When I first heard of Sigma’s 40mm F1.4 Art lens, I was a little confused. Sigma already had both 35mm and 50mm prime lenses in its high-end Art series; was a 40mm necessary? And wasn’t this focal length reserved for things like compact pancake lenses?

Sigma developed this lens with cinema applications in mind, where 40mm is apparently a sought-after focal length. On a cinema camera using a Super35 sensor, 40mm is equivalent to a full-frame 60mm, a good mid-telephoto length. The lens is also designed to resolve enough detail for 8K video.
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https://www.digitaltrends.com/camera-lens-reviews/sigma-40mm-f14-art-lens-review/