At small apertures you get diffraction in the mix so you can loose critical image sharpness, wide open like Gareth said you looks sharpness too. That is why a lense that is sharp wide open costs a fortune!
Also at 300mm on a digital slr (unless an eos 5 or 1ds) you are shooting in old 35mm film terms at 450mm! You need around a 1/500 shutter speed to get rid of camera shake at those lenghts so in low light you will have a tough time! The old 35mm rule of reciprocal shutter speeds still holds true for digital and catches me out all the time.
Also at 300mm on a digital slr (unless an eos 5 or 1ds) you are shooting in old 35mm film terms at 450mm! You need around a 1/500 shutter speed to get rid of camera shake at those lenghts so in low light you will have a tough time! The old 35mm rule of reciprocal shutter speeds still holds true for digital and catches me out all the time.
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