Audii-Dudii Offline Upload & Sell: Off
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Thanks, genji, for your kind words! (Have you seen my latest camera project? For my admittedly very much niche purposes, this camera is an even better performer than the Actus, especially now that I'm applying a small, fixed amount of tilt to increase the amount of DoF for most of the scenes I photograph with it. And I have yet another project in mind using the Cambo WRC-400 as the base camera that should perform even better still!)
Before I answer your question, though, let me note that I'm not an optics expert, so the following is speculation on my part. That said, I do have quite a lot of experience working with with larger formats -- prior to my switch to using digital cameras, I was using a 4x5 and 8x10 view cameras, and prior to that, I also used several different medium-format cameras (as well as since, because I have a medium-format digital outfit built around a Contax 645 body and lenses, too, although I don't use it much these days) -- so I would like to believe it's informed speculation, but it remains speculation nevertheless.
So with that disclaimer out of the way, I believe the reason why certain of the Sigma Art-series lenses have larger-than-necessary image circles is because a key part of Sigma's design approach is to make the "sweet spot" of their lenses as large as possible.
One way of doing this is to design the lens to cover a larger format than necessary and then effectively crop the size of the image circle it projects by using the lens hood to block the unwanted/uneeded light from ever entering the lens. (Note: Allowing non-image-forming to enter the lens and bounce around is potentially problematic and can degrade the performance of a lens, so great care must be used when a lens is designed to avoid these issues.)
As lens reviewers are continuoually reminding us, the performance of most lens designs isn't consistent across the full dimensions of the frame they are capturing. Generally, their center performance is quite a bit better than their edge performance, so by making the image circles of its lenses a bit larger than needed, Sigma has been able to effectively expand the size of their center "sweet spot" and therefore improve the performance of the lens away from the center of the image circle.
In effect, the Sigma Art-series lenses are essentially (mini-)medium-format lenses being used on a 35mm format body, just as many of their APS-C-format lenses are essentially 35mm format lenses. Which also at least partially explains why many of their lenses are noticeably larger and heavier than similar 35mm format lenses from other manufacturers.
In the case of the zoom lens I'm using (the 24-35/f2 zoom), this is likely just a byproduct of designing a zoom lens, because the size of the image circles projected by most zoom lenses will vary across their focal length range. The manufacturers then add baffles to the lenses (which are sometimes quite easy to remove, btw) to reduce the size of the image circles so they just cover the format for which they were designed and don't allow extraneous, non-image forming light to reach the sensor.
Speaking of which, because I use my Sigma lenses without their OEM lens hoods, I have to be careful to avoid veiling flare and other artifacts that can appear whenever non-image forming light is allowed into a lens or camera body. For this reason, whenever it's possible, I use compendium shades and mount them on my camera bodies. I also wear a large hat or carry around with me a piece (or pieces) of black foam-core board that I use to flag the lens as necessary. And sometimes even this isn't enough, so I have to pass on taking some photos.
And thanks -- again! -- for your kind words. I work in a very narrow niche that generally isn't appreciated by many viewers, so it's always nice to hear that my photos resonate with at least a few of them.
P.S.: Although I haven't bothered with putting up a website, I do have a photo-blog where nearly 1800 of my (mostly nighttime) photos have been posted over the past eight years. Check it out!
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