Mounting Nikkor (Nikon) lenses on Pentax K Mount Cameras
Mounting Nikkor Lenses on a K Mount Camera
I recently posted the following information on "that other" Pentax forum. I thought some Pentax users here might also be interested.
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.
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.)
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)
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