Street photography is the photography of street life, be it people, animals, architecture, nature, or public spaces where they all combine. Street photography is built on basis of candid photography, which is defined as subjects having a unposed appearance often with a lack of acknowledgment to the cameras presence.
Cameras like the Fujifilm X100T and Ricoh GR are modern classics in the street photography scene. With full frame focal lengths equivalent of 35 and 28mm, they sport focal lengths that are widely accepted as the best for street photography.
With such wide focal lengths, it is essential to get physically close [compared to standard and tele. lengths] to your subject if it is the focus of your composition, especially if you are trying to capture an emotion through a portrait.
Getting close without breaking the candid relationship with your subjects can be extremely difficult. A skilled street photographer will find the composition, adjust settings, get close, frame the shot, capture the image, and move on all in a matter of seconds and without being noticed.
It's the fast pace of street photography that can sometimes lead to dutch angles, slightly blurry shots, and improper exposure. While unacceptable in a studio situation, these elements add to the charm candid nature of street photography.
What is a Candid Image?
Being that street photography is so closely related to candid photography, let's take a closer look at the meaning of candid photography. Here are some excerpts from the Official Nikon Nikkormat Manual (1977).
"So much of the best 35mm photography today is unposed, that the term 'candid' has almost lost its meaning. However, we'll limit our discussion of candids to those shots you are attempting to take in a natural surrounding with your subject almost or entirely unaware of your intent."
"Try avoiding confusing backgrounds. Make use of your depth of field to throw confusing backgrounds out of focus."
"Wait for peak expression or peak excitement to occur before shooting"
"If you intend to shoot many candid shots, investigate the 85mm f1/.8 Nikkor Auto, which is a lens with exceptional speed and sharpness and can make close-us from some distance."
So now it seems we have a bit of conflicting information. The best focal length for street photography is widely accepted as about 35mm, but the essence of street photography is based on the candid, which is most easily achieved by a telephoto length.
Perhaps it's best to settle somewhere between. Say a nifty fifty (50mm lens)? Like most of photography it's a little more art than science. So there is no official best focal length for street photography, the best focal length is what works for you, but 28mm to 85mm is a great place to start.
Street Photography with A Telephoto Lens
Taking the idea of introducing telephoto lengths to street photography a bit further, I wanted to personally explore the candid in a most unobtrusive way. Completely leaving wide angles in the dust, I chose a (Canon FD) 200mm lens on a Sony A6000, giving an equivalent full frame focal length of 300mm.
With an 11 mile walk through the city, I was able to capture a small glimpse of the most iconic and diverse street life of Manhattan.
Advantages and Disadvantages of Using a Telephoto Lens for Street Photography
Using a large telephoto lens for street photography presented several new challenges and advantages. Firstly the physical size was a challenge. It's no secret you are taking photos. Though it is not easy for a bystander to pinpoint what is it you are gazing at. Additionally, with the reach of a telephoto lens, extreme changes in lighting are easy to come across with a simple pan of the lens. From blinding sunlight to a heavily shaded stoop, these situations are not new to a street photographer, and are ones where a camera with a wide dynamic range will prove its value.
The biggest advantage of using a telephoto lens for street photography is of course the optical reach of the lens. With the narrow field of view of a 200mm lens, even standing across the street may be too close to capture a head-to-toe image.
Being further away from your subjects make it much easier to maintain a candid relationship. This also lends the ability for the photographer to take their time waiting for a peak moments of interest, in a way that wide angle street photography often does not.
Conversely though, putting distance between you and your subject can make it difficult to get a clean shot in crowded areas. And with these distances the subject is more in control than the photographer. If you are about to capture a portrait and your subject turns around completely, you may have to walk half a block to get on the other side of them to mimic the composition you intended to capture but just missed.
Looking back at the idea of street photography and the results from shooting it with a 200mm lens:
With its narrow field of view, a 200mm lens captures very compress images, that is your foreground and background appearing to be compressed together. A 200mm lens is commonly accepted as yielding 'unflattering' results when taking pictures of people. Though the ability to take your time composing images [unnoticed] is a clear advantage of using a telephoto lens for street photography.
Considering the pros and cons I think there may be a sweet spot in using an 85mm to 105mm lens. These two focal lengths are telephoto lengths that still support a realistic and flattering amount of compression when photographing people, and put enough distance between you and your subject that you may get a bit more time to wait for the moment of peak excitement.
 Amphora Editorial Board. Official Nikon Nikkormat Manual. Garden City, New York: American Photographic Book Publishing Co., Inc. 1977
Technical note: Other than slight cropping and some highlight adjustments, these images are mostly as-is from the camera. Shot on 'Soft High Key' picture setting which gives a low-contrast look similar to that of the RAW video from a Black Magic Pocket Cinema Camera (BMPCC).