Laowa 4mm f/2.8 Fisheye!

[Growltiger]

Yep. The widest rectilinear APS-C lenses I know are 8-16s. For m43 it's the 7-14s. I may have missed a wider rectilinear prime someplace. Additionally, it may be worth noting this announcement from Laowa includes a 10-18 f/4.5-5.6 for Sony full frame.
Physically possible and economically impractical, most likely. Rather than wait I tend to stitch or defish when need arises. Mathematically this can be somewhat problematic as the angle of view of a rectilinear lens goes to 180 degrees as the focal length tends to zero. It's easy to exceed that with in camera or phone panorama and 360 modes and, depending on the gig, I might do so many times per day. It also happens combining three vertical 12mm m43 frames gives about the horizontal angle of view of 4mm full frame. With a little care in composition, stitches in this range often don't look too crazy and you can get away with five images.

I've done quite a bit of stitching too, but have had the best results stitching far more images, taken with a lens at about 18 to 25mm rather than UWA. Vertical like you. The advantage is that with say 15 images, even allowing a 1/3 overlap, you get a lot of pixels and detail in the panorama. An example: Panorama with 15 images
(Mouse wheel to zoom, drag to move, or use controls.)

 

[wjiang]

UWA are difficult to stitch due to the inevitable barrel/moustache distortion that they have. It's much cleaner to stitch using more moderate FoV lenses that exhibit less inherent distortion. The FF astrophotographers I know tend to stitch using 35mm and 50mm primes.

 

[archaeopteryx]

I've done quite a bit of stitching too, but have had the best results stitching far more images, taken with a lens at about 18 to 25mm rather than UWA.

I typically stitch with 12mm from my 12-60 3.5-5.6 and 50-65% overlap. Wider would probably not work as well. It happens my latest phone's angle of view is only a little narrower than 12, so mostly I've taken to using the phone as it's less fuss.

As much as I enjoy the idea of rectilinear hyperwides I don't know I'd actually use one. Having worked in the range of the Laowa 7.5 quite a bit I feel that's about where many images tend to transition to ultrawide for ultrawide's sake rather the width making the image better. Somewhere in the 7-8mm range seems a good practical maximum, though there's always exceptions. Wider than that I tend to feel rectilinear isn't the most useful projection. It's never a choice I've selected when defishing, for example.

 

[MarylandUSA]

Opteka markets a 6.5mm f/3.5 diagonal fisheye designed for APS-C but available with a micro four thirds adapter on eBay and Amazon.

 

[tkbslc]

There are 12mm FF lenses, but I think that is the widest. So that would be 6mm for us, a whole 1mm wider than the 7mm we have today. It should be possible.

1mm doesn't sound much but it would be a huge difference.

Canon has an 11-24 zoom for full frame SLR and Voigtlander has a 10mm f5.6 prime for FF mirrorless. That's 5.5 and 5mm in m4/3 terms.

10mm on FF is WIIIIIDE...Review: Voigtländer 10mm f/5.6 Hyper-Wide Heliar (Sony E-Mount) - Page 3 of 3 - Admiring Light

 

[Bytesmiths]

Here it is, serial number #262! Delivered two days early!
No post processing at all — right out of the camera. Looks a bit dark, though. (EDIT: I had -1EV dialled in by accident.)
Note that the ceiling light is actually behind me, showing off the Laowa's 210° angle of view.

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