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Originally Posted by prostie1200
My Nikon TC's the 1.7 and 2 work on all the AFS lenses I use from the 70-200AFSVR through all the Primes from 200 and the AFSVR 200-400 from max aps of f2.8 - min f32 giving full support on AF and metering regardless of the aperture setting.
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That's because you may have set the aperture on the camera to take the picture at f32, but when the camera is taking its metering readings and performing AF, it will have the lens wide open, not at the final intended aperture.
If you have a Depth of Field preview function try this experiment:
Set the aperture to f32 using aperture priority or manual mode.
Look through the viewfinder without using Dof preview.
Press the DoF preview button, watch the viewfinder go dim, very dim.
The viewfinder screen is normally viewed with the aperture wide open, the camera performs AF in this mode. When the DoF preview button is pressed, the camera stops the aperture down to the intended final setting so that you can see the resulting DoF in the viewfinder. When taking the picture, the camera does not actually stop down the aperture until the shutter release button has been pressed. If the camera varied the aperture directly as the aperture setting was adjusted on the camera, it would be possible to see the DoF varying as you made the adjustment, it would also be clear that the brightness of the image in the viewfinder was changing as you varied the aperture. Neither of these effects are seen when not using the DoF preview function.
When performing AF and letting the camera change its focus point, you do not see the brightness of the image in the viewfinder vary, nor do you see the DoF vary, this is because the AF is performed with the lens wide open. If you stick on a 2x TC to a lens that has a max aperture of 6.3 it is unlikely to AF unless the subject is exceptionally bright and contrasty, the max aperture just isn't enough for the AF sensor to work with under most circumstances. Unless you have an extremely bright subject with lots of contrast, AF will struggle if presented with a smaller effective aperture than f8 when wide open.