I also like looking at graphs. What the charts that get published by manufacturers mean, I have no idea. Not only that, most manufacturers charts are derived from the design specs, not from actual shooting. Actual shooting brings sample variation into play. Whether that's here or there is another whole debate.
The one thing I'd mention....
The better the lens, the lower the ƒ-stop at which it achieves maximum resolution.
Looking at the DFA 50 1.4
you can see it's sharpest from ƒ2 to ƒ4. That's a hallmark of an exceptional lens.
Most consumer lenses are sharpest at ƒ5.6. But ƒ8 is still quite good if you need more depth of field. Less so on APS-c than on FF, the the diffraction limit is between ƒ5.6 and ƒ8 for both but the larger the format the lower the diffraction limit, which of course ends with Ansel Adams shooting ƒ64 of 8x10 film. So you might want to be a little quicker to use ƒ8 on your K-1 than you were on your APS-c sensor.
To me the whole argument for the DFA 50 1.4 is sharp edge to edge at wide apertures. Because landscapes need wide depth of filed, I'm not sure that even matters to me.
DA* 55 chart
For me, sharp edge to edge from ƒ2 to ƒ4 but falling off on the edges from 5.6 on would be of limited use. AT 5.6 to ƒ8. the DA could be the better lens, if you're looking edge to edge. But not necessarily. You still have to do some image comparisons to be sure.
I use the graphs to tell me what I should be looking for when I test.
Why would I need further testing? From the graphs you know what the numbers are, but you don't know how that correlates to real world images. I don't think any of us are so smart we know how images will compare based on a bunch of numbers. IN the above examples I know the DA* is more even edge to edge at ƒ8, but because they are on different formats, I really know nothing. I don't know if the DA*s edges fall off considerably after the APS-c circle. Where the DFA 50 looks weaker on the edge, for all I know it's still stronger on the edge than the DA* is in the middle.
The first thing I'm looking at comparing these two would be how many images do I take that would take advantage of the DFA's strength. Not including wildlife, of my 900 keepers this year, 15 would qualify as f2-ƒ4 images. For me the DFA would be a waste of money. It would be rare I could actually see a difference. So what I would be looking at on the charts would be ƒ5.6 and ƒ8, and those are the places I would look at comparison images.
To me the charts are a good way to narrow down your search and focus on those situations you are actually buying the lens for.
Looking at the DA 20-40 chart, its a traditional Pentax design. Soft edges where the depth of field is narrow, meaning large areas of the image will be out of focus and soft edges won't be as critical. Good edge to edge from ƒ5.6 to ƒ11 for landscape. A very functional lens for the way I shoot.
The DA 20-40 is definitely in the spirit of the FA 31 ltd
When I see a lens like the DFA* 50 1.4 I wonder "who shoots like that?" Who shoots from ƒ2 to ƒ4 and needs edge to edge sharpness? Do people shoot walls in dim light (and why?)
But everyone is not me. Having better edge to edge would effect 1.6% of my images. But I can imagine that other people might shoot more in that range. That's what you should be buying a DFA 50 for. If not, and you're like me, you're buying capacity that you won't use.
Of course people can buy lenses just because they can. No other reason is needed. I do wonder if anyone actually makes use of ƒ2-ƒ4 on their DFA 50 1.4, or if it's just a prestige item. Personally I look at the DFA and DA* images and decided the DFA* wouldn't help many of my images.
And I wonder, who are the folks who benefit from the DFA and what are they shooting that makes use of it's qualities.
And I seriously like graphs , so I can quickly look for numerical differences that highlight what exactly I should look at when comparing the images taken by the lenses. But you still have to compare images to see exactly what the numbers mean in terms of how each lens design was executed. I look at the graphs as the key to intelligent evaluation, letting you focus your attention at lens differences, and not going through the areas where they are pretty much the same. Every tool used to filter information helps.
Especially for those of us with short attention spans.
We have to get to the important part quickly or we might forget what we're doing.