400 mm focal length is 400 mm focal length on any camera. The sensor is about 2/3 of the size of 35 mm film, so the picture looks like a 35 mm film shot cropped to show the center 2/3 and then magnified to the size chosen.
I cannot comment much on the FA400 (other than I'd love to try one
), but I do have a DA*300 and a Sigma 150-500 HSM zoom. The Sigma is a big, heavy beast to carry but does work well. Note that this one
does have HSM. I do not find that it beats the DA*300 and a Tamron 1.4x TC for IQ, but it is pretty close.
Here are a few bird shots with the DA*300 on the K20D (no TC)
Western Meadowlark
Not sure who
Bald Eagle (the zoom would have been nice - I underestimated wingspan while waiting for takeoff
)
Killdeer
And with the Sigma
Marsh Wren (one of our tinier birds) - Sigma at 340mm
California Quail - Sigma at 370mm
Western Kingbirds - Sigma at 400mm