Well, at least how I choose a lens for street portraiture. So that we are on the same page, I am talking about meeting strangers and asking permission to make a portrait. This is not about candid street photography. This involves walking around with the camera (and lens!) on a wrist strap (as I do), or some kind of a shoulder strap. Clearly a light rig is preferred, and I will photograph people from about 6 to 15 feet away. As I am trying to establish a relationship, I want to stay fairly close – at least within easy earshot. :-)
I have been following the 100 Strangers Flickr group for about two years. The posted images usually have the camera type, and the text often mentions the lens and metadata. I can say that all kinds of gear can be used, and any number of lenses can get the job done. Nevertheless, there are some common themes that emerge, and the aesthetics of the images are often dictated by limitations of glass. And being opinionated, after having photographed hundreds of strangers, I have fairly firm ideas about what makes a good lens for street portraiture. :-)
OK, so the first question is whether to use a prime lens or telephoto. This, for me, is easy: prime all the way. There are a few obvious reasons, and one not so obvious.
1. Prime lenses are smaller and lighter than telephotos with similar maximum apertures. When walking around for any length of time, this is significant. Also, the smaller lens is less likely to be intimidating to a stranger.
2. By and large, prime lenses are cheaper than telephotos. As I'll mention below, a fairly wide aperture is useful, and telephotos that open up to f2.8 become pricey.
3. In the lower price range, prime lenses tend to be sharper at wide apertures. In portraiture, overall sharpness is not always desirable, but sharp eye sharpness is hard to beat.
Now for the not so obvious reason:
4. With a prime lens, you never have to worry about what focal length to use. The decision has already been made. This is not trivial. I consider it important to have all camera-related decisions made before encountering strangers to approach. Then the mind is focused on the available light, background, and the personal interaction. That's plenty! It is counterproductive to also have to consider focal length, ISO, aperture, etc. These should be set in advance.
OK, so a prime lens, but what focal length and maximum aperture? For focal length, the camera (system) and sensor size become important. It is easiest to relate sensor size to full frame cameras, even though I have never owned one. :-) For head shots, or head and shoulder (my favorite), 85mm is considered ideal for full frame cameras. Much shorter than 85mm leads to unflattering distortion; the nose and cheeks look large, and the ears small. Longer than 85mm gives good results, but the photographer has to get progressively farther from the subject. For full frame cameras like the Nikon D600, or Canon 5D Mark III, 85mm is (arguably) ideal.
But what about cameras with smaller sensors? DSLRs with APS-C size sensors, have a crop factor of approximately 1.5:1 (Canon and Nikon differ only slightly here). I used to use a Nikon D90 with this sensor. A 50mm lens is equivalent to 75mm for full frame: pretty close. I used 50mm and sometimes an 85mm. The 85mm is equivalent to 127mm in full frame. The results are nice, but I did have to stand back a bit more than I like.
Now I use an Olympus OM-D E-M5 (who dreamed up that name?) with a micro 4/3 sensor; the crop factor is 2:1. So the 45mm prime lens is equivalent to 90mm full frame. Close enough. Other camera systems may have different sensor sizes. Relating them back to full frame and 85mm equivalent will help choose the appropriate focal length.
So, lastly, what about maximum aperture? A couple of factoids:
1. A lens with a wide maximum aperture can always be stopped down to f8 or f11. A lens with a smaller maximum aperture cannot be further 'opened up'.
2. As a general rule, the wider the maximum aperture, the larger and more expensive it will be.
So what is the best maximum aperture for portraiture? Ah, it depends on the aesthetics that you are trying to achieve and your skill/limitations in controlling the depth of field. The trick (for me) was to get a shallow enough depth of field to render the background out of focus, but not so shallow that one eye was soft. I had purchased a 50mm f1.4 for my Nikon D90. But I found that under about f2.4, I usually got one eye out of focus. At f1.4, I got poor results, and that is with the APS-C sensor. So I probably would have been just as well off with a 50mm f1.8, which is smaller and cheaper.
The full frame sensor has a more shallow depth of field, and the subject has to be PERFECTLY square to the camera to get two sharp eyes at a wide aperture. My Olympus OM-D E-M5 has a smaller sensor, so the depth of field is a bit larger, and I can shoot at f1.8. But the greater depth of field limits how soft the background can be rendered. You pays your money and takes your choice!
So, in conclusion, my not-so-humble opinion is that a good lens for street portraiture is a prime, 45 – 85mm depending on the camera sensor size, and maximum f stop 1.4 to 1.8 depending on sensor size and personal preference. Some cameras have sensor sizes different than mentioned above. The Nikon1 system has a 2.7:1 crop factor. As long as you do the arithmetic and relate it back to full frame, they can be compared properly.
Now, after all this, in the 100 Strangers Flickr group, some members use an inexpensive kit zoom lens, f3.5 – 5.6. Some use point-n-shoot pocket cameras, and a few even use phones. Anything can work, as long as you don't mind giving up aesthetic control.