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I'm looking for a lens to focus on one thing, and the background is really blurred out. I also want really good quality. I was looking a 50mm prime.

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Having one thing in focus with the background or other things blurred is due to small depth of field. This is a function of aperture. You therefore want a lens that has a wide maximum aperture.

Providing a wide aperture costs money due to the larger lens elements. To keep the quality up at the wide aperture costs even more money. Be prepared to spend a premium for something like a f/1.2 50 mm versus a f/2.8 or even f/2.0 50 mm. The price goes up fast with the maximum aperture, especially when the quality is kept good at that aperture.

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  • \$\begingroup\$ Important to add is that most 'fast' lenses are not acceptably sharp at their maximum aperture. Before buying, one should investigate the sharpness at maximum aperture and make sure it is acceptable. i.e. using DXOMark. \$\endgroup\$
    – Myridium
    Commented Apr 30, 2017 at 14:31
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Buy the brightest aperture lens you can afford and, among those the longest focal length. Something like a 85mm F/1.2 produces a really shallow depth-of-field.

Third party manufacturers also offer F/1 or F/0.95 lenses which are completely manual. Those are usually 50mm lenses but there may be others. There is also a Canon 50mm F/1.2 if you really want your focal-length to be 50mm.

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If you are looking for main subject vs. background separation get a long fast prime. Blurry background is function of aperture, focal length, and ratio of subject distance to background distance.

By taking the aperture approach you run into diminishing returns pretty fast - in the Canon range there is 50mm f1.8, f1.4 and f1.2. The 1.2 is a beast of a lens, weights a ton and costs a fortune. Hardly anyone owns one (but all Canon shooters should be required to carry the plastic f1.8).

By taking the focal length approach the costs don't rise as quickly. You might run into close focussing issue, an extension ring will help you with that.

If you feel adventurous try a old medium format lens on an adapter. 180mm Carl Zeiss Sonar in Pentacon Six mount is a good candidate. It is not expensive, has nice subject separation and interesting color rendition.

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