Author Topic: Backfocus on Nikon F3  (Read 319 times)

JeffBlack

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Backfocus on Nikon F3
« on: July 03, 2020, 14:20:04 »
Hello, I just bought my wife a pristine-condition Nikon F3 with a 50mm f1.4 AI-S lens from a reputable dealer. Having put three rolls through the camera now it's clear that it is suffering from backfocus. At f1.4 the focus consistently comes out 5cm or so in front of where it's intended, and there are similar errors at smaller apertures. It focuses fine at f11 and beyond. The lens functions perfectly as I've tested it in precisely the same conditions on my D850. I'm about to take it back to the dealer for, I assume, adjustment, but before I do: Has anyone else experienced this problem, and what can the cause be? I don't quite understand how the image can be perfectly in focus in the finder, and accurate according to the distance scale on the lens, but still out of focus once it hits the film plane. Any explanations gratefully received, thanks!

Birna Rørslett

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Re: Backfocus on Nikon F3
« Reply #1 on: July 03, 2020, 14:33:36 »
Take the camera to a qualified repair shop. Your observations might be due to an incorrect mirror angle. It might also be user error, see next paragraph.

Do check you have the corret dioptri set to the finder so focus is on surface of the screen. Eiter use a screen with focusing aids (split-screen or similar) or at least have some dirt on the surface of the screen to help your eye align properly.

JeffBlack

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Re: Backfocus on Nikon F3
« Reply #2 on: July 04, 2020, 02:31:39 »
Thank you for your reply. Am using the standard K focusing screen, and the shots appear sharp in the finder. There doesn't seem to be any issue there. Back to the repair shop we go!

Steven P.

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Re: Backfocus on Nikon F3
« Reply #3 on: July 07, 2020, 20:12:26 »
A small piece of frosted mylar can be used on the camera's film plane to confirm focus.


A piece of ground glass (pulled from an old transparency viewer) works even better, if the mirror needs adjustment.


As Birna said, mirror adjustment is tricky. Make sure the mirror is not sticking or binding on anything first. The foam cushions turn into sticky goo.