I shoot regularly in the studio with a Canon 85 1.8. The majority of the shots are shot at F8 under studio lights. It's my preferred lens because of weight and its a good balance of creating a repore with the person sitting. The sweet spot for the 85 1.8 for shallow depth of field is at F2.2. The 24-105L would be better suited as an all around lens but in outdoor environments I prefer to shoot portrait at 2.8 or less. Thats when I enjoy the lightweight of primes and their very strong ability to blur any outdoor distractions during portraiture work.

tldr; Portraits, indoor sports, shallow DOF and sharp handheld available light at mid range distance

I recently purchased the EF 85 1.4 IS for my 5DS. I primarily purchased it for portraits and other situations where I want a shallower depth of field.

I saved my pennies and sold off some gear I wasn't using because I really wanted IS. Unless you really want an ultra-shallow DOF at shorter range, IS, or really need that extra 1/2 stop the 1.8 is probably just as good for far less $.

Upon recently reading a UHH thread on indoor sports, I may try it out for that application as well to get a higher shutter speed with available light when a short telephoto is useful.

I discovered I especially like it on my Rp. The DOF preview with the electronic viewfinder is at full brightness and really lets me dial in DOF perfectly every time.

I'm glad I bought it - it does what I expected but there was a bit of a learning curve as it was my first 85mm lens.

It's basically a very Good portrait Lens that is 1/3 rd the cost of it's more expensive cousin the 85mm 1.2 that runs about $1800. It has a gold ring around it's barrel and that indicates a higher status than the kit lenses they also make and retails for about $300.00. It has a 28 degree viewing -angle and will also do landscapes nicely as well. You already own the 70-200 L which is a major player in the wedding scenario and would compete with that 85. I would borrow or rent it and shoot it before I bought it if I had all of the lenses you currently own. You have a great selection of glass and a very Good Camera to Boot !

The Canon 85mm f/1.8 USM is a fine lens that you will probably enjoy using. I've had one for many years. It dates back to the film era! But it works fine on modern DSLRs, too.

The f/1.8 simply can't "obliterate" backgrounds the way the much more expensive, much bigger EF 85mm f/1.2L can. But it can do shallow DoF effects pretty darned well and is at least one stop faster than any EF zoom... two or more stops faster than many of them.

The f/1.8 is quicker focusing than either version of the f/1.2L. All of them are fine for portraiture and other relatively slow, sedentary shooting. But for sports and other action, the f/1.8 lens is a better choice.

It's not nearly as expensive, but also not the build quality of the L, of course.

The build is quite similar to EF 28mm f/1.8 USM, EF 50mm f/1.4 USM, 100mm f/2 USM... what I'd call "mid grade" quality. Not L... but also not "micro motor" or "STM" style, and certainly not "kit lens" with excessive plastic. It's not a big or heavy lens, but not a lightweight either. Using pretty standard-for-Canon 58mm filters and slightly under 1 lb., it's a little under half the weight of any of the Canon 85mm L-series (including the latest EF 85mm f/1.4L IS USM, the first of them to have image stabilization).

Like most (all?) Canon non-L-series, the 85mm f/1.8 doesn't include the lens hood. I highly advise getting one. It's unusual for Canon hoods, in that it "clips" onto the lens (it's not bayonet mount). When I saw the little tabs that hold the hood on the lens, first thing that came to mind was "Well, those won't last long". However, after many years of use mine have held up fine and never broken, even though I've knocked the hood right off the lens more than once. The OEM hood works well and holds up. I recommend it. I've never compared with the clones.

I also have not compared the latest 85mm f/1.4L IS USM. Only know what I do about that lens from the specs online.

Like a previous poster, I'm an old "prime lens only" shooter. That's largely because I learned photography many decades ago, when zoom lenses were new and reeaaallllly sucked! I now use a number of zooms... modern ones are much better. But I still appreciate the smaller size, lighter weight, less intrusiveness, usually faster apertures and often lower cost of prime lenses.

Prime lens shooters tend to develop preferences for specific focal lengths.... my usual kit included 20 or 21mm, 24mm, 35mm or 40mm, 85mm, 135mm, 200mm and 300mm (with 1.4X or 1.5X teleconverter). I rarely carried all at the same time (though all would get packed and taken on trips, along with 3 or 4 camera bodies, a couple flash, lots of batteries and many rolls of film). For example, I wouldn't need the 300mm and TC unless I were planning to shoot wildlife or sports. If I were shooting portraits I'd be sure to take the 85mm and 135mm (for full frame/film).

Whether or not the EF 85mm f/1.8 USM is "up to the task" with the 5DS-R is difficult to say. That's a very demanding camera, with it's ultra-high resolution. (But just wait until the 75MP to 83MP successor for it is announced. Be it a DSLR or, more likely, a mirrorless R-series, it will be even "tougher" on lenses!) The 85mm f/1.8 is sharper wide open than the EF 50mm f/1.4, and the 85mm stays sharp as it's stopped down. It has a little chromatic aberration, but that's pretty easily fixed in post-processing. To my eye, color rendition of the 85mm f/1.8 tends to be a little cool (cooler than some of the other similar grade Canon lenses). Again, that's easily changed in post-processing, when wanted.

I use my 85mm f/1.8 on both crop and full frame cameras... though usually for different purposes. It's my "short portrait" lens on full frame. Particularly with non-professional models, family, street photography, it's a lot less intimidating and intrusive than a big zoom or a massive L-series lens.

On crop sensor cameras it sometimes serves as my "long portrait" lens, but I'll also use it for sports, particularly for indoor venues.... basketball, volleyball, hockey, etc.... where the 85's extra stop or two (compared to a zoom) can be is very helpful. Even when making exposures with a smaller aperture, the viewfinder stays brighter thanks to the larger aperture lens. That helps autofocus performance in some cases too (many Canon cameras have AF points that perform best with f/2.8 or faster lenses). Plus it's small and light enough to be easily hand held for hours on end.

Unless it’s low available light without a flash, and then that >2 stops faster is the difference between getting the shot and not. Recently, I went to my youngest son’s wedding. He had a professional photographer, but I wanted my own shots. It was a well lighted indoor venue, and I took my Canon 5D3 with my 24-105L, but after taking a couple of readings, I realized that even at ISO 10,000, the fastest I could shoot would be between 1/30 and 1/60 - too slow. Fortunately, I had my 85 f1.8 in the car, and I shot the wedding with that at 1/125 (which was adequate) and typically f2.8. That 2+ stops made the difference between a couple hundred excellent shots and leaving the Camera in the car.