Being that previous poster, I did state that the charts had one useful purpose, and that is for comparing lenses within Canon's range _as long as they are of the same focal length and speed_!> -----Original Message-----From: [EMAIL PROTECTED] [mailto:owner-eos@;a1.nl]On Behalf Of Bill Jameson Sent: Monday, October 21, 2002 10:26 AM To: [EMAIL PROTECTED] Subject: Re: EOS I too can understand MTF data- almostHenning Wulff <[EMAIL PROTECTED]> wrote (in part): ... > The tests are usually done at various spatial frequencies, such as 10 > line pairs per mm, and 30lp/mm. As Canon doesn't tell use which they > use, these charts become a lot less useful. The performance at > 10lp/mm is usually much more important than that at 30 or 40lp/mm, > and testing at 5lp/mm and 15lp/mm would of course give a lot higher > values than testing at 15lp/mm and 40lp/mm. Therefore the _most_ one > can do is assume that Canon tested all lenses at the same > frequencies, in exactly the same fashion, and then one could, with a > fair bit of experience, compare lenses of the same apertures and > focal lengths. > > In this case, the graphs are almost useless. ... Hopefully someone will correct me if I am wrong, but Canon did give the frequencies for the spatial pairs where the MTF charts were originally published: Lens Work II. Problem is that the charts have been reproduced on the web without the rather lengthy explanations, including the fequencies, that were published in the back of Lens Work II. Bill JamesonHi Bill, Contrary to the previous poster's comments, the charts are for exactly the purpose of comparing relative lens performance within Canon's lens line. Nope you are mostly right, the charts show data for each lens for 10 lp/mm and 30 lp/mm both wide open and at f/8. The charts as shown on the Canon web site are fully explained just as they are in Lens Work II, just click on the "how to read mtf chart" at the bottom of each chart. Cheers/Chip
Also, if the spatial frequencies are mentioned on Canon's web pages, I was not able to find them. I missed any reference to the Lens Work II. The 'how to read mtf chart' link doesn't tell you much. 10 and 30lp/mm seem possible values, but if you are going to use these charts in any kind of meaningful fashion, it would be nice if the parameters were stated instead of inferences having to be made from some of Canon's other promotional literature.
As presented, they are not useful except in a very limited way. It's nice that they provide some information, but they should have a look at Zeiss or Schneider info that is provided on the web. I just had a look at some of the Schneider info on the SA-XL lenses, and besides giving a full range of MTF curves, with spectral weighting etc, it also provides information on transmittance vs. frequency, distortion at various magnifications and illumination falloff. Altogether another ballpark as far as information is concerned.
_Still_ you cannot compare lenses of different manufacturers' with this data readily, as the test conditions are likely to be different.
Again, the charts as presented by Canon on the website are almost useless, and only the most obvious inferences can be drawn from them. But maybe that's all they were intended for. That's not necessarily a bad thing for Canon to do, but it would be helpful for people unfamiliar with MTF charts to be informed of this on the website.
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* Henning J. Wulff
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