A few days ago I received the latest Canon 40mm F/2.8 STM Pancake lens. The lens runs under $200 dollars on B&H (here) and delivers some stunning images. A little dark around the edges on full frame cameras, but it's very nice and lightweight for traveling photographers. A few people have commented about using it for video, and there's focus noise that you need to account for.
If you're shooting with an external audio recorder, or even a remote wireless LAV, you need not be concerned. I'm usually aware of in camera noise that gets picked up, especially when shooting with the loud Image Stabilization motors on the 24-105mm F/4L. The 40mm F/2.8 noise is heard in auto focus and even when the lens is switched to manual focus. If you're planning on using in camera audio, then check out video.