Mounting Nikkor Lenses on a Pentax K Mount Camera

A while ago I modified some Nikkor prime lenses for use on Pentax K mount cameras, both the film and digital versions. There is a description as well as some photos.

A Few Preliminaries

I have adapted four Nikkors. All are prime lenses. Three of the four are non “AI” Nikkors.

(A bit of history. From 1959 and the introduction of the Nikon F, Nikkor interchangeable lenses had an aperture coupling prong – the “ears” on the rim at the rear of the lens. On pre-1977 cameras, it was necessary to "index" the lens to the camera's meter each time a lens was mounted. This entailed rotating the lens to its largest aperture and back. This action ensured that the meter was properly calibrated. In 1977, Nikon introduced Automatic Indexing [AI] linking lenses directly to the meter without having to dial in aperture information. Nikon accomplished this by adding a coupling ridge on bottom of the aperture ring. Older, non-AI lenses could be modified to be AI either by Nikon or many independent camera technicians.)

With the one AI lens I have modified, I removed the coupling ridge on the back of the lens. I did this as the ridge prevented the lens from seating flat on the Pentax camera’s lens mounting surface.

My first attempts to modify Nikkors for use on a Pentax simply involved trying to insert the three prongs of the Nikon mount into the three receiving notches of the Pentax mount. This generally worked but there were issues. These were:

1. The amount of rotation was very limited – about 1.5cm or .5 inch. The lens always seemed to be at risk of falling off if the camera were bumped for instance.
2. The Nikkor locking notch did not line up with the locking pin of the Pentax mount.
3. On some of the Nikkors, the back of the lens – the aluminum section which surrounds the rear lens element, has the three mounting prongs and is screwed onto the lens body – is flush with the back of the aperture ring. On other Nikkors , the back of the aperture ring is higher than the aluminum back. This leads to problems.
4. The distance between the back of a lens and the film or sensor plane of the camera is known as the registry distance. On Pentax lenses (both the old M42 as well as the K mount lenses) the distance is 44.6mm while on Nikkors it is 46mm. The good news for Pentaxians is that Nikkors mounted on a Pentax will focus at infinity without needing any optics to change the plane of focus. Because of the different lens registries, however, the distances marked on the Nikon lens from closest focus to infinity will be incorrect. Accurate focus at infinity will be achieved when the marked distance is well short of infinity.

Before I realized that there were differences between the backs of Nikkors as noted above, I thought the solution to creating a notch for the Nikkor lens to mate with the Pentax body was simple. Just drill a small hole in the aluminum back at the appropriate spot and the Pentax pin would mate with the hole and hold the lens in place. This action worked for the first lens I modified (the Nikkor-P 105mm f2.5) but, on the other three Nikkors, the protruding back of the aperture ring prevented the locking pin on the camera from mating with the drilled hole. The pin is not long enough.

I discovered a second problem caused by the protruding back of the aperture ring on some Nikkors. The distance from the aluminum lens back to the camera’s lens mounting surface prevents the necessary electrical shorting of the Pentax’s lens information contacts which is necessary for “catch-in-focus” shooting.

My Solution

Upon careful examination of various Nikkors I thought of a fairly simple solution to all of the issues mentioned above. A thin metal ring attached to the aluminum back of the Nikkor extending from the ridge below the mounting prongs to the inner edge of the back of the aperture ring would solve two issues. Namely, to correct the registry difference between Nikon and Pentax; and, create the electrical shorting necessary to permit catch-in-focus. Further, if a notch were included in the metal ring that lined up with the Pentax locking pin when the lens was mounted, the lens would be held much more securely in place.

I chose to have the rings made from stainless steel 1mm thick. The outer diameter of the ring is 5.6cm (56mm); the inner diameter, 4.7cm (47mm). There is also a locking notch which is 2mm wide and 3mm deep. (See photo.) I decided to use a glue to hold the ring in place as this would not require making any changes to the Nikkor and thus allowing its restoration to Nikon use if desired. (See photo.)

