Many of you know that I'm reviewing lenses in my spare time (see e.g. my post in the Nikon, Sigma and Tamron section). I'm trying to give you an overall impression mostly of sharpness (and contrast, as both factors are deeply intertwined).
Now here's the 10,000$ question: How do you measure sharpness which depends heavily on quality of focus?
This in turn splits into several sub-questions:
- Should I use AF or MF?
- Should I repeat AF coming from different starting distances? Like if the subject is 2m away should I defocus the lens first to 1m and/or 5m and then let it find it's way?
- If using MF how can I be sure that I focussed correctly?
- Should I "bracket" focussing? How do you bracket? Certainly not by moving the focus-ring
Some people must already have answered all these questions! Yes they have: In a professional lab, the resolution-test-charts are shot with the camera (+lens) moved in small increments relative to the test-chart to make sure that you nail the focus at some position.
Now, what happens if the lens has field-curvature (that is, the plane of sharpest focus is not a plane but some curved area)? There are two principle possibilities:
(1) The lab analyses only the shot that is sharpest at its centre and measures the resolution of this single shot at the borders and corners. If the lens produces a flat field of sharpest focus this is ok. If the lens has field-curvature the borders and corners show a weaker resolution in this single shot than at some different distance.
(2) The lab takes the best (=sharpest) shot for each section of the image. In this case the sharpest corners may come from a different shot (=shooting distance) than the sharpest center. If the lens produces a flat field of sharpest focus the selected shot(s) will be the same as in (1). If the lens has field-curvature, the lab will measure better results at the borders and in the corners than with method #1 because it analyses different shots.
Now the question is: Do you shoot flat subjects or is it not really relevant for you whether the lens has field-curvature or not?
Well, I cannot answer this for you, but I can tell you, what answers you get from my reviews:
- I always
analyse a single shot of a flat subject
- But I select the best AF-shot from a bunch
- I always
- If the results from the different corners yield different resolutions I use the corner/border with the best results.
The consequences are:
- If the AF is consistently off, my results will not show the potential of the lens even if the lens could produce fantastic results with MF. My take on this: (a) nobody can focus manually as good as a good AF and (b) most people use AF so this is the most likely scenario!
- If the AF is just like a scatter-shot I take the best result for analysis. If you own such a lens, make sure to try multiple AF-efforts to have a good chance to nail the shot.
- If the lens is decentered (which most lenses are more or less) taking the best corner gives you at least an approximation of what this lens(-design) is capable of if it only were well centred.
- If the lens has field curvature, my review shows you, what to expect when shooting flat subjects. But I will/can not indicate whether a lens has field curvature. So it just might be that some of the lenses where I complained about weak borders/corners are fantastic in the field with non-flat obbjects
So that's about it and how I decided to cope with these problems. You now know about it - and you can never say I didn't tell you
If you'd ask me what I think many test-labs use I'd say: mostly #2 but with Gordon's tests of corner sharpness I think he uses the image that gives the best centre-sharpness (= method #1). But we can ask him!