I'm confused. I'm not up to reading MTF charts right now, but I was agreeing with you in my last post. Or I thought I was.
I think we are agreeing Strolls

The 'opposite' bit was that you had heard the DO was better, even though I think you were sceptical of this. I just never heard anybody ever say it was better, rather the opposite, and quite a few people then went on to prove it by pointing it into the sun and saying, look, it's got some flare and if you look very carefully you can see the DO rings. Well, it's not hard to devise a test that will show weakness in any lens, and my other point was that when I used it on a crop camera I got some very impressive results. I sold it purely for practical reasons.
MTF is Modulation Transfer Function - that is, how efficiently the image target (modulation in lines per mm) is transferred to the sensor. It looks complicated, but it is basically very simple. MTF is a true test of
sharpness which is a combination of 'resolution' (the fineness of detail) and 'contrast' (how clearly those details are shown). In optics, when resolution goes up, contrast goes down, and those are the two key characteristics that are measured. Perhaps surprisingly, contrast is generally more important in that if you look at a low resolution but high contrast image, it will look sharper than one with much higher resolution, but lower contrast.
Basically, you decide the level of resolution that suits your purpose, and then go for the lens with the highest percentage contrast at that figure. In a simple MTF graph (such as those published by Canon, Nikon and Sigma) they take 10 lines per mm as basic resolution which is fine for small prints and on-screen images (don't confuse lpmm here with dpi or ppi) and then 30 lines per mm which is much more demanding. That will give you a very crisp A4 print.
With crop format digital, it would be handy to take that figure up to 50 lines per mm to compensate for the greater degreee of enlargement you need with crop cameras, but fortunately a lens that does well at 30 lpmm will generally show very similar characteristics at 50 lpmm, but at a correspondingly reduced contrast level.
If you want a crude-ish short cut to reading MTF, a really good lens will usually show its class with a high % MTF at the lowest f/number, at the edges of the frame (roughly 10mm out for crop, or 15mm out for full frame) at 30 lpmm resolution. 90% (or 0.9) is obviously excellent, 80% is very good, 70% is good, and 60% is acceptable. When MTF dips below 50%, it's getting mushy. This is quite a tough test, as you're looking at the edges where it could be argued critical sharpness doesn't matter so much, but this is what sorts the men from the boys and any lens that does well in this area is going to be extremely good indeed in the centre. This is really where primes tend to score, with really strong edge sharpness compared to zooms while there's not much in it at the centre.
Not all graphs present MTF in the same way, some tests are conducted differently, and some are not worth the time of day. But those Canon tests are pure MTF of the lens alone (no camera involved). Well actually, I believe they are computer simulations generated from the lens design only, so they represent a 100% perfect example of that lens, but a good production sample should be able to replicate it.
Sorry, got a bit carried away there