Further Observations

In various online discussions I have read about using Nikkors on K mount Pentaxes, there have been differences about where the “top” of the lens should be. Given that there are three ears on the lens and three notches on the camera, the Nikkors I am familiar with can be mounted in any of three orientations. The aperture information can be set at roughly the 10 o’clock, at 2 o’clock or at 6 o’clock positions when facing the camera.

I much prefer the 10 o’clock orientation. (See photo.) That puts the aperture information on the right hand side of the camera as you are taking pictures. It is thus on the same side as most of the controls including the LCD panel. For me this “off center” lens orientation is preferable to it being at the 12 o’clock position. The built-in flash overhang makes seeing aperture markings difficult.
Some may have noted that the stainless steel ring I have chosen is only 1mm thick while the difference between Nikkor and Pentax lens registry is 1.4mm. My experience is that the added 1mm ring results in the viewfinder manual focus indicator lighting when the object is at infinity and that is also where the lens focus ring is set. I suspect the explanation is threefold. There is some tolerance in the camera choosing “perfect” focus. The cutting of the inner and outer ring diameters in the sheet of stainless steel added slightly to the thickness. The glue has added a bit of thickness as well. In any event, the 1mm thickness of the ring provides me with excellent focusing results.

Finally, some care is needed in setting the ring and getting the locking notch in the right place. Initially, I simply noted where the notch should be by mounting the lens and marking the position of the pin on the lens barrel. I failed to consider that the extra thickness of the lens with the ring would prevent the lens from turning as far when mounting. Set the ring slightly short of the mark where the lens would mate with the pin without the ring.

Hope this proves useful.

(In addition to the photos of the stainless steel ring, the glue and the Micro-Nikkor mounted on my K20D, there are shots of the following Nikkors with the ring added:

Nikkor-O 35mm f2.0 [2 photos]
Nikkor-S 50mm f1.4
Micro-Nikkor-P 55mm f3.5
Nikkor-P 105mm f2.5)


Richard

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Attached Images



View Picture EXIF
PENTAX K200D Photo

[Kayaker-J]

Photos to accompany the modifications to Nikkor lenses.

It appears I can't figure out how to add the photos. If you'd like to see them, please let me know how to do this.

Many thanks.

Richard

Yes -- Someone, please assist Richard here. I really want to see these "how-to" photos! Sorry I can't help with the technical details, Richard, as I'm in the same boat in that respect. But I do have quite a few AI, AI-S, and multicoated "K-Nikkors", + some other pre-AI's; and I am starting to use Pentax (plus Oly. m4/3) and not Nikon for my foray into digital photography... after a long , long layoff from the hobby/profession. Specifically, I see the 24/2.8, 35/2.0, 40/2.0 CV Ultron SL-IIn, 105/2.5, 135/2.8, 200/4.0, 50-135/3.5, 70-150/3.5 Series-E, and 100-300/5.6 (classic "bread and butter" models) to be of prime interest [groan...] in my own case. Thanks for your contribution! -- Fred

[jadedrakerider]

Very interesting. Where did you pick up the metal ring?

[rrodgers]

I had the rings custom made. I have a brother-in-law who works in a metal shop so I gave him the specifications as noted in the original post. These are: fashion from 1mm thick stainless steel. The outer diameter of the ring is 5.6cm (56mm); the inner diameter, 4.7cm (47mm). What the cost would be I can only guess - maybe $15-20? The unit price would likely drop if you had more than one manufactured. Richard

[UncleVanya]

I had the rings custom made. I have a brother-in-law who works in a metal shop so I gave him the specifications as noted in the original post. These are: fashion from 1mm thick stainless steel. The outer diameter of the ring is 5.6cm (56mm); the inner diameter, 4.7cm (47mm). What the cost would be I can only guess - maybe $15-20? The unit price would likely drop if you had more than one manufactured. Richard

Do you have a CAD file or a maker file for these?

