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I shot tens of thousands of
birds in flight photos from 1999 to 2014. Mostly swans, geese and ducks. I started using Nikkor manual focus prime lenses on Nikon bodies. Transitioned to AF Nikkor telephoto primes.
Shoot thousands of birds in flight with a MF lens, and your skills in time will greatly improve and so will your keeper rate. The sharpness and IQ of an in focus image taken with a quality MFlens is just as good as the IQ of an in focus image taken with an AF lens. HOWEVER, when I switched to AF after years of MF experience, my keeper rate went way up.
The quality of your images and your keeper rates are largely a function of how much money you spend on your equipment. I had Nikon equipment that was moderately priced. Where I shot BIF there were many other photographers (even some National Geographic ones). Some shooters I got to know personally shot the best and most expensive Canon cameras and prime telephoto lenses. Their rigs were much more expensive than mine. They took slightly better images and they had higher keeper ratios than I.
It's not only the lens, but also the body it's on that dictates how well a lens will perform for fast action and BIF (buffer, etc.). Shooting fast, one always uses the continuous mode (AKA machine gun type shooting). The more images you can capture in a burst, the better chances of obtaining the shot where the subject in the perfect pose. For example, a flock of birds would fly in front of me. I'd shoot continuous mode with my Nikon equipment and get 10 - 15 images. The guys shooting top of the line Canon gear next to me would get many more photos (maybe 50 or more) during that same pass than I would.
I can't comment on the Pentax DFA 150-450 with the new PLM motor in it. Maybe it's the equal to the best from Canon. I don't know and I suspect few would have extensive experience with both.
But from over 10 years of shooting tens of thousands of BIF photos next to my professional Canon shooter friends this is what I've learned. The top of the line, expensive AF Canon primes on the best Canon bodies have proven themselves over the years when it comes to fast action sports and birds in flight. I don't shoot birds in flight seriously anymore and even recently sold my Nikon system so I don't even have that anymore to shoot BIF with. But, if I had unlimited money to spend, I would absolutely go with the top of the line Canon equipment to shoot fast action and BIF. While the DFA 150-450 very well might be adequate or even superior, I don't believe it's got the years of experience proving itself like Canon lenses have.
P.S. Just as important as the lens is, the body may be even more important. No Pentax body can shoot in the continuous (machine gun) mode as fast for as long as the top Canon bodies can. Also, there's been much written about how fast and accurate the Canon auto focus system is. Conversely, much has been written about shortcomings of the Pentax AF system. You can research this further if you choose.
I love my K1 bodies and Pentax in general. But no one camera maker is best at everything. When it comes to BIF and super fast action, there's a clear winner between Pentax and Canon and it's Canon. Heck, Canon is even better than Nikon's best when it comes to super fast action.
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