the higher up the chart the 10 LP/mm line is (the thick lines), the higher the contrast reproduction capability of the lens will be.

— the higher up the chart the 30 LP/mm line is (the thin lines), the higher the resolving power and thus subjective sharpness of the lens will be.

— keep in mind that the black lines show the lens wide open while the blue lines show the lens stopped down to f/8, so the closer these sets of lines are to each other the better the performance of the lens when used wide open. The very best lenses will have the black and the blue lines close together.

— generally speaking a lens whose thick lines (10 LP/mm) are above .8 on the chart should be regarded as having excellent image quality. Above .6 is regarded as "satisfactory". Below .6 is, well, below.

Answer 2

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

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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:

1. Small differences in higher MTF values are particularly significant at high object contrast levels.
2. 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.
3. 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 too optimistic.

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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.