First, the focal length is different. If you wanted to take a head and shoulders shot of someone, you'd have to stand further back from the subject with the 85mm than you would with the 50mm.
Second, the 50mm is slightly better in low light situations than the 85mm, though both are excellent. Both lenses are outstanding lenses (although not in the same league as Canon's L-series lenses). Both are very sharp with excellent bokeh (background blur) and work well in low light. Hope this helps.
I own both lenses ... and depending on whether you have a digital full frame camera body (like the 5R) with a 1:1 lens ratio (same as a 35mm film camera body) or the more popular small sensor camera bodies (XT series or 20/30/40/50D) series giving a 1:1.6 magnification to your focal lenghts ... may determine your choice ...
My opinions are bassed on use with the 40D and the original Digital Rebel (300d) - both 1:1.6 aspect camera bodies
The 1:1.6 aspect found on the most popular camera bodies gives the 50mm lens a focal length of about 80mm and the 85mm an focal length of about 102mm ....
The 50mm is a slightly better low light lens - but very few shots I take are used wide open The 85mm is tac sharp - and is comparable to L glass (imho) ...
Both are good overall - - the 85mm an awesome piece of glass - but 102mm is not ideal for some portait work ...
both offering desired bokeh -
There is no 'better lens' -depends on your intended use
With the 85mm coupled to a 1:1.6 aspect camera body - you would indeed find yourself backing up to capture many shots ... also the face flattening - often used by portrait photographers (to enhance subject faces) is more pronounced in the 85mm ... again a matter of taste ... some prefer the 50mm ...
Unless you have cash to just drop and a stop you problably wont often you I highly recomend the 50 1.8 as opposed to the 1.4 I have the 1.8 cuurently and have borrowed my friends 1.4 and other than the stronger and thougher build of it I found no real benefit of the extra stop. Most my shots were above f/2. Now that states my opinion between both 50 1.8 and 1.4
Now, between 50mm and 85 1.8 its a different ball game. 85mm is of course more pricey but it is just an INCREDIBLE lens. If you want something for sports or somewhere were you need reach this is a must have lens. It creates a nice blurr background and super nice for portraits. Good for sports if you want a prime lens and for a situation where you have space to backup.
For something like a social event where you can walk up to your subject a 50mm might be a better choice.
If you must have just one (assuming you dont own either) I say the 50mm is a must to obtain first. Then look into the 85mm down the road. you will NOT regret it, trust me.
For someone just starting and hasnt made their decision yet, the 50mm 1.8 is an AMAZING lens for VERY cheap. I would recommend getting that first, you will be very happy. Its one of my favorite lenses for indoor/outdoor family activities like birthdays. For outdoor/indoor sporting events, I think you will want something closer, like the 100mm/f2 or the 135mm/f2.8
I got the 85mm f1.8 a few weeks ago for my son's indoor volleyball games. I love the lens. The focus is fast and I can get pretty good shots even in the dim gym lighting. I went back and forth between the 50mm f1.4 and this lens. For me the length of the 85 has really helped when taking pictures from the stands. If I could stand court side all the time the 50 would be nice - but most refs don't like that.
If you really need the extra stop at f1.4 then go for the 50 but so far for me f1.8 has been fine. In some of the better gyms I can take pics at f2.0 or 2.2 and get a little sharper image and still keep a pretty fast shutter speed.
My two cents would be if you're just starting out and in the market for a prime lens, go with the 50mm (possibly even the 50mm f/1.8). The optical quality is gorgeous and it's an excellent choice for portraits. The 85mm is a bit more specialized and is more ideal if you have a full-frame sensor. On a crop sensor (such as the Rebel series or 40D, etc) you will constantly be backing up to fit your subject in the frame, at least for your average indoor shooting. That said, they both perform extremely well with regards to resolution and bokeh and will open up your photography to a new level of quality.
I have the digital rebel xsi....I want to photograph my daughter at gymnastics meets, and this lens was recommended (the 85mm) since no flash is allowed during the event. Of course I'd love the expensive lens, but as a single mom--no can do! This is pricey enough for me, but what do you think?! Thanks for your help! jana
Mom of 3, The 85 f/1.8 or the 50 f/1.4 are ideal for indoor sports. You should not consider anything smaller than f/2.8 aperture. You may also want to consider the 100 f/2, although I have never used this lens, the reviews say that it is a great indoor sports lens, and about the same price as the 85 if you need the extra length. The 50 is a little less expensive, but may be to short for what you need. Try the kit lens first and see what focal length you use the most and then decide. All three should work great for what you would like to do with them, and as a bonus they are all great for portraits, especially the 85, wonderful bokeh. JMHO :)