[rrodgers]

I do not have a CAD or any sort of template for the ring. I simply measured where the ring would go and gave the inner and outer dimensions along with the required thickness of the stainless steel (plus the size of the notch) to my brother-in-law. He took it from there. He gave me the impression that it was pretty straight forward.

Richard

[UncleVanya]

I do not have a CAD or any sort of template for the ring. I simply measured where the ring would go and gave the inner and outer dimensions along with the required thickness of the stainless steel (plus the size of the notch) to my brother-in-law. He took it from there. He gave me the impression that it was pretty straight forward.

Richard

Did you post the inner and outer dimensions and the dimensions of the slot? If I overlooked it I apologize,

[rrodgers]

In the original post I indicated, "the locking notch is 2mm wide and 3mm deep. (See photo.)"

If you have other questions or concerns, please let me know.

Richard

[yucatanPentax]

Did you post the inner and outer dimensions and the dimensions of the slot? If I overlooked it I apologize,

This is in the original post under My Solution:

I chose to have the rings made from stainless steel 1mm thick. The outer diameter of the ring is 5.6cm (56mm); the inner diameter, 4.7cm (47mm). There is also a locking notch which is 2mm wide and 3mm deep. (See photo.) I decided to use a glue to hold the ring in place as this would not require making any changes to the Nikkor and thus allowing its restoration to Nikon use if desired. (See photo.)

Anyone with a CAD program could readily create two concentric circles of the specified sizes and place a notch of the size indicated anywhere along the way. Or even just cut the notch with a Dremel or a file once the ring is cut.

[UncleVanya]

Thanks. Late nights lately, just missed it when I looked for it.

[Lowell Goudge]

I have a couple of observations here.

If you are already attaching a ring with glue, I am not sure the lens is "restorable" to original with any ease. But any way.......

You need to be very careful with glue around lenses as the solvents or outgass img products of many adhesives attack lens coatings. For me, this is a serious risk, but it is easily avoided. Just remove the lens mount from the lens. And then work with the glue away from the lens. You might think this makes the mount impossible to attach again, but that is a simple solution. From the opposite side of the mount, drill new pilot holes, using the screw holes as the guide, and then drill the larger counter sink holes from the mount side. Again, since you are away rom the lens the operation can be done, and parts cleaned so everything goes together clean,

There is, of course another option, but it requires having an assortment of rings of different thicknesses. Simply replace the Nikon mount with a ring, plus flanged K mount M42 adaptor. Glue it all up, and drill and counter sink the mount, using the original mount as a drill template. The only possible problem to encounter is on some lenses, the mount also holds part of the aperture mechanism, this was the case on my nikkor-H 300/4.5. I had to attach to the mount a small lock to accept the aperture return mechanism, converting the lens into a manual aperture lens. But it all worked.

I have also converted a kiron 28/2 and 24/2 this way.

The only thing you need to consider is to measure the thickness of the Nikon mount you remove, and replace it with appropriate rings to make the correct final regestry distance. Errors, if made, should be on the short side, so the lens focuses just a little past infinity. This is preferable to not getting to infinity

[Sliver-Surfer]

I did this up. Something like this may work without gluing. Tab for lens, divot for camera. I might have to 3D print this to see

[UncleVanya]

I did this up. Something like this may work without gluing. Tab for lens, divot for camera. I might have to 3D print this to see

That's an awesome idea. I would love to have the maker file or whatever format you end up with if you do.

[Not a Number]

There's always the PK+Multi-Mount
Pentax K+ Multi-Mount - Adaptist

[UncleVanya]

There's always the PK+Multi-Mount
Pentax K+ Multi-Mount - Adaptist

Certainly true but this is simpler if the only adaptation desired is the use of the Nikon lenses that can be adapted this way.