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| Re: lens adapters for landscpape use/ real world vs. test exp... |
Just to put some numbers in.
Let's imagine a 21/2.8 lens which we use with an adapter in a different mount camera.
Let's imagine also that the adapter's thickness is a bit uneven and skews the lens inwards 0.01mm in one corner.
If we focus a subject located 3mt, away using the centre of the frame, the 'faulty' corner -being 0.01 mm. nearer to the focal plane than the centre- will focus at 3.2 mtr instead of 3.0 mtr.
With a 21mm lens open at f/2.8, even using a very small CoC of 0.015mm. the DOF will give sharp enough images from 2.3 to 4.2 mtr.
Granted, sharp enough isn't perfect but I bet that those possible small tolerances in adapters are almost always less noticeable than the inherent corner softness of most wideangles full open.
Of course if the adapter had a tolerance in the order of 0.3-0.5mm., that'd be a different question.