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I'm a professional photographer & use the Canon L Series lenses regularly, & while this lens is not a regular feature in my camera bag, I've used it enough to give it a pretty good review I think.
For starters this is an extreme wide angle lens, it is an 8-15mm zoom lens, with a maximum aperture of f4, which at that focal length acts as a fish eye lens. If you're unsure what the term 'fish eye' means it is basically a distorted curved image, the focal length is so short that it curves that would otherwise be straight lines of perspective (tall buildings for example, will appear curved in the frame). It's a type of image which has often been used on record covers in the past & produces an effect similar to a curved mirror (often seen on public transport such as the London underground or on buses).
Essentially this is 2 lenses in 1, as it can photograph in a circular fish eye manner & also as a diagonal fish eye.
As usual for an L Series lens it's sealed to prevent dust & moisture getting inside & comes with the usual exceptional build quality you would expect from a lens at this price point.
One quibble would be that the zoom ring is pretty loose, meaning it can move without you knowing, which at this depth of field can be very important as a slight change can result in distorted proportions in the finished image. Part of the appeal of a lens at this range is the ability to distort the frame of view, but I would have liked a lock to prevent it from doing that if required.
The auto focus is as fast & accurate as all the top shelf Canon lenses, I've used these zoom lenses for years & it's very rare when the focusing isn't instant & deadly accurate. There will be aberrations at the edge of the frame occasionally, but that is simply a quirk of using a fish eye lens & has little to do with the quality of the lens, it's just an anomaly you will encounter when photographing that wide.
On the 8mm end of the frame you will get a circular image, completely curved, whereas on the 15mm end you will get a full frame image yet it will still be distorted & straight lines will appear curved.
You can get a unique perspective photographing this way, however many photographers consider a fisheye lens something of a gimmick, it has it's uses, but I'd think vary carefully before spending the £1000+ it will probably cost you to acquire one of these. I don't own this particular lens as it's use for me is too infrequent to justify the expensive price tag, I will hire this lens as & when I need it & prefer it that way. My workhorse lenses (24-70mm, 16-35mm, 70-200mm) I always have in my bag, but a lens like this has only niche interest & is by no means an essential purchase.
There are some quality lenses made by Canon (not L Series lenses) & Sigma available for £300-500 that would easily provide a good enough result, so I would consider one of those if you wish to buy a lens of this focal length. Having said that, for it's kind this is the best lens you will find, however I'd be sure you will get enough use out of it before making the considerable investment.