da* 16-50 f2.8 artificially restricted to f2.8?

Meuh it is normal for the aperture blades to be the light restricting factor and not the internal barrel of the lens when wide open. At 50 mm f2.8 the hole should measure 17.8mm the same as any other 50mm lens so try to check that with a ruler if possible.

Your 50 - 135 should measure the same - compare the 2.

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Regards Dean - Capturing Creation
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