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| p.3 #15 · Canon EF 24-70mm f/4L IS MTF Chart comparison |
I see now that you just used the key that you referenced and I just reasoned which lines were which based on first principles and used the graphs on the first page of this thread. I am used to looking at actual measured MTF versus theoretical MTF, however, and that threw off some of my judgments. Basically if you measured the MTFs for f/2.8 or f/4 zooms then sagittal/tangential pairs should be in this order. The top pair should be f/8 10mm, then wide open 10mm, then f/8 30mm, then wide open 30mm. If the key you provided is correct, which I assume it is, then the theoretical graphs suggest that the f/8 30mm line is higher than the wide open 10mm line, which would be implausible with measured MTF.
There is another important aspect to the MTF graphs as well. Normally you would expect sagittal/tangential pairs to be close to each other and follow a fairly similar curve across the graph although these two sets of lines would typically diverge toward the right of the graph (i.e., in what would be the corners). Note this is the way the lines look in your key. This is an important aspect of MTFs because the extent that these lines diverge, the lens is likely to show lateral chromatic aberrations. So based on first principles, I was assuming that blue and black in the graph was indicating sagittal vs. tangential rather than f/8 vs wide open. Now that you have provided the correct key, what is particularly striking is how separated the sagittal and tangential lines are. None of the lenses are great, but the two new lenses are notably better and should have less lateral CA.
One last point is about thinking about the corners. It is probably not best to think of the far right of the graph as the corners. The extreme right is actually only the last pixel or so in the corners. Corner performance is probably best understood as from about 18mm to 21.6mm from the centre on the x-axis. With this in mind we see that the new f/2.8 is clearly the best at 24mm, but the new f/4 lens falls about half way between the new f/2.8 and the old f/4 lens.
I am sorry, I didn't look at the key and therefore misread some of the numbers, but I hope this discussion is helpful. In general, it would be much better if Canon would provide real measured MTF graphs as these theoretical graphs differ significantly from what they would actually be in practice.
S Dilworth wrote:
Steve, looking at the corners alone is insufficient for a complete picture, but people are widely concerned with corner performance at the wide-angle setting, presumably because itís often poor in this type of zoom lens.
I hear fewer complaints about the long end, since it's usually better, and the corners are often out of focus at longer focal lengths (in portraits, etc.).
In any case, the Canon MTF charts have too much information to absorb at once, so we have to concentrate on one aspect or another of performance to take in anything at all.
Back to the corner numbers, then. Iíd...Show more →