Roy Pertchik
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I've updated my matrix posted above to include more non-L's for comparison. To read and MTF chart, In a nut shell, goes like this:
The left side is the center of the frame, the right side is the corner of the frame. Up is good, down is less good. The blue lines represent performance at f8. The thick blue line is how well the lens can "see" a target of 10 line pairs per mm, and is a measurement of contrast. The thin blue line represents the same measurement using a 30 lpm target and is a measurement of resolving power. What we perceive as "sharpness" is a combination of these two factors. The dashed versions of the lines are measurements repeated with the target lines ortiented radially, (solid means the lines are perpendicular to that, or "tangential".) In all of these charts, the f8 performance is very similar: the thick solid blue line is plastered to the top, the thin blue line is a little lower, but basically straight across, which means great contrast and great resolution. The right ends dip down a bit, representing corner performance. The radial line tests (dashed blue) are similar but curve down sooner, and the wider the focal length lens, the more severly these begin to dip towards the edge of the frame. But basically, the blue lines do not really set an L lens apart from a non-L counterpart... at f8, they are all great.
The black lines represent wide open performance. Again, thick is 10 lpm, contrast, and thin is 30 lpm, resolution. Dashed is radial lines, solid is tangential. In every case, the wide open performance is noticably worse than the f8 performance. Also, it' s hard to compare the 85 1.2 to the 85 1.8, for example because, "wide open" is different for these lenses. Basically, the 1.8 wide open graph looks as good as the 1.2 wide open graph, but if you stopped the 1.2 down to 1.8, it's graph lines would move up, so you have to compenstae in your evaluation.
The truth is we know all of this form trying lenses and gabbing on line:
The longer the lens, the sharper, the wider the lens the less sharp.
Stopped down to f8 almost any lens is good, wide open degrades things a bit, more and more so for wider and wider lenses. Expensive L's open a little more, sometimes as much as a stop. The performance of a wide L wide open, is about as sharp as the non L wide open, but at equal aperturese nearly wide open, the L will win. That's all she wrote, re: sharpness. That's why I say the most outstanding thing I see in the sample images posted for the new 50 f1.2 is the creamy smoothe bokeh. Can't see that in the charts!
Here is a link to a Luminous Landscape description of MTF charts:
http://www.luminous-landscape.com/tutorials/understanding-series/understanding-mtf.shtml
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