Originally posted by Anvh But 54mm f/2.8 is very different with DOF compared to 300mm f/2.8. You actually need something around 54mm f/0.5.
You are correct, it's different, but instead of this being a negative as you seem to imply, it's actually a positive, IMO. At this FOV, the deeper DOF of the Q with a 54mm f2.8 wide open is more useful since the very thin DOF of an actual 300mm f2.8 on an APS-C or 135mm format body is something of a handicap. At MFD (6 feet for a typical 300/2.8) wide open, the DOF is about 1/4". Even at 20 feet, the DOF is under 2", and that's with the deeper DOF of an APS-C sensor compared to a 135mm format body. For a small bird facing me, I try to get everything from the beak to the feet in reasonable focus. This takes at least 2" of DOF, and at 10-12 feet, even f11 gives you only about 2.5", and getting the 4 stops back with high ISO usually isn't possible because of detail loss to noise. Also at f11, APS-C is getting into the area where diffraction starts to become noticeable.
Here's an example with the K20 with my Sigma EX 300 f2.8 APO at @ 8 feet. f4.5, 1/200. I was shooting handheld, so needed as high a shutter speed as possible, and i was already at ISO 1000, my limit for that camera. Only the plane including the eyes and chest are reasonably in focus.
Here's one that illustrates the feet OOF, which is something of a pet peeve for me. This one was shot from about 12 feet with my DS and Tamron SP 300 f2.8 with the F 1.7x AFA. f8, 1/80 ISO 400.
It's not unusual to come across situations where the wide apertures are needed for bird photography. Shooting at Ev 12 (heavy overcast or open shadow on a clear day) at ISO 100, f2.8 requires a 1/125 shutter speed. For me, this is about as slow as I'd want to go for birds or wildlife since they don't seem to realize that they should stand still for the photographer
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The Q at 54mm and f2.8 gives about 4" DOF at about 6 feet -- allowing shooting wide open with some DOF room to spare. Even at 10 feet, it's about 11", which still allows for good subject isolation unless the bird is standing right in front of a wall.
Here's a Q shot with the FA* 300 f4.5 wide open from @ 20 feet. Notice how the whole bird is in focus, yet there's decent subject isolation from the background, and the small Sparrow still more than fills the frame despite the distance.
For fast ultra tele (300mm and longer) and macro work, deeper DOF can result in significantly better images. For these, I'll trade some detail loss from the smaller sensor for subjects that are more completely in reasonable focus anytime.
Scott