Let’s start at the beginning. It is commonly accepted that a prime lens in preferable over a zoom in street photography. But why? There are several good reason:

• First of all, a prime lens is smaller than a zoom and being inconspicuous is important. The camera is also smaller and fits into a smaller bag.
• Prime lenses are lighter. You shoulder/neck/back will thank you.
• Framing and shooting needs to be done quickly because in street photography a scene can be very fleeting. Sometimes you can find yourself moving your camera to your eye and before you can press on the shutter, the scene has changed, the moment is gone.
• Prime lenses are reputed to have a better image quality than zooms, although IQ (image quality) is good to have, there a few bad lenses. More importantly, prime lenses are faster : ie. they let in more light. A good (expensive) professional zoom will open to f/2.8 but  a “nifty fifty” (cheap 50mm prime) opens to f/1.8. A better quality prime will open to f/1.4 and it will let in 4 times more light than more expensive f/2.8 lens at a lots more that the kit lenses usually sold with cameras.
• Some prime lenses are very special in the way they “draw” the photo. Leica lenses have this reputation. I own the fuji x35f/1.4 and in my opinion, it has an extra something…

  

As you see the 50mm lens is a great candidate to be the street photographers friend. It is also the cheapest prime lens.

The first difficulty when starting street photography is getting close enough. When you are close, people will see you. That makes it all the more difficult to raise your camera and take a photo. The 50mm is a “normal” lens, it is wide enough to let you capture a scene and at the same time it is “telephoto” enough to let you stay far enough so you are not noticed.

The longer the focal length, the better bokeh you get when you are close to your subject. Bokeh is a Japanese word that means he visual quality of the out-of-focus areas of a photographic image. It is noticeable especially in the highlights (the bright areas) of a photo. With a lens that has good bokeh, the subject is well separated from the background and stands out well. With a 50mm prime lens open at f/1.8 or f/1.4 it is relatively easy to get a good background separation. This is not the case with the standard kit zoom lens that is bundled with a lot of consumer grade cameras.

The 50mm focal length is great for portraits as well. Be it a head shot or a head & shoulders shot, this focal length gives great rendition of faces. Beware though, it is not the focal length studio photographers use for portraits usually : a small telephoto (90mm to 135mm) is often preferred because you can stay further away from your subject while keeping a good bokeh and getting much less distortion on the face.

In conclusion, the 50mm prime lens is cheap and lightweight, versatile in the sense that you can capture a scene and grab a portrait, and you get fabulous image quality! If you are starting out in street, travel or documentary photography , you should seriously consider getting one.

A few words on equivalence…

Photographers speak in focal lengths when talking about lenses. When I talk about the 50mm lens, I am talking about a focal length on a 24×36 film camera or on a “full frame” digital camera. Most of us own a camera with a small sensor called apps-c, micro four thirds, 1 inch or even smaller. There is a “crop factor” to consider in these cases so that the apparent focal length stays the same. (That also goes for the aperture of the lens in the way it blurs the background). A smaller sensor give less blur.

• On an aps-c sensor (Fuji x series, Nikon D3000, Nikon D5000, Nikon D7000 series, Sony 6000 series etc) the crop factor is 1.5 and 50/1.5 is roughly 35 so a 35mm lens on this sensor will give you the same field of view as a 50mm on a full frame. I own the Fuji 35mm f/1.4 and I say it is my 50mm lens.
• Canon apps-c sensors have a crop factor of 1.6. Their crop sensors are smaller.
• On a micro-four thirds sensor, the crop factor is 2 so you need a 25mm lens.
• On smaller sensors there are no interchangeable lenses to my knowledge now that the Nikon 1 series is discontinued…(just a rumour so far…)