The 40mm f/2.8 STM is a lens I rather like; it makes an excellent body cap, weighing only 125 grams, and is corrected optically in a pleasing way. At about $149 with instant rebate it’s a go-anywhere lens that fits into just about anything. Might as well use it over a body cap, and it’s a very nice lens to shoot and carry. Just get one—you won’t regret having it handy.
This series is enjoyable to view, showing vignetting and bokeh characteristics as well as sharpness and the peripheral focus shift to be aware of. The 40/2.8 STM has a nice feel to it.
This aperture series from f/2.8 - f/16 includes many large crops as well as entire-frame images up to 24 megapixels.