Canon joined the ranks of manufacturers that produce pancake lenses, small, fixed focal length lenses that have a length of a couple of inches from your mount at most, in 2012 with their 40mm f/2.8 STM. I picked up this lens as my first non-kit lens, and it's well worth the money. Read on to see why.
Measuring just 22.8mm (0.897") thick and 68.2mm (2.685") across, the 40mm f/2.8 STM is just over half as thick as Canon's slightly cheaper 50mm f/1.8 II. With a suggest retail price of $199, this two-hundred dollar lens sports a metal bayonet and a quiet stepping motor, good for video, over the 50mm f/1.8 II, which has a similar focal length, but with the added cost savings of a plastic bayonet and a conventional motor, which tend to be rather noisy.
Build Quality: 8.5/10
There isn't much to the pancake lens. The plastic barrel is solid, but one would expect it to be solid, being so thin. There are no issues with the mount, and the AF/MF switch gives solid feedback. My only complaint with this lens' build quality is that my copy's electronic focus ring gives a little when you wiggle it. Other than that, there's not much to break or fall apart given the pancakes small stature.
Whether I mount this lens to my full-frame 6D or my crop sensor T2i, focusing is quick and accurate. The stepping motor gives a little bit of whine, but nowhere near the amount of noise that a conventional focusing motor produces. I'd relate the sound to a very quiet printer from the 90's. I can hear the focusing motor in videos, but that can be easily corrected by using an external mic.
Wow, this lens is a performer! The front element is miniscule compared to almost any other Canon lens, but don't let that fool you. Sharpness is phenomenal, as well as contrast. Aberrations are kept well under control, and saturation isn't overwhelming. This image was taken with the pancake on my T2i:
On a crop body, you can expect a 35mm equivalent focal length of 64mm, which is a little tight for a walk-around lens, though the lightweight 40mm might just make it on your camera as a walk-around lens because it's fun to use!
On full-frame cameras, expect a bit of vignetting wide open, but if you have a 5DmkIII, 6D, or 1Dx, there's lens correction data available.
There's virtually no distortion, and any that pops up is taken care of by correction data in post.
At $199, this lens is one hell of a bargain. Only $74 more than the $125 50mm f/1.8 II, the 40mm f/2.8 STM gives you all of that extra cost. If you've got the money to spare, give the 40mm a shot. If you can't quite afford the pancake yet, save up another $75 and get it. Seriously, I have no idea why people are still buying the 50mm f/1.8. It's only 1 1/3 stops faster than the pancake and is more difficult to work with. Though, I guess they don't call it the plastic fantastic without reason.
I'm not rating this section, but I'll include it. Bokeh produced by the 40mm f/2.8 STM is ok. Slightly hard edges detract from it's appeal, and a 7-bladed aperture keeps things smooth when stopped down. Love it or hate it.
With an electromagnetically controlled aperture, aperture changes while recording are smooth, unlike some of Canon's lenses (the 35mm f/2 I have comes to mind). The electronically controlled focus motor makes focusing smooth and easy, however, the lack of image stabilization poses some issues if you are shooting handheld. A great performer.
Final Score: 45/50 = 90%
All in all, the 40mm f/2.8 STM Pancake lens is one of my favorites. A low price tag and competitive optics make it an easy decision to buy.
There's not much I can say. Buy this lens and try it yourself. It'll be worth it.