Without a doubt, one of the requirements of a cine/video zoom lens is that it holds focus throughout the zoom range. What this means is that with a cine zoom, when you zoom in and focus, as you zoom out, the focus holds and doesn’t drift or require adjustment. In the cine world, this has led to strong spring-less lens mounts and collimation to tolerances of hundredths of an inch. Even in the ENG world, lenses are fitted with a back-focus adjustment, allowing for in-field adjustments, because news cameras didn’t have mounts that were quite as strong as cine cameras and the constantly changing conditions could wreak havoc on the very precise flange focus distance required for a lens to perform its best.
Creating a parfocal lens is a complicated, expensive, and demanding task. Recently, Sigma released its High-Speed zooms, an 18 to 35mm T2 and 50 to 100mm T2, which are in the Cine Lens lineup, but Sigma doesn’t refer to the zoom lenses specifically as cine zooms, and nowhere in the company literature on the lenses does Sigma use the term “parfocal.” This has led to confusion, especially because these lenses have been tested by some to be parfocal in performance and, in some cases, exhibit better parfocal performance than cine zooms from other companies. In studying the situation, it appears that these lenses are based on a still photo lens design, and still photo lenses are not designed to be parfocal. Although I’m not a lens engineer, I wonder if perhaps it is the relatively short 2:1 zoom range of the Sigma lenses that aids in the parfocal performance, which is certainly desirable in any lens, but especially so in a lens with a such a fast aperture.
So, to resolve this conundrum, I contacted Sigma, at which point I was directed to the FAQ page on those lenses, which addresses the parfocal question this way. “In general, a perfectly parfocal lens production is nearly impossible. SIGMA goes through great lengths to reduce focus shift throughout the zoom range. However, as is the case with most cine zoom lenses, the SIGMA cine zooms are nearly parfocal.” What this means may be open to interpretation, but it seems to me that Sigma cares very much about entering the cine lens market, and wants to be completely honest about its own expectations for lens performance, even if the lenses, in general, perform better than designed. Are the lenses parfocal? Sigma staunchly denies this, insisting that they are only “nearly parfocal.” So technically, no the lenses are not parfocal. However, in practice, which is what really matters after all, it seems the lenses perform as you would expect a cine zoom to perform.
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