I had not realized that Canon does their own spec publishing. Good discussions. I know know what Sagittal and Meridional lines are the difference between contrast and resolution. But the quick summary is that:
- Canon publishes lense specifications and MTF for its lenses on its website
- It measures both the lense at the center and at the corners. It isn’t quite an Imatest surface plot, but gives some indication as it goes from the center to the edge of the lense. It shows the fall off of lense quality, the the Y-axis is the MTF where 1.0 means perfect and most folks think 0.5 is the edge of usable
- Contrast means how well the edges resolve. They test this with contrast with a target which is 10 lines per millimeter (10lpm or in a 24x36mm full frame that is 240 lines per height or 120 line pairs per height (lph)) and resolution which is how fine detail can you see and resolution with 30l/mm or 720 lines per height (in the realm of high resolution, that isn’t so much as a 10MP sensor has a theoretic resolution of more like 2100 lph) . They are not the same, so a lense with poor contrast but good resolution would show detail but it would look muddy. These two factors tradeoff, so you can have one or the other but not both. Interestingly, many folks perceive good contrast as being more important to apparent sharpness than good resolution.
- Finally lense performance also varies by aperture, with being wide open being much more difficult than F/8 (which is what they measure), so they show both
- It also varies by how you orient the target. Most lense do very well with lenses that are parallel to the diagonal line drawn from the center to the corner of the lense. These parallel lines are call Sagittal. They are like spokes of a wheel radiating from the center of the lense. Lines that are perpendicular to these lines (they are at a 90 degree angle to the Sagittal spokes) are called Meridional and have lower MTF typicaly.
In summary, here is the legend for those Canon curves, but the important limitrations are they don’t tell you the aperature where the MTF is at it maximum (important in real shooting as you want to know the sweet spot of aperture), it doesn’t tell you about other distortions like chromatic aberrations, comatic, vignetting, zooms are only tested at their widest and longest so it won’t tell you the sweet spot for focal length.
Black curve lines: lens performance at maximum, widest aperture. Blue curve lines: MTF curves with lens stopped-down to f/8. Thick lines: MTF data showing contrast — measured with thicker line sets on test target. Thin lines: MTF resolution data — tests measure performance rendering thinner lines on test target. Dashed lines: Lens’s MTF performance with harder-to-reproduce Meridional line sets. Solid lines: MTF performance with Sagittal lines, arranged parallel to a diagonal from image center to corner.