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I recently bought a Nikon Df camera. The auto focus point is sharp in photos, but when I focus manually with the same lens and settings the photos are not as sharp as photos shot in auto mode.

I have tried this with two other Nikon lenses and I get the same results. Although I am far sighted, I can manually focus the microprism with a +2 diopter with ease. I don't understand why the inbuilt electronic rangefinder is accurate, but when I focus manually with the microprism it is not.

I hope someone can help.

mattdm
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MICHAEL
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    The original Nikon Df ground-glass does not have focusing aids like a microprism ring or split image field. Can it be that someone has replaced the ground-glass and inserted the new glass incorrectly? – jarnbjo Oct 30 '19 at 15:25

2 Answers2

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A possibility is that your ground glass isn't positioned correctly in the viewfinder.

As far as I understand it focusing is really focusing on something which is by design as the same light-distance(*) from the lens as the sensor or film.

In you camera, the focus on the AF sensors is correct, so they are at the same same distance as the sensor, so the mirror and the secondary mirror are likely OK. The only other distance that can go wrong is the ground glass, and this would means it has shifted slightly.

(*) Ie. including mirror bounces...

xenoid
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The diopter should be set so that the alpha numeric data projected in the viewfinder is sharp. It should not be based upon an image in the viewfinder because the setting is then dependent on how sharply the image was initially focused.

Assuming that's not the issue, another possible issue is the use of wide aperture lenses. There are three paths of light in a DSLR, and each has it's own aperture restriction. For the viewfinder path the aperture is typically between f/2-2.8 and if you are using a wider aperture lens you cannot see the shallower DOF of the lens. And that can make precise placement of the focus/DOF more difficult.

The third possibility is a misalignment; probably of the focus screen as has been mentioned. It could also be the main mirror, but that would almost certainly affect AF as well.

Steven Kersting
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  • After reading all your replies I went online and checked the specs of the DF focusing screen,it describes it as being a BRITEVIEW CLEAR MATTE SCREEN and that's not what I am seeing in the view finder,so thank you gentlemen for your help,I will take it to Nikon and have the original screen installed. – MICHAEL Oct 30 '19 at 21:51