The great thing about street photography is that there are so many different ways of doing it. Some people stick with a rangefinder with a 35mm or 50mm lens. Some use mobile phones. Other prefer using a longer lens or telezoom on a professional DSLR. Whichever lens you decide to use affects the process and ultimately the look of the photos you take. A conscious choice of lens is a conscious choice of the style you choose to shoot and the scenes you want to portray. But there are also many practical reasons for choosing one lens over another.
Primes versus Zooms
It could be that all you have is the kit zoom which came with your camera. Zooms are fine for starting out, they give a flexibility in framing, reduces the need for lens swapping, and maybe all that you have. If you’ve shot a bit with your zoom, you can have a look at the focal lengths you’ve used more often. I found when I did this that the wide end was dominant. I preferred to get in close and take a more intimate photo of the subject, or else take in a larger scene incorporating greater context.
Lots of street shooters tend to work with single focal length lenses (so-called prime lenses). There are practical and psychological reasons for this. Practically, single focal length lenses can be made with larger apertures for low-light work, and also for greater background blur. On conventional rangefinder cameras, primes are your only choice (Fuji X-Pro1 excepted or a Leica M digital, or Leica M with Visoflex). Psychologically, it’s one less decision factor to take care of when making photos. A single focal length can act as an artistic constraint forcing you to concentrate your creative energies on positioning, lighting, framing, rather than zooming in and out of a scene.
Focal Length Choice
So if you do decide to shoot with a prime lens, then it boils down essentially to whether you want to use a wide, medium or long lens. It’s tempting when starting out to use a long lens as this allows you to stand back and “pap” (act like a paparazzo) people from afar. It is tempting to keep away from your subjects if you are not confident but the perspective often has the effect of being impersonal. Also, a large telephoto lens does make you stand out more, especially if attached to a big DSLR. A short tele, e.g. 85mm on film, makes a good portrait lens though so could do double duty.
A favourite lens is the 50mm (~35mm on APS-C, ~25mm on M4/3), the so-called normal lens. It’s gives an in-between angle of view, neither very much telephoto or wide angle. This can make it either versatile or boring depending on how you look at this things. It’s popular partly due to the fact that large aperture 50mm lenses are relatively cheap compared to other focal lengths. Cheap 50mm/1.4 manual focus lenses are easy to come by. Even AF versions of normal lenses aren’t terribly expensive. It’s probably a good place to start, if you find yourself stepping back to capture a scene, or wanting to crop in a lot to isolate an individual, then you can step up or down in angle of view by choosing a different focal length lens. But if you can live with tweaking your frame by stepping a little bit back or forwards, then the normal prime may be good for you.
Going wider (but not especially wide), the 35mm (~24mm on APS-C, 17mm on M4/3) is also a very popular focal length/field of view. It gives a moderately wide angle of view, but not obviously so. It allows approaching the subject a bit closer with the same approximate framing as a 50mm, or else capturing a usefully larger scene at the same distance.
Even wider lenses can be used for effect, 28mm (~19mm on APS-C, 14mm on M4/3) or even 24mm (~16mm on APS-C, 12mm on M4/3) allows really close approaches to the subject. This suits a fairly intimate style of candid photography best done in crowded scenes where even though you’re within a metre or so of the subject, they do not notice you taking photos.
Autofocus versus Manual Focus
A lot of the time, the choice is driven by necessity. Cheap manual focus lenses may be all that one can afford. AF versions of desirable focal lengths and apertures may not be available. You may be using a conventional rangefinder camera. One is forced to use manual focus lenses in these cases. Luckily some AF cameras have focusing aids for manual focus. On SLRs, you may be able to get a focusing screen optimised for manual focus, usually with a split prism. On LiveView cameras, many offer a magnification function blowing a region of interest allowing fine focus. Other cameras also offer focus peaking, a coloured outline of areas with high contrast usually indicative of in-focus regions. Using manual focus lenses for street photography is an acquired skill, it can take a bit of practice to use it effectively.
In some circumstances, AF may be more appropriate. It is usually faster, allows those snaps shots and reduces the time you have the camera up to your face and the possibility of alerting your subject. AF allows completely unseen operation, from the hip for instance. In bright light, with manual focus you can approximate the results by using a small aperture and “zone focusing” but this becomes generally infeasible in low light.
Your Own Style
After trying different options, you may find that you are drawn to a particular “look” given by a particular lens used in a particular way. My advice is to explore the range of possibilities in order to find your own personal style. Just because many street photographers use a rangefinder with a 35mm or 50mm lens doesn’t mean that this combination necessarily suits your own photography. I can imagine someone being attracted to using telephoto lenses up close for ultra-intimate candids (though I don’t personally recommend it, I will not be held responsible for any arrests), or using ultra-wides or fisheyes for taking vistas. The important thing is to make it interesting, engaging, and compelling.