The problems I have with the M 400/5.6 are a direct result of user error in 99.99% of the cases. It does produce PF and CA in abundance under some circumstances, but it can still come up with some very nice images. This is a 1500 px (25%) crop from the K-3. The image was taken through a double paned patio door which is not the cleanest at this time of year. The snow drift outside it is 1 m (3 ft) deep, making it a bit difficult to get to it to clean it.
Exposure was at ISO 800; 1/500 @ f/8. Shutter speed was set by green button so Mr. Pentax would have applied centre weighted metering to the feeder. Post processing was to slightly increase exposure for detail in the head, remove green and purple fringing and slightly reduce the noise in the dark bird's feathers. Given a bit more light so I could reduce the ISO to 400 and stop down to f/11, this would be a much better image. It would have helped a bit if my focus was tighter also.
The minimal depth of field can be seen with the beak out of focus slightly and the eye almost perfect.
[EDIT] I just used my DoF Pro App and the depth of field of 400mm @ f/8 from 30 ft away is from 6" !!!! No wonder I have focus problems with the old girl. The feeder is 10" from front to back. I said most of my problems were user error