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Sharpness - Bokeh
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But for a background not to call the viewer's attention, it is not enough that it is out of the depth of field and therefore blurred: it is best if that is a special kind of blurring, particularly unremarkable.
We say that a lens has good bokeh if it generates pleasing out-of-focus backgrounds that don't call for the viewer's attention, are "soft" and "creamy". Conversely, we say a lens has bad bokeh if it generates nervous backgrounds, full of artificial detail.
Not great bokeh (Carl Zeiss Jena Pancolar 50mm f/1.8)
Exceptionally good bokeh (Leitz Elmarit-R 50mm f/1.4, set at f/1.8)
And we have to distinguish quantity and quality of bokeh:
* we have more quantity with higher aperture
* we have more quality with some lenses than with others
Quality of bokeh is a very important characteristic of a lens (specially for portrait lenses, like an 85mm) to which most people pay too little attention, but which is often the reason for a pro to spend five times more on a lens that, in every other aspect, is not really better.
It is always best to get quantity and quality, but, if that's not possible given your budget, you usually can just live with lots of mediocre bokeh. For example, the canon 50mm f/1.8 is a cheap and badly built lens (just holding it you can feel its low price), but it is very sharp. What sets it below its bigger brothers,the 50mm f/1.4 and the 50mm f/1.2, is that the f/1.8 is a bit slower (not much) and has a pretty bad bokeh. But, being f/1.8, it will usually deliver better results than a zoom with a better bokeh but smaller aperture, so it is the main portrait lens for a lot of people using APS-C cameras with a tight budget.
A lot of not-great bokeh (Carl Zeiss Jena Pancolar 50mm f/1.8)
Very little good bokeh (Canon 18-55 f/3.5-5.6, puesto en 50mm y f/5.6)
Basically this bokeh thing is a matter of the lens not generating artificial detail in out-of-focus backgrounds, and a quick but imperfect way to look into this is examining out-of-focus highlights: if they are homogeneous and soft, without artificial elements, bokeh is good; if they show several rings, colored borders, bright center, etc, then bokeh is bad. Also, though this is not always the case, it can be bad if the iris in the lens (the mechanism that controls aperture) is not circular, but pentagonal or hexagonal (because, away from maximum aperture of the lens, out-of-focus highlights will inherit this shape, and its repetition can generate aritificial detail; this problem is generally avoidable by using the maximum aperture of the lens).
These are my bokeh tests at f/1.8 with the CZJ and the Leitz lenses used above:
The rest of my tests can be seen here.
Most lenses with good bokeh are expensive, so the usual compromise is to live by with fast primes, which have lots of bokeh, even if it is not of great quality. There are two cheap alternatives that deliver good bokeh, but they both require manual focusing: some vintage lenses (like most of mine) and Samyang primes (you can read more about them in my equipment recommendations, for Canon and for Nikon).