Regarding blocking the viewfinder..I use an external finder (the Zeiss 25/28) . But honestly I wasn t looking thru the viewfinder at all for many of the photographs taken with the 21. For example ....I saw the 2 dogs coming and guessed there path. I focused on a spot on the ground . I lowered the camera to my shoe tops and waited ...when the dogs crossed the spot I took the picture. Same with the picture of the girl with the RED umbrella...taken from waist high..prefocused on a spot on the ground. None of the 1st five images where framed through the viewfinder.....even the eiffel tower...I braced the camera on a ledge ...took a few to guage the composition and then settled on what you see. I look down at the camera to see thats its level and line it up .
I will use the external finder to frame the scene and then anticipate the location of the subject.....and then watch carefully for the moment.
But to answer your question the 21 didn t bother me when focusing or when composing with an external finder. The only thing I wasn t thrilled about was the size(length) of the 21/1.4 .....I still have my 21/2.8asph and used it at the 4th of July celebration where I had exceptional light .