Nikon F4 focusing "blurb" in viewfinder

[Åsali]

Hi!
I have struggled a bit with my Nikon F4 with a DP-20 attached. The camera is beautiful, and works flawlessly- but when looking through the viewfinder I feel that I need to place the eye in an exact position, or else the center spot could seem a bit blurry.
I checked everything and cleaned very bits and pieces. Then I finally found the issue. If I detach the DP-20 and look up, I can see a microscopic line in the prism where two mirrored planes meet. I guess that's the point where the image gets from inverted to the right way around. This line is perfectly in the center... and it is this line that make the center spot a bit blurry if my eye is aligned over this.
Ive googled several times, and I can find some people describing the same issue... but those post are old, and I guess trying to contact them is no point.

Is this familiar with somebody in here? Is this normal for the F4 and DP-20, or is this a malfunction? Everything else is superb, and I really can't see if the DP-20 is damaged or anything like this.....

 

[Christiaan Phleger]

The line is just the center "corner" that formed the pentaprism angle, that's not the issue, which is most likely the Diaopter Adjustment optics, I would fiddle with that first to check with your own eyesight and then to see if that adjustment shows the results of either an impact damage or incorrect reassembly after a sloppy service. If the previous owner shot without an actual eyepiece on the DP-20 then dust and fungus gets into the that diopter adjustment and then it needs a CLA. It is possible with reasonable skills to get into there to clean it but there is one element that is really easy to re-install flipped and then you get exactly like you described, good decent focus but only in the center and straight on. Proper is your can see all about the screen clearly, and with me even with glasses. If your not comfortable with self-inflicted repair then see a pro, APS in Illinois has done good work on Nikons and would know how to fix.