Received the new Canon 40mm F/2.8 STM pancake lens from B&H today. Wow, I really like it. Sure, it was made with their STM tech for full time auto focus in video mode for the new EOS T4i, but put all that aside, there's just no other options for Canon shooters who want a super low profile lens with auto focus.
I mounted the 40mm to the Canon 5D Mark III Full Frame camera, and you can see how the edges have a slight vignette (darker around edges). I personally like the look of this, and might consider this to be a more stylized lens for full frame cameras. If you want to have even exposure throughout the image, you can always program this lens into Adobe Lightroom's Lens Correction feature. On a T4i or other cropped cameras, you will see less of this vignetting.
It's too bad Canon doesn't have a smaller mirrorless body available (yet). As you can see from the image below, when placed on to the Canon T4i, the camera is about the same width as my Sony NEX-7 + Sigma 30mm (seen on the left). There's another example of how small this 40mm pancake lens looks on a Canon 5D (on the right).
The 40mm focal length is a good distance for this type of lens, and it was a focal range Canon was missing in their lineup. It's not too close and not too wide. It's lightweight, fast, and low profile to fit into small bags for traveling.
Rokinon 35mm vs Canon 40mm. Close focal distance, major size difference.
With a max aperture of F/2.8 it's not the fastest lens in Canon's line, but the build quality is great and the price is right at only $199. I highly suggest trying this lens out, and because it takes up minimal space in the bag, there's a good chance you'll want this with you all the time. They were out of stock on initial release, but are currently available via B&H (Click Here).
Canon 40mm F/2.8 Pancake STM Lens