Please note that the Canon EF 85mm f/1.2 L USM Lens has been replaced by the Canon EF 85mm f/1.2 L II USM Lens in Canon's lineup.
One of the primary reasons to own the Canon EF 85mm f/1.2 L USM Lens is to get amazing background blur! In the hands of a good photographer, the results from this lens can be stunning!
There were some features I didn't like about this lens when I first acquired it. First, it focuses slowly. Even manual focus is strange. The Canon EF 85mm f/1.2 L USM Lens uses Electronic Manual Focusing. You must have the camera body powered on to manually focus. A motor changes the focus distance as you turn the focus ring. I personally don't like this.
Anyway, I generally do not recommend this 85mm lens for fast action sports that require fast, widely varying focus tracking. Although some people use it for such, the 85 f/1.2 L won't keep up with a subject quickly moving in and out of the plane of sharp focus. Sports with subject distances that do not rapidly and greatly vary can be covered very adequately with this lens.
Second, this lens is fat and heavy. And the fat lens hood wobbles.
Well, those were my first impressions anyway. I have since come to like the weight of the lens. Because I am frequently using the 85 f/1.2 L in low light conditions, my shutter speed is often dragging (long). The weight of the lens helps it to remain still during the shot. I am also getting used to the fatness feature. It seems to anchor well in my hand - also aiding steadiness. And the hood wobble? Well, the hood is very easy to get on and off - it is very forgiving to installation misalignment. And the wobble does nothing to hinder performance.
The high price tag is another detriment to this lens. You could buy 4+ Canon EF 85mm f/1.8 USM Lenses for the price of 1 Canon EF 85mm f/1.2 L USM Lens. 'Nough said about that topic.
The front element moves in and out during focusing - not a lot, but it moves. The rear element is flush with the rear of the lens requiring care during lens changes.
The 85 f/1.2 L does not provide the camera with distance information, a parameter used in Canon's E-TTL II flash exposure determination. This does not mean you can't use an E-TTL II flash with this 85, but there will be one less parameter for the camera to use in optimizing flash exposure.
So, by now you are asking "Why does anyone own this lens?!" It's all about the aperture - and sharpness, and image quality.
The Canon EF 85mm f/1.2 L USM Lens is currently equa to the fastest (widest aperture) EF lens Canon makes.
Open the 85 f/1.2 L up and watch the background disappear. The result is very dreamy looking. Background distractions are gone. Keep in mind - the DOF (depth of field) is razor thin at f1.2. Unless you are far from your subject, you must hit your focus perfectly.
Shooting handheld at lower ISO settings - or just very low light conditions - is accomplishable at this focal length when using the 85 f/1.2 L.
The Canon EF 85mm f/1.2 L USM Lens is very, very sharp. Even at f/1.2, the 85 f/1.2 L is sharp and it gets even sharper when stopped down a small amount.
Vignetting is very noticeable at f/1.2 (even on a 1.6x FOVCF body. Full frame users will see vignetting nearly disappear by f/2. The 85 f/1.2 L has about 1 stop of vignetting advantage over the Canon EF 85mm f/1.8 USM Lens. A bit of CA (Chromatic Aberration) is noticeable wide open at times.
The 85 f/1.2 L is most-closely comparable to the Canon EF 85mm f/1.8 USM Lens.
I found the Canon EF 85mm f/1.2 L USM Lens at f1.2 to be as sharp as the Canon EF 85mm f/1.8 USM Lens at f1.8. From f1.4 through f4, the 85 f1.2 was slightly sharper than the 85 f1.8 at the closest comparable apertures. The difference was not big enough to make it a big factor beyond f/2.8 in my opinion.
The Canon EF 85mm f/1.8 USM Lens rules in the size, weight, cost and autofocus speed categories.
The major advantage the Canon EF 85mm f/1.2 L USM Lens has over the Canon EF 85mm f/1.8 USM Lens is the wider aperture. There is a significant difference in the background blur at f/1.2 compared to f/1.8. If you really need significant background blur in an 85mm or so lens, the f1.2 is perfect. The difference in shutter speeds at these two wide open apertures is also big.
What about the Canon EF 70-200mm f/2.8 L IS USM Lens? I can typically handhold the Canon EF 70-200mm f/2.8 L IS USM Lens at a shutter speed of 1/30 - far less than I can handhold the Canon EF 85mm f/1.2 L USM Lens. But - subject motion blur can quickly become a problem at these shutter speeds. The faster aperture on the 85 f/1.2 makes up for most of the IS feature. And the background blur is much more significant on the 85 f/1.2L at 1.2 than the Canon EF 70-200mm f/2.8 L IS USM Lens at the same focal length. The Canon EF 70-200mm f/2.8 L IS USM Lens is far more versatile however.
The Canon EF 85mm f/1.2 L USM Lens excels at portrait photography. The 85mm focal length combined with excellent bokeh is perfect for portraits. Slow focus speed is generally not an issue in portrait photography - and the 85 f/1.2 L lens is easily sharp enough for this task.
The 85 f/1.2 L is very capable of stopping action in low light - but don't ask it to quickly focus on a subject distant when a close subject is currently in focus. You will likely miss the shot while waiting for focus to lock.
The Canon EF 85mm f/1.2 L USM Lens is an excellent low-light lens for handholding at events without fast-distance-changing subjects. Many consider the 85 f/1.2 L one of the best wedding lenses available. Taking pictures around the house using available light, shooting church events, photographing a speaker or business event, still life photography ... There are many uses for the 85 f/1.2 L.
At one point, I sacrificed this lens to buy the Canon EF 300mm f/2.8 L IS Super Telephoto Lens. But, I kept looking at the portrait shots I liked best - and a significant number of them were taken with the Canon EF 85mm f/1.2 L USM Lens - and many were at f/1.2. So, I added this lens back into the kit as soon as funds were available.