Experiments with Sigma Art 20mm f1.4 underwater
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    First shoots with a Sigma 20mm f1.4 on a Sony A7R II with Metabones adapter, in a Nauticam housing.
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The details and sharpness on this lens are great: check out the pigments of blue and the ampullae of Lorenzini. I find the bokeh quite pleasing as well, with lovely transitions into the softness -even on something as rough as shark skin.
20mm can be an odd angle to work with -not quite wide enough for underwater overview shots when the visibility is murky. But you can make those conditions work for you if you want to play with the mist.
The lens deals with backlight really well. The camera is no slouch either; Sony's A7R II is known for it's dynamic range. The lens seems to compliment it nicely, even with the Metabones adapter attached.
20mm is about wide enough to get a blue shark in at fairly close range. I was hoping to get a shot like this, maybe with both pectoral fins in frame, but at least with the eye in focus and the rest nicely blurry.
Main reason i got the lens: had a feeling it would be great for portraits underwater.
The Sigma had no problem with head-on moving objects, like this blue shark investigating.
A jellyfish in a rough shore break. Lots of action and movement but no trouble focusing.
For big freediving action like swimming through bubble rings the lens might be a bit too narrow -i have a feeling i would have gotten a better result on this one with my Zeis 16-35 f4 at 16 mm.
But for things like the bubble rings themselves it is perfect, with the front of the ring nicely in focus and the back blurred.
The bokeh holds up in B&W portraits as well -even if it focused on the eyebrows here. Probably my mistake: have yet to figure out the Sony menu and how to focus on eyes.
Minimal focusing distance is tiny: 28 cm. Which means you might get stung by the jellyfish before you lose focus.
One more for the bokeh: little bubbles catching sun makes for a sweet display. The Sigma handles it beautifully both in the foreground and in the background.
As an experiment, i also cranked the f-stop up to f14 and worked with a shutter of 1/15th. As a fast lens this is not its natural habitat, but i wanted to take it and myself out of the comfort zone. It held up, but this might be more for wider, slower lenses.
Overall, i think this is a great lens to have in your collection if you want to do portraits underwater, or add diversity to fish-eye and 16 mm shots. It fitted into my existing Nauticam housing with dome and extension for a 16-35mm Zeis, even though the Metabones adapter added a good 3 cm to the already enormous lens. If you play to its strengths and let it do its f1.4 magic, it is unique and will yield gorgeous results. I have yet to try it in low light, but so far the vignetting is not too bad so i reckon that won't be much of a problem. It won't replace my 16-35 for action shots, but as it says on the box and on the lens itself, this is an 'art' lens. State of the art, if you ask me. Brilliant piece of kit.