The EF 85 f/1.8 USM is a wonderfully sharp lens. I've used it for portraits and a walkaround lens in landscape situations. But, for me, it's not a natural focal length. I too use primes, typically a 35mm or 50mm. The 24-105 zoom can be more useful when I want one lens with a longer focal length. One of the limits I found with the 85 as a 1-lens walkabout is that it doesn't focus very close, so its great for distance subjects, but not for close-up details. Have an extension tube for a 1-lens solution to also focus on close-up details.

We're all different and we all "see" things differently. When I used a cropped body, I liked the 75mm ish field of view of the EF 50 f/1.8 on a cropped body, say at museums. The 85mm on a FF body would be much the same configuration and at f/1.8, is useful for indoor situations. An EF 12 or 25 extension tube fits in your pocket and pops on the lens quickly, for a close-up / macro ish lens.

If you have a searchable image library, see how often you naturally shoot at 85mm for the two zooms you have already that cover this focal length. This might confirm your 'need' for this lens. But, it also might show a hole that needs filled, or a hole that doesn't actually exist. Below are two examples of the 85 in use, one out hiking in Colorado and another at the Graceland Museum in Memphis.

Red Rocks Park by Paul Sager, on Flickr
https://live.staticflickr.com/65535/35581748684_e9142f5869_b.jpg


Graceland by Paul Sager, on Flickr
https://live.staticflickr.com/65535/49392725661_e34109d282_b.jpg

You should buy the Chief’s 85 f1.8 ASAP. It is one of Canon’s sharpest lens if you take a look at the MTF charts. It’s fast and a perfect low light and portrait lens on a FF body. I have 5 “L” series lenses, and the 85 f1.8 is easily the equal of them in sharpness and certainly sharper than you 24-105 as well as being >2 stops faster and very light weight. $80 is 1/4 the going price of one from Canon - a great bargain. I use mine all the time. There’s nothing faster from Canon at this FL except the 85 F1.4 (which has IS) and 85 f1.2, both of which will set you back ~$1500. Both great lenses if they fit into your budget. If you want something longer and similar speed, the 135 f2L is a great lens even though it’s an older design (sells in the $800 range) and of course the 100 f2, but it’s not in the league with the 135. There’s nothing else faster in the Canon EF lineup from 100mm up except the 200 mm f2 at 6K$. Btw, the 35 f2 is another (non L) very sharp lens at ~$500.

HMM? First off, the 85 1.8 is indeed a super sharp lens in the 5.6 to 28 aperture range but is a little lacking in what it can do it terms of shallow depth of field. If shallower depth of field at greater focal lengths are what you are after then you should look at faster lenses in longer focal length ranges. To that end, I recommend you peruse all the great info. and esp. the reviews on the b&h web site. All that being said, I will send you my 85 1.8 for you to try. I will pay for the shipping your way, if you don't like it, you pay for shipping it back. If you want to keep it I would ask for $80.00 for it. As a post note, it is essentially a new lens. Just wasn't quite all that I wanted in a shallow depth of field lens. The 1.4 is much more to my liking!

HMM? First off, the 85 1.8 is indeed a super sharp lens in the 5.6 to 28 aperture range but is a little lacking in what it can do it terms of shallow depth of field. If shallower depth of field at greater focal lengths are what you are after then you should look at faster lenses in longer focal length ranges. To that end, I recommend you peruse all the great info. and esp. the reviews on the b&h web site. All that being said, I will send you my 85 1.8 for you to try. I will pay for the shipping your way, if you don't like it, you pay for shipping it back. If you want to keep it I would ask for $80.00 for it. As a post note, it is essentially a new lens. Just wasn't quite all that I wanted in a shallow depth of field lens. The 1.4 is much more to my liking!