The Venus Laowa 12mm Zero D is a pretty amazing lens... for full frame... if you're willing to pay $849 for a manual focus and manual aperture control lens.
However, 12mm on a 90D will not give as wide angle view as 16mm on full frame. So you'd be better off just using your 16-35mm on your 5DIV.
You need a 10mm lens to have the same angle of view on an APS-C 90D as you are already getting with 16mm on a full frame 5DIV.
If you want autofocus and auto exposure, as of now there are only two lenses that will go that wide... Canon EF-S 10-18mm f/4.5-5.6 IS STM (~$300) and Canon EF-S 10-22mm f/3.5-4.5 USM (~$650).
If you're okay with manual focus and manual aperture, there's one more possibility: Rokinon 10mm f/2.8 ED AS NCS CS. That's currently on sale for $340 (reg. $429). Note: The Samyang 10mm f/28 is exactly the same lens. Samyang makes both, but they sell under the Rokinon name in N. America.
No current lens goes any wider on 90D, except for fisheye lenses with very strong curvature distortion in images.
In the past Sigma offered an 8-16mm f/4.5-5.6 DC HSM FLD lens for APS-C Canon DSLRs like the 90D. If you can find one used it would give you a wider view on your camera, But even though it's not a fisheye, it does have fairly strong curvature distortion and cannot be used with standard filters due to a convex front element.
There also is a Tokina AT-X 10-17mm f/3.5-4.5 AF DX lens available in Canon mount, but this is a fisheye lens at the short focal lengths, largely uncorrected for curvature.
Your best bet for ultra wide lenses is your full frame 5DIV. There are 15mm, 14mm, 12mm and 11mm lenses that go wider than 16mm on full frame. Canon themselves offer an EF 11-24mm, while Sigma has made 12-24mm and 14-24mm lenses for use on Canon full frame DSLRs. Of course they all can also be used an APS-C 90D, but will be less wide on that camera, than a 16mm is on full frame.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7DII (x2), 7D(x2), EOS M5, some other cameras, various lenses & accessories
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