Can't get infinity focus with Nikon 16mm fisheye

[Stephen Pruitt]

Hi all. . .

I have exactly zero experience with fisheye lenses, but I just got a used Nikon 16mm AIS from KEH yesterday and it won't give me an infinity focus. Indeed, it only seems to focus within a very narrow range up to about 5 feet. Beyond that, the only way I can get things into focus is to stop the lens down to a tiny aperture.

All of my other Nikons focus perfectly.

I should probably add that I never did adjust the backfocus on the Nikon mount, as all of my other lenses hit focus perfectly right out of the box.

Any thoughts? This lens is totally unusable as it is.

Stephen

 

[bjonsson]

filter

filter

Doesn't that lens come with a screw in filter on the back? If it's not there, that could be the problem.

 

[Steve Phillipps]

Or yet again it could be back focus, the wider the lens the quicker you lose infinity focus if the back focus is not set properly.

 

[JohnF]

Just to clarify that this isn't a pure mechanics issue... Does the focus ring actually turn to reach infinity? Are the mechanics of the lens still moving after reaching the 5feet mark?

If the above is yes then it does sound more like a back focus issue.

Wide angle lenses have a much shallower depth-of-focus than longer lenses therefore back-focus is critical for getting sharp images.

JohnF

 

[Jorge Díaz-Amador]

Very wide lenses have very short depth of focus. If the Nikon mount on your RED or the lens itself is off, this can happen. In this case you are too far away from the image plane to reach infinity.

 

[Stephen Pruitt]

Thanks for your comments. I'll try the back focus when I get back into town.

It does turn mechanically to infinity focus. . . but it definitely isn't focused to infinity.

I believe that this lens does have some sort of an odd bayonet filter mechanism, but I don't have any of those filters.

I hope I can, indeed, get this lens to work, as the field of view is just sensational for the RED. I could really become one of my favorite lenses!

Stephen

 

[JohnF]

Good luck! If it's just a back-focus/depth-of-focus issue then it shouldn't be too hard to sort out. Though as CinemaTechnic mentioned ultra-wides have very short depth of focus so getting the BF bang on might be a bit fiddly.

Out of interest could you post some "before and after" images once you get it working? (don't bother if it's a hassle!!!)

JohnF

 

[Stephen Pruitt]

Oh, geez, John. . . before is a cloud of haze. . . but I'll definitely see what I can do. I'm really hoping this fixes the problem, because that field of view with the 16 is just terrific.

Stephen