Originally Posted by
John Layton
A couple of possible issues: One, if the camera's focussing screen is not original - it may have been replaced incorrectly... with a front-mounted separate fresnel lens without a proper re-positioning of the actual ground glass. There is a formula for this and when I purchased a separate fresnel from Bill Maxwell for my 5x7 he shared this formula with me so I could make my own adjustment. At any rate...this possibility, combined with a possible field curvature of your Nikkor lens (many lenses have very slightly (or more pronounced) curved fields), might conspire to give you the result you show...where the now incorrect focus plane would still allow a small area of "focus" when the lens is stopped down...due to its field curvature. Then again...the wide open result looks too similar to be fully explained by this. Hmmm...perhaps your lens' elements are out of proper position? I once was given a "defective" enlarging lens to mess with...and discovered that one of its elements had been installed
backwards! Probably a DIY repair gone wrong or accompanied by mind-altering substances.
Or...maybe you have a combination fresnel/groundglass which is plastic, and you are putting enough pressure on this with your focussing loupe and this is deforming the screen slightly? This actually happened to me when I briefly owned a Linhof Technikardan - which has (IMHO) a lethal design flaw in that the supplied plastic fresnel/screen is (or was, if Linhof has since corrected this) supported by four relatively small plates, and not by the proper recessed shelf which would support the entire perimeter. (why, oh why, did LINHOF do this?) At any rate...I found that only sight pressure with my focus loop was deflecting the screen!
Or...perhaps one of your lens cells has gotten a bit loose?
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