Sagittal and Tangential lines in MTF tests

[Orio]

I have studied how to read the MTF tests and I have to say that, much to my surprise, I managed to understand almost everything, and now I am able to actually understand in full those things!

One residual doubt I have is this: what are exactly the sagittal and tangential lines? I understand that they are light rays that come from different directions, and that the more the respective curves diverge, the higher is the lens astigmatism.

But if I had to explain to someone else what are exactly the vectors of those sagittal and tangential lines, I mean their direction, I would not be able to.

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[Flor27]

http://www.luminous-landscape.com/tutorials/understanding-series/understanding-mtf.shtml

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To quote from Canon's explanation of what meridonial and sagital mean....

Fine repeating line sets are created parallel to a diagonal line running from corner to corner of the 35mm frame, directly through the exact center of the image area. These are called sagital lines, sometimes designated “S” on Canon’s MTF charts. At a 90° angle to these, additional sets of repeating lines are drawn, called Meridonial (or “M”) line sets.

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[patrickh]

If they aint square they is bent and that is distortion?


patrickh

[ChrisLilley]

[Orio]

So they are basically two sets of opposed diagonals?
Then I got it wrong in my previous understanding, I thought they could be light rays either perpendicular or lateral to the front glass
OK so they're not light rays - however the utility of having two sets of diagonals, one basically the specular of the other, escapes my understanding.

[LucisPictor]

Orio, this is a very good question and one that I also need to ask.
At the moment I would understand it the way you describe in your second post.

[ChrisLilley]