Pentax P30t & P50 - Nikon F Body Mount Conversion

[Scheimpflug]

//Edit: This converted mount has now migrated from my P30t to my P50. I have updated the thread title to reflect the change, and both cameras are covered here in this thread.

Time for another project.

Even though the advance lever is still broken, I have decided to put my P30t body "under the knife" for another operation. You see, almost my entire lens collection is Nikon F mount, but fully manual Nikon film bodies are quite a bit beyond my "hobby budget". I've been wanting to get back into film, and a while back this poor broken down Pentax P30t fell into my lap... so... what the heck. Let's see what we can do.


Here's the P30t with the Pentax mount. There are six electrical contacts and no AF drive shaft, so I believe this is the "K-A" mount.



With the mount plate removed, we have a thin ring with the mount leaf springs that simply lifts out, and we can turn the camera over to let the other parts fall out: 6 contacts and their springs, and the lens release button and its spring.


There is then the aperture sensing ring, which turns a few degrees and then lifts straight out, with two long springs to unhook from the body.


With these parts removed, we are left looking at a bare camera body with no remaining K-mount parts. The stop-down lever is still in place, but it sits far enough back that it shouldn't be a problem. It is just stamped steel, so if it does cause problems, I can just flatten it down without needing to disassemble the camera any further.



Now enter the mount donor- a Komura Telemore95 teleconverter, which had long ago been turned into a budget extension tube.



With the mount plate for the teleconverter placed over the Pentax body, I can see that the holes are unfortunately not quite spaced ideally. The Pentax had 5 holes, three of which formed the corners of a square, which I was hoping to pick up on. But the Pentax mount is a bit larger in diameter, so the Pentax holes would be right at the edge of the Nikon flange. They are very close, probably just 1mm difference in radius.


This pretty much gives me two options. The first option would be to mount the flange in the Nikon orientation, and either epoxy the flange in place, or drill four new holes in the Pentax body and find some longer screws. The Pentax body had raised flats for the Pentax mount points, and has slight recessions where these new screws would go, so it isn't quite ideal. It would however allow me to use lenses with rectangular masks without issues, