Canon L Lenses - MTF Graphs (very technical)

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I'm looking for a simple explanation of what a saggital (S) target is as well as a meridional (M) target.

Canon uses these targets to create their MTF Graphs for all of the L lenses (found in the L Series brochure). The MTF Graphs illustrate the transfer function of each lens with respect to contrast as a function of spatial frequency for 10 lines/mm and 30 lines/mm with wide open apertures as well as for f/8.

After reading and re-reading Page 12 of their brochure, I still don't have a complete understanding of what exactly is the "target".

(I have a BS in Electrical Engineering and a desire to understand optics). Thx!

Sue

-- Susan Butler (sbutler2@ameritech.net), April 15, 2001

Answers

Sagital lines run towards the center of the frame (also called radial lines). Meridional lines run at 90 degrees to that (also called tangential lines). If you shot a picture of a spoked wheel, the MTF for the spokes would be sagital, for the rim meridional.

When sagital and meridional MTF is different, you have astigmatism.

BTW the MTF plots are theoretical, not measured on real lenses, and appear to ignore the effects of diffraction and flare. Don't put too much faith in them as predictors of real world lens performance.

-- Bob Atkins (bobatkins@hotmail.com), April 15, 2001.


You can see some actual MTF tests at:

Photodo

They also have some information on MTF tests.

-- Marc Bergman (mbergma2@ix.netcom.com), April 16, 2001.


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