Originally posted by Na Horuk Not sure about 645, but if they are the same as K-mount, then A setting surrenders the aperture choice to the camera. So you can choose the aperture number manually, but the camera won't know what you chose! And then you need to use Av mode (which will not stop down the aperture to your choice, but will shoot wide open. Note that auto ISO or any more other than M defaults to Av), or M mode - where you can use the green button to let the camera stop down meter and select the shutter speed.
With A series lenses, you can push the little lever on the aperture ring and choose A mode. Now the camera has full control over the aperture. So you can choose the aperture on the camera, use Av mode, P mode, and so on. The camera can now continuously meter and even write the aperture into exif. Basically, if your lens has A on aperture ring, you should just always keep it in A.
(note: I speak from experience with K-mount, 645 could be different, so feel free to look for threads on using manual lenses)
You can use the aperture ring on the 645D with no limitations, since the mount isn't "crippled". Even away from A, the camera will know what the aperture value is. However there isn't much of a reason to use the aperture ring except in very specific circumstances.
Adam
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