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I've now spent some time with three new lenses on the A7s: M-Hexanon 28mm f/2.8, Zeiss 25mm ZM f/2.8, and the very intriguing M-Hexanon Dual 21-35mm f/3.4-4 and I wish I had better news to report. On the upside, there is next to no color cast on all three. However, they all still have significant field curvature at short/medium/long distances.
I was most hopeful of the Dual Hexanon as it's a retrofocal design (by M-mount standards). It really is an incredible piece of glass - extremely well built and with a wonderful feel. It would be a great outdoor walkaround lens for the A7s.
In testing it though I've learned a few interesting things. Nothing not known to others I'm sure, but for me it was a bit of a lightbulb and will force me to retest some other wide lenses on the A7s. In my normal shooting with MF glass and an EVF I set the lens wide open (or one stop down), focus, and then stop down to my desired aperture and shoot. This makes focusing easy and is great for normal shooting. Now with a lens that has strong field curvature in the corners, this technique will likely result in very poor corner sharpness. What I've found is that with a lens sufficiently wide enough (21mm on the Dual Hex is very acceptable, but at 35mm it's not great), you can combat the field curvature with smartly placed DOF. For a subject at infinity, this would mean focusing at the hyperfocal distance for your desired shooting aperture. For a subject closer than infinity, first stop down to shooting aperture and then focus at the shortest acceptable distance where your subject is in focus. This slows down the entire process a bit as focusing is much harder when stopped down, but it does lead to better corner performance.
I am going to take a fresh look at the lenses that are piling up in my apartment to see if any of them can perform decently with this technique. If you're looking for reasonable cross frame performance at larger apertures below 35mm with rangefinder glass outside of the Leica WATE, the A7s probably isn't going to be the Holy Grail you want it to be. However, if you're willing to change your technique and are mainly shooting medium to long distance subjects outdoors, there may be hope.
-Dustin
Edited on Sep 09, 2014 at 09:48 PM · View previous versions
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