12mm Ultra Wide.

Now here's something a bit out of the ordinary which you don't often see on ebay or the sales adds.
A 12mm uwa fisheye full frame lens in an m42 mount. Admittedly a tad pricey @ £175 but interesting never-the-less for someone who wishes to experiment or who has a purposeful need for such a quirky lens.

I'm tempted myself just out of curiosity for the unique imagery that it can render but - hey-ho!

https://www.harrisoncameras.co.uk/used/used-camera-lenses/?filter=brand:used-sig...

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C.O.L.B.A.S victim
(Compulsive Obsessive Lens Buying Addiction Syndrome)

What you need are lenses, more lenses, bigger lenses, better lenses, faster lenses, and when you have these, your pictures will be perfect!