Trouble with manual focus 300mm F4L IS at long distance.

I have recently been trying to get good shots of some Ospreys, Swamp Harriers and other large raptors in flight at distances of between 200 and 500 yards.

I am using the 300mm F4 IS plus 1.4X converter and I find it almost impossible to use auto focus on these birds in flight so I have gone over to manual focus.

My problem is, I set the lens on infinity and use F8 and around 500sec but the focus is all wrong, what am I doing wrong.

What is infinity on this lens with the 1.4X? and am I missing something important.

Any suggestions gratefully received.

Are you sure you're focusing on infinity and not BEYOND infinity? Lenses can focus beyond infinity to compensate for changes in temperature, or for shooting infrared film, which requires a different focus plane to get things in focus.

I have the lens set right on the start of the infinity "L" not past it but even so does not the DOF of F8 compensate.

Here's your answer, Chris: The birds you are shooting are just too far away. I shot Bald Eagles last winter with my 300 f2.8 +1.4 combo. In this case the birds were approx 100 yards away and sometimes more. While it may have focused, the resulting pictures were basically useless. Even though I was shooting with the 1DsMKII, a 16 mp camera, the Eagle occupied maybe 1/20th of the frame, if that, so less than 1mp of the frame was the bird. Once viewed at 100 percent, the pics just sucked. I learned here that some things are just too far away for the lens to work well. It did come as a surprise.
Here is an example. And this is 100 percent the resulting size:

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I don't understand , why are you focusing on infinity? 200-500 yards is not infinity on a long lens. Like Mark just mentioned, if they are but a dot in the viewfinder = so small you cannot focus on them, you need to get closer or use a longer lens.

Mdjak nailed it.

Thanks folks I suspect that you are right.

Thanks for your help, it would appear that patience is the answer.

This is the best I got at about 250 yards.

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If it makes you feel any better, manual focusing on bird's eyes does take a lot of pratice. Even with the best equipment.

I think that this is a fact of life with wildlife shooting! There is always something that is just farther away than we'd like! I still like to take the shot, though!

Thanks, I see a 500mm Sigma 4.5 in the equation as soon as my wife leaves her credit card lying around.

Learn how to get closer. Always a good solution.

Manually focusing based on the distance scale is a bit imprecise, especially with today's autofocus lenses with relatively short scales (except the 85/1.2 which has a very long vernier focus scale). It's better to do so with the eye through the viewfinder, but that is also difficult when trying to track a moving object.

I find autofocus superior for anything that is moving or is small in relation to the overall size of the viewfinder.

This is about the best I got at about 400 yards with a 2X Kenko converter 600mm hand held manual focus but I deleted a lot of fuzzy ones (and I clipped his bloody wing on this one)

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