Tokina 12-24 & Sigma 8-16
I occasionally photograph houses for builders, interiors designers, etc. I've been using my Tokina 12-24mm for years, I really like it, but I needed something wider for smaller spaces. I bought a Sigma 8-16mm and it doesn't seem to focus as well and I'm not getting the sharper photos I'm used to. Did I get a bad copy or does the Sigma need to be focused in a particular way?
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One appears to be a considerable field curvature, exacerbated by close focus. What this means is that for subjects closer than infinity, the field of focus (parts of the image in sharp focus) will probably curve inward toward the image edges and corners. If you are shooting a relatively flat surface and focused on the center, the edges and corners will be out of focus at large apertures. You may be able to improve the situation by stopping the aperture down/smaller, which will increase shutter duration for the same exposure.
Even then I would expect some chrominance aberration in the same areas, which may or may not respond to post processing corrections.
A possible option: Multiple exposures at different focus distance settings, combined for maximum sharpness in post-processing. Not really valid for moving subject matter. There are also diminishing returns and limits for this technique.
In short, it's not that the Sigma 8-16mm f/4.5-5.6 DC HSM is that bad, it just trades different optic properties for the sake of AOV, compared to the Tokina 12-24mm f/4 AT-X 124 AF PRO DX SD.
I don't think that any other zoom lens exists with this wide an AOV, unless you consider moving up to a FF body. For instance the Nikon AF-S NIKKOR 14-24mm F2.8G ED has a very similar 114 degree diagonal angle-of-view. It still has some field curvature at close distances but I don't think as bad as the Sigma 8-16mm f/4.5-5.6 DC HSM (at similar apertures). Rental and testing would tell the tale.
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