JonPB
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I prefer shooting film, partly because it discourages "testing" and encourages "photography." However, I recently acquired a NEX and Speed Booster, so now my testing addiction can recommence. I hope these remarks are appropriate here despite being more test than sample and being about a Leica/Metabones combination rather than just the Leica lens.
The Leica Vario-Elmar-R 21-35 f/3.5-4 has been criticized for having soft corners, especially at its widest angle and wide open. I tested this at a close focus distance, and found that, yes, at 15mm f/2.9 (indicated 21mm f/3.5), the corners might be charitably described as "overcooked oatmeal" in terms of fine resolution. They do become reasonably detailed at f/6.7 (indicated f/8), but I wanted to investigate more.
I tested for field curvature and found quite a bit of it. Using the corners to focus (at 14x magnification) rather than the center of the frame, I found that optimum focus on a cupboard in the corner of the image, about 7 feet away, required setting the lens to about 4 feet. Thus, this lens at an indicated 21mm has its corners focused further away than the center. This can lead the corners to appear quite soft when only the center of the image field was considered during focusing. When focusing for the corners, though, even wide open, the lens draws a fair amount of detail in those corners; the center, of course, will be softer due to being out of focus.
With a less extreme situation, I focused on a neighbor's chimney approximately 90 feet away, again wide open and set to an indicated 21mm. The corners were focused when set to about 8 feet; by way of reference, the center focused when set to the side of the infinity symbol. After some global edits--see the Flickr page for more detail--here is the result:
21-35 Comparison by jpb@pdx, on Flickr
The corner (on the left) has lower contrast, stronger chromatic aberrations, and notable sagittal blurring. Definitely inferior, but there is plenty of usable resolution; not oatmeal by any stretch.
The upshot for my photography is that, if I want sharp corners, I will stop down a bit and pull the focus slightly closer than would be optimal for the center frame. On the other hand, if I'm shooting close subjects, I can readily take any distracting corner details and blur them away by opening up the aperture and pushing the focus a bit.
In other news, at 25mm (indicated 35), when compared to the most recent Elmarit-R 35, the 21-35 has better corner performance--about three stops of improvement, even when not compensating for focus plane curvature--whereas the Elmarit is sharper in the center wide open than the 21-35 ever becomes.
Overall, I think the 21-35 is a great compromise for folks like me who want a general purpose wide lens that has great haptics. The drawbacks--limited range and speed, focal plane curvature, and lower peak performance than comparable prime lenses--are worthwhile considering how well everything else works out.
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