Here it is.
My List is actually even shorter then i remembered for 4/3" Format, so i did some more digging online on my own on top of what i had Listed.
Sadly, most Large Sensor C-Mount Lenses top out at 1.1" Sensor Size (4/3" is 1.3", so just barely bigger), and while i am positive that quite a bunch of those 1.1" Lenses might even fully Cover 4/3" Sensors, i wouldnt be able to say for sure unless i found someone who tested that, or would own one of these, which both is not the Case, so i did only write down Lenses, that are Specifically designed to work for 1.3" / 4/3" Sensors and Rated for that Sensor Size and sold as such.
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Kowa XC Series Lenses
https://www.bhphotovideo.com/c/product/1320778-REG/kowa_lm8xc_fixed_focal_length.html https://www.bhphotovideo.com/c/product/1385292-REG/kowa_lm12xc_4_3_megapixel_xc.html https://www.rmaelectronics.com/kowa-lm16xc/ https://www.rmaelectronics.com/kowa-lm25xc/ These are Available in 8 / 12 / 16 / 25 / 35 / 50 mm Focal Length for 4/3".
Horizontal Angle of view is 40,9° on the 25mm one, and 93,5° on the 8mm one, so thats the Links i listed above. 8 and 12mm are of course also available at RMA, but B&H is considerably Cheaper.
Aperture is 2.8 max. on the 8mm, for all the other ones its 2.0 max.
Rated Resolution is 8MP, so they would still resolve 4K even wide open, according to Rating by Kowa.
Those are also probably the most popular of the 4/3" Rated C-Mount Lenses i will list here, so you can sometimes find a some of them used on Ebay, currently there are bunch used 25mm ones on there.
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Schneider Kreuznach Compact Xenoplan C-Mount Lenses
Now while i highly assume, your Vignetting is mostly due to Mechanical Vignetting because of the Adapter you use,
https://de.wikipedia.org/wiki/Vignettierung#Echte_Vignettierung https://en.wikipedia.org/wiki/Vignetting#Mechanical_vignetting There is still Pixel Vignetting on Digital Cameras, that can be a Problem.
Schneider is building Lenses, that are especially made to solve that Problem:
https://www.bhphotovideo.com/c/product/400373-REG/Schneider_21_1001972_21_1001972_1_3_28mm_F2_0.html https://schneiderkreuznach.com/application/files/4115/4114/8809/1001972_Xenoplan_2-0_28.pdf https://www.bhphotovideo.com/c/product/715062-REG/Schneider_21_1056472_Apo_Xenoplan_2_0_Lens_20mm.html https://schneiderkreuznach.com/application/files/2715/4089/1969/1056472_Apo-Xenoplan_2-0_20.pdf https://schneiderkreuznach.com/application/files/2215/4089/2869/1071371_Apo-Xenoplan_2-0_24.pdf https://www.tesscorn-analysis.com/wp-content/uploads/2018/01/Accessories-APO-Xenoplan-24mm-f2.0-Ruggedized-lens.pdf They just Specify AoV here (= Angle of View), which i highly assume would then by Diagonal Angle of View, if you want to make sure, give them a quick call (their customer Support is crazy good, called them a few times myself because of some of my Schneider Lenses, can recommend), or write them a Mail if you want to know for sure.
Anyways AoV as given by Datasheet is 44° for the 28mm; 52,17° for the 24mm APO; 60,67° for the 20mm APO.
Aperture is f/2.0 max.
Now while the 28mm is still far from cheap, the APO Lenses of that Series start to cost some Real Money (couldnt find an actual online Listing for the 24mm with an Price, but from the 3k Price Tag on the 20mm, i assume it wont be much cheaper also).
However, strangely enough, you can find some utterly Insane Deals for some of those on Ebay sometimes, so if you are not afraid of Used Lenses from Ebay, give that a Try. Just make very sure, you know what Lens you are Buying Exactly, and that that one Actually Covers MFT, as there are a ton of Different Versions of these Schneider Xenoplan Lenses around, some of which wont even cover 1" fully (as Per Specification).
Here are some Links to German Ebay Listings, i found from a very Quick Search that seem to offer some of those at really Reasonable Prices in different Conditions, MAKE SURE TO DOUBLE CHECK ANY OF THOSE LISTINGS YOURSELF, IF ITS ACTUALLY THE CORRECT LENS BEFORE BUYING!
https://www.ebay.de/itm/154706595876?hash=item24053b5024:g:gkEAAOSwL3FfA4y- https://www.ebay.de/itm/265422415583?hash=item3dcc68a6df:g:q60AAOSwKeBdd75U https://www.ebay.de/itm/124244183747?hash=item1ced87cac3:g:cRAAAOSwH4Be~XLe https://www.ebay.de/itm/274440653603?hash=item3fe5f00b23:g:oJAAAOSwqeBfHt~A According to my List, the Non-APO versions are rated for "3-5MP" which should technically still render a Sharper Image than the 2.1 Sensor can capture, and the APO-Versions are Rated to resolve 12MP.
These should be Available in 20 / 24 / 35 / 50 mm Versions as APO and 28 / 35 m Versions as Non-APO.
Also very short Minimum Focus Distance, meaning you could use these for Closeups or even "Macro" Work.
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Goyo Optical
http://www.goyooptical.com/products/industrial/GM12HR61220MCN.pdf http://www.goyooptical.com/products/industrial/GM12HR61620MCN.pdf http://www.goyooptical.com/products/industrial/GM12HR62520MCN.pdf These range from 40,6° to 75,46° Horizontal Angle of View.
Aperture is also 2.0 Max on these, and they would also have a 35 and 50mm Version of that Lens.
No idea what ever these would cost, as their Site does not list any Prices, and cant seem anyone else offering these anywhere.
Rated Resolution is Specified as 12MP, and Minimum Focus Distance is also short on most of these.
I assume you would need to buy from them directly, and Request Prices if that would actually be something that would fit your need, but given they are 12MP rated and for such a Large Sensor, i guess they wont be cheap either.
#EDIT#: Found someone Selling them, but no Price either here:
https://www.visionlighttech.com/product.php?hoofdgroep=lenses&subgroep=12-megapixel&id=GM12HR62520MCN .
They however offer some VERY Fast Lenses also:
http://www.goyooptical.com/products/industrial/GMHR62409MCN.pdf http://www.goyooptical.com/products/industrial/GMHR62408M42N.pdf These are both 24mm, one of them is f/0,9 max the other one even is f/0,8 (f/0.8 is M42 However).
As they dont even give any Rating regards Resolution, i wonder how sharp those are, and if they are Sharp, probably not up to the Corners, otherwise they pretty sure would have at least put "Megapixel Lens ...." somewhere in there (which usually means 1+ Megapixel, so it would properly resolve 720pHD).
Also not able to find any Price for those, but also cant imagine these being cheap at these Specs.
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Zeiss Dimension Series Lenses
https://www.stemmer-imaging.com/en/products/series/zeiss-dimension-lenses/ Available in 8 / 12 / 18 / 25 / 35 / 50 mm Versions.
Could only find A Resolution Rating from Stemmer:
https://www.stemmer-imaging.com/en/products/zeiss-dimension-288/ Which is "56.2" without any unit given, but i assume Megapixels, which would be pretty intense.
Given that Pixel Pitch on the 2.1 is 10µm and Zeiss Rates these good to use for a pixel Pitch of 2µm, that would seem possible at least.
They apparently Have some Kind of Advanced Backfocus Adjustment built into the Lens itself.
Price is Substantial, but these are about as good as General Use Lenses for 4/3" get, if you have to Stick to C-Mount.
Aperture again 2.8 on the 8mm, and 2.0 on all others.
https://www.rmaelectronics.com/zeiss-dimension-2-8-8-4-3-8mm/ https://www.rmaelectronics.com/zeiss-dimension-2-12/ https://www.rmaelectronics.com/zeiss-dimension-2-18/ https://www.rmaelectronics.com/zeiss-dimension-2-25/ .
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Computar
https://www.vision-dimension.com/en/lenses/ccs-mount-lenses/monofocal-lenses/12-mm-c-mount-computar-45mp-objektiv-f1228-mpt/1232 Also found this Lens, but its out of stock. 12mm f/2.8 and apparently rated for 45MP Resolution, but currently out of Stock and has a Price tag of 9999,00€ if you actually make an Account on the Site to see the Price (they Sell to business Customers only also).
Not sure if that is a Real Price or just some Placeholder Price, or why anyone would spend 5 times the Price of what that Zeiss Lens with basically better Specs costs on that, but this apparently exists, and should fit what you looking for (except for the Price, i guess).