Would the 40mm pancake or 50mm prime be better for street photography with a Canon APS-C camera?

You already span the range 18-250 mm with the lenses that you've already got. They are zoom lenses however and there is a big advantage with prime lenses - their fixed focal length can with some practice be known by heart.

To know how the picture will turn out before even looking through the viewfinder is a huge advantage when already facing a situation that from a photographers point of view can be as short as an blink of an eye. By using a prime lens you can put all your effort into nailing the focus and composition and not having to worry about also getting the focal length right. This can be accomplished with duct tape and a zoom lens, but primes generally have larger apertures which you benefit from both in auto focus properties and low light situations. Also their one focal length only design allows it to be optimized for that focal length and almost certainly will give you higher quality than a similarly priced zoom lens.

The 40 mm pancake lens is an excellent choice in street photography in the sense that you won't look as intimidating using it compared to larger lenses. It's also rather cheap.

You didn't specify which 50 mm you where thinking of, but I assume the 50 mm f/1.8 considering what other gear you're using. I can't say anything else than that it's a great lens that (considering it's price) and that you certainly will make good use of it as a street photographer. As noted below its focusing speed in not the best and it can result in not locking the focus before the opportunity already has already passed.

Otherwise all reasonably fast auto focusing primes in the range around 35-50 mm are great options. You are using a cropped sensor camera though so I would try to lean towards the shorter end of the spectrum. Wide angle lenses (shorter than around 30 mm) are an alternative, but they tend to be rather expensive.

Only you can answer which is best for you.

The EF 50mm f/1.8 II or EF 50mm f/1.4 give wider apertures that allow better images in low light conditions if you can handle the narrower depth of field that comes with wider apertures. And sometimes that narrower depth of field allowed by wider apertures is highly desired. The f/1.4 is faster in terms of auto-focus than the f/1.8.

Update: The newer EF 50mm f/1.8 STM introduced in 2015 is now my recommendation over the much older EF 50mm f/1.8 II (and often even over the more expensive EF 50mm f/1.4). There's not much difference between the two in terms of cost and optical performance. The newer STM, though, corrects a lot of the older lens' shortcomings: the AF is faster and more accurate, the mount that attaches to the camera is metal instead of plastic, and there's a usable manual focus ring that allows manually focusing without needing to move the AF switch on the lens to the manual focus position.

The EF 40mm f/2.8 STM gives a wider field of view but losses two full stops of aperture to the 50mm f/1.4. That could be the difference between a usable 1/100 second or a hopelessly blurry 1/25 second in low light.

I second that it depends on your style of shooting -- is your taste and eye more towards wide shots or close-ups? What helped me decide was to take my 18-55mm and shoot a few hours with it fixed at respectively 28mm, 35mm, and 50mm. Having said that, 50mm to me does sound a bit too long for street on a crop sensor.

I own both the 50mm 1.4 and the 40mm 2.8. From my point of view both lens are fantastic in terms of image quality and build quality.

Autofocus of the 50mm is a little bit faster, but not to much. The difference in aperture is not to underestimate. Big advantage of the 40 mm is the unobtrusiveness, weight, the price and the better macro capabilities.

If I had to decide for one in street photography I would choose the 40mm, but as Michael, Hugo and mortendahl (+1 for all) have said: only you can answer the question.

For me, street photography is often about taking my camera when I might otherwise not bother. It's kind of opportunistic, rather than premeditated.

For that reason I would think the 40mm pancake would be the way to go.

It keeps the physical size of the camera right down, and with a small front element (not sure if we're comparing to 50/1.4 or 50/1.8) it's pretty discreet. Compared to the 50/1.8 it's a bit quieter, focuses a bit faster, and more easily fits in a pocket/smaller bag. Unless you're shooting at night, then f/2.8 will be plenty (I rarely shoot below f/2.8 on my 50/1.8 anyway).

Both of those lenses are rather fiddly and slow. For street people are not going to pose for you. So you need something you can prefocus and stays there, ie. internal focusing. If you want auto focus, preferably FTM (full time manual).

If you are shooting candid, you might want to get closer with a longer focal length as well. 50-85mm is adequate.

If you are going to shoot at night, and that's where the most interesting shots are to be found, you need F1.4-1.8.

Here a 50mm prime F1.4 full manual lens (Pentax M 50mm F1.4)

Trying to do it with a F2.8 std zoom 17-50mm:

These were a few photos that worked that night, while the 50mm 1.4 manual lens had mostly keepers.

You are on the right track with primes, but I agree with M Clark that you should look for F1.4, which on top of better light transfer also comes with better mechanic designs to help you in this particular shooting style.

Within the two lenses you are looking at, I suggest 50mm F1.8. In that budget you can also get a vintage prime lens with chipped adapter.

You might find this inspiring, it was what made me try it out:

Lens review for street photography

I used to shoot a lot of street, and it's still my favourite type of photography to do for fun. I started with a 35mm lens on an APS-C body. That was already a bit too long, so I got an X100 which was perfect at 23mm (35mm Full Frame equivalent). I recently got another APS-C body with a 35mm lens, but ended up having to get the 23mm as well since I prefer the length.

50mm would be much too long for me, although I could see 40mm being good. As others have mentioned, it being a pancake would be very convenient to carry around.

A good way to find out what length would be good for you would be to use your 18-55 zoom lens for some street, then go home and look at the EXIF data to determine what the focal length was for your favourite shots. In Lightroom, hit the "I" button on your keyboard while having an image open to show different information. I only have experience with Lightroom, but I'm sure other programs can show it, too.

You also might find out that you prefer shooting with a zoom lens. I have a friend who shoots excellent street photos with an 18-200 or something on his Nikon. I only shoot with primes, but that's a personal preference.

I wouldn't worry too much about focus speed. A common technique with street photography is to zone focus to about 2 meters, then set the camera to f/8. Everything between 1 to 4 meters or so will be in focus.

Of course, all of this depends entirely on what your personal style is, which you may still be developing. Again, go out and experiment with your zoom lens!

You may use as follow:

1. 40mm STM(pan cake). for day street photography

2. EF 50mm f/1.8. for night street photography

The human eye is somewhere between 35mm and 50mm. Street photography masters like Henri Cartier-Bresson built their entire careers on 35mm and 50mm because these focal lengths resemble the human eye.

My preference is 35mm. For APS-C bodies with the 1.4x crop factor, this means getting a 24mm lens. I like to optimize for light and composition. Most of the real estate is reserved for non-human elements. I find 50mm to be a little tight for my style. If you shoot a lot of candid photography and portraits, go with the 50mm, or 35mm for APS-C bodies.

Here's a good example of a decisive moment seen by the human eye, captured with a 35mm lens (24mm for APS-C):

The weight is also hugely important, because it is the difference between having your gear with you or not. And you won't be taking any photos if you don't want to lug your gear with you. The Canon EF-S 24mm f/2.8 STM is a cheap and tiny lens to try. I wrote about how carry convenience impacts street photography here.