Compare image quality of 85 f1.8 to 24-105 f4?

Currently, I have the 24-105 f4 USM and the nifty fifty 50 f1.8 for my XT. I get a fair amount of requests from friends/relatives to do senior pics, family shots and other portrait-type work (I usually go with a more candid, informal style), as well as pics of my kids . I love the nifty fifty for the bokeh I can get with it, but I also recognize that it is not the ideal focal length for portrait type work. Conversely, the 24-105 gives me some good focal lengths to work with, but with it only going to f4 I just can't get the dramatic background blur that I can with the fifty.

I have tried to read as much as possible about the 85 f1.8. From what I have read, I believe that I would be happy with the image quality from it as compared to my 24-105; and I am certain that I would appreciate the f1.8 at that focal length. I would like to hear from anyone who can specifically compare these two. I just can't afford L glass at this point in my life, but I could swing an 85 1.8, and I would like to get some opinions to confirm my suspicion that I should go that route.

not sure about the 24-105 but here's a couple shots one at f2 and one at f4 of my 85mm. I was told you can't get any better sharpness than a prime at F4-5.6 so I would think you would have better IQ than the zoom and like you said capability to open up to F1.8 EDIT: the flower in the F2 shot looks soft but only because it's out of focus, the center flower thingy is whats in focus. Compare the green leaf to the right of the flower it's in focus in both shots.

Are you on a crop or full frame?

50mm on a crop is an effective 80mm.
85mm on a crop is an effective 135mm

Of course the lens focal length characteristics are the not the same, but in terms of how far you have to stand to fill the VF with a full length portrait, the effective focal length is what's important.

Also, the 85 1.8 has min focus distance of about 3'. This pushes the min focus distance for FF cameras for tight headshots. On a crop, it would be plenty.

You can also look at the Sigma 84 1.4 or the Canon 135L. Both are about 1k, twice the price of the 85 1.8, but are both a bit higher quality.

That said, the 85 1.8 is a very highly regarded prime and considered a great bang for buck lens.

I own a 24-105 and used a 85/1.8 before buying a 100/2 for a great price.

For portraits, since you aren't worried about the edge sharpness, I think the 85/1.8 is an excellent choice on a full frame DSLR. With a bit of post processing tweak, you can get excellent color with the 85/1.8. It performs pretty well when slightly stopped down.

I was never happy with the 24-105's ability to blur backgrounds --- need a lot of space behind my subjects to get the same effect.

http://www.the-digital-picture.com …omp=0&FLIComp=0​&APIComp=1

Comparison of Sigma 85 and Canon 1.8. Sigma generally sharper.

facedodge wrote in post #14366227
http://www.the-digital-picture.com …omp=0&FLIComp=0​&APIComp=1

Comparison of Sigma 85 and Canon 1.8. Sigma generally sharper.

if you read his post you can see he mentioned he can't swing another L glass but he could swing an 85 1.8... that sigma lens is priced as an L glass if i'm not mistaken around 900 bucks so it does not compare price wise to a 350 dollar lens.

Yeah, that Sigma looks great but $900 just isn't in the budget right now. By selling some guitar gear, I can do $300 to $400, which is why the Canon 85 1.8 was looking appealing to me. It seems to be a good bargain for the price, and will be an upgrade over my 24-105 for the purposes I will use it for.