MTF Charts - What am I looking at?

Hello All,

I've been looking around at new lenses, and noticed MTF charts, but don't know how to read them.

Can someone explain how to read the charts?

Thanks,

David

P.S. *Edit* I'm re-learning the trade from when I started back in 1991, so I had forgotton some points since I last shot SLR back in 1998. **Edited because some people like to bring posts like this back up to make one look like a fool***

Essentially the higher the line the better the lens, but of course it's not quite that simple. MTF (modulation transfer function) graphs usually have two sets of four lines for a given focal length. These two sets are for two apertures, one wide open, where the lens is probably at its poorest, and the other usually f/8, where the lens is probably at its best. Canon usually shows maximum aperture lines in black and f/8 lines in blue. Then, they use thick lines to show the MTF for a "spatial frequency" of 10 lines per millimetre, which is fairly coarse, and thin lines for a spatial frequency of 30 lines per mm, which is quite fine. This shows how well the lens can reproduce details at these two levels of structure. Then solid lines are used for subjects whose structure varies "sagitally", or in the direction radially from the centre of the image and dashed lines for structure perpendicular to these directions, so-called meridional structure. As you can see it gets a bit difficult to describe in words.

The lines are drawn left to right, where the left edge of the graph represent the cente of the image, where most lenses perform best of all, and moves to the right to represent increasing distance from the centre. Note that if you are using an APS-C format camera, you will not be interested in the MTF lines past about 15mm, as that is the extreme corner of the sensor. If you are using a full-frame camera, the corner is at 22mm.

Lines that drop sharply to the right signify a lens that is much sharper in the centre than it is at the corners. If not even the thick blue line gets above 0.9, the lens is relatively soft. The thin black lines will normally be the lowest ones and the thick blue the highest.

It is useful to think of the thick lines to be a measure of contrast and the thin lines to be a measure of resolving power and sharpness. If the continuous and dotted lines are close to each other (for a given thickness and colour) then the background blur looks better.

Does that help?

Perhaps I should have added that a zero would represent a lens that does not respond at all to the input structure and 1.0 is a perfect, unattainable score.

^^ Very good explanation, thanks for writing that out.

Higher flatter lines are better as a really simplified simplification. Mark did a good job with his explanation.

Here is a tutorial on the topic
http://www.luminous-landscape.com …s/understanding​-mtf.shtml