Modulation Transfer Function.....yikes, how confusing. So, I have produced a couple of images which could be helpful in the clearing up the confusion of exactly what these mean. My understanding, the charts measure the resolution and contrast by looking at thin (30 lines per mm) and thick (10 lines per mm) in orientations which are lined up with a radius from the center of the lens outward (sagittal) and lines which are tangential to a circle of coverage, i.e, at 90° to the sagittal lines (meridional).
The measurements are taken at varying distances from the center of the image, and distance from the center vs. resolution of the images on the chart used. The higher the resolution, the higher on the vertical axis the point shows up. This is highly simplified as the mathematics are complex.
In general, the thick (10 lines/mm) are a measurement of contrast and the thin (30 lines/mm) are more a measure of sharpness.
One caveat is to avoid comparing MTF charts for lenses which vary widely in focal length as this has no meaning. The usefulness is looking at perhaps wide angle lenses or telephoto, but not to attempt to compare the two.
How do we remember all this. Unfortunately MTF charts vary in how they show the data. Here are two examples which I have drawn, hopefully to help anyone who has an interest in this.
A different style MTF Chart:
Please...if there are errors or omissions in this post, help me out and correct me.
Here is an MTF chart from Nikon to see as an example: