Hi there,
I recently purchased a N-AF 2x Teleplus MC7 from Kenko to pair with my cheap Sigma 70-300 f/4-f5.6 lens. I did this knowing full well that my lens and ancient D60 is more than capable of focusing at /f11 and I took 90% of my contrails at f/11 without the Teleconverter so I assumed (bad idea) all would be grand. I also stumbled across this video: https://www.youtube.com/watch?v=1qQDLR6p77o Which indicates to me that AF would work well with my new configuration. I finally got to test my new Teleconverter to and much to my disappointment the lens would just continue to hunt with no end. I'm just wondering what is restricting me from getting a half decent focusing situation and if getting a new body would help in anyway.
Thanks for your help. Jamie.
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Re: Ground - Air Photography with a 2x Teleconverter @f/11
The reason why you were able to take shots at f/11 without the teleconverter is because the lens was set to its biggest aperture before you took the shot. Without the teleconverter at 300mm and f/11 dialed in on the LCD on your D60 the lens is actually at f/5.6 before you press the shutter and when you do it stops down to f/11 before opening back up to f/5.6. Most camera bodies can only AF at effective f/5.6 while some higher end bodies can AF at effective f/8 but I'm not aware of any that can AF at effective f/11. A 2x teleconverter takes away 2 stops of light so an f/5.6 lens becomes an f/11 lens which is too slow for the camera to be able to AF as there is not enough light reaching the AF points to allow it to AF. When not using the teleconverter you can AF and take a shot at f/22 because your 70-300mm will be at f/4-f/5.6 depending on your focal length. For example if you're at 70mm then you can be at f/4 at maximum and if you're at 300mm then you can be at f/5.6 at maximum which is fine for the AF system but when you add a 2x teleconverter it becomes an f/11 at the 300mm end which pushes it beyond the AF realm. So again if you're not using the teleconverter and want to take a shot at 300mm and f/22 then the lens will be at effective f/5.6 before you release the shutter as at that aperture the camera will AF as it has enough light coming in. Then when you release the shutter after getting AF it will immediately stop down to f/22 before opening back up to f/5.6 after taking the shot.
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