A nifty-fifty is the first lens I ever bought, which is a familiar story for many beginner photographers. They are incredibly cheap, fast, and rewarding to use. In fact, I'd be tempted to credit the 50mm with a key role in my falling in love with the craft in those early days with my second-hand Canon 350D. It wasn't until a few years later that my curiosity for the 85mm grew to be insatiable.
The truth is, I'm no longer much of a fan of the 50mm focal length. I don't dislike it, but I haven't reached for a 50mm for a long time; it has always seemed a bit like a half-measure to me. If I want to bring in a lot of the scene around my subject, I would opt for a 35mm f/2, and if I wanted more subject isolation from the background, I would go for one of the 85mm, 100mm, or 135mm primes. Nevertheless, there are millions upon millions of world-class shots with the 50mm and it suits some photographers perfectly. Which do you prefer?
From far away, I like most 50mm because the background is more in focus and you put the person on a location.
From closeup, I like most 85mm because the background is more out of focus (more bokeh) and anyway that background (in 50mm) doesn't give you useful information.
I've been doing outdoor portraits since 1975 when I got my first Nikon FTn. It came with a 50 f/1.4. It was sharp, but I never liked the portraits I was shooting with Tri-X. The perspective of a 105 Nikkor seemed too flattened, so I ordered an 85 f/1.8. I still have it, but since then have a D850 with the 85 f/1.8 and now a Z6ii with the Z mount 85f/1.8. I'm old school I guess, but I never considered the 50mm a portrait lens, but then again, I never did full length portraits except of Brides. For those, I used the 80mm on my Hasselblad.
Simple comment, if you are doing a 3/4 to head shot, go for the 85mm. If you want full length, go with the 50mm.
Back in my younger days, shooting 120 roll film, an 80mm lens was "standard" but 120mm was commonly used for portraits.
With 35mm/FF, that became 50 (or 55) for standard, and 85 for portrait.
It's not a "rule" - it's a starting point - a habit perhaps.
The only "rule" is to choose the lens that "makes" the image you are trying to capture.
These days, this has become much easier. Because while zooms historically weren't as good as the fixed focal length lenses, mostly these days there's little to choose between them. Almost nothing, in fact.
So now we have another option - a quality zoom. And we can adjust it to suit exactly what we're chasing. In fact, this "uptick" provides a trade-off against any "downtick" from any quality difference in the use of a zoom.
Your question is which do I prefer?
Generally, the 85mm. Your own split image shows a very good reason why. The subject of the portrait is lifted out of the background better on your 85mm shot, but kind of cluttered by background in the 50mm shot. This is partly due to the shift in perspective - and partly due to the change in depth of focus. Whichever one I choose, or prefer, I do it for a reason - but as I've said, that generally favours 85mm or thereabouts.
I think that as we become more competent shooters we gravitate towards wider focal lengths for portraits. 85mm has the tremendous benefit of decluttering the background for us with a tight field of view and out of focus rendering. As we introduce more background elements we need to account for them more consciously in our compositions.
All that aside, love having the combo of an 85 and a 35. With the 50mm splitting the difference, it's no wonder people recommend it as the lens to get if you can only have one good prime.
50 so meh… it’s good all around, but between the two it’s 85 for me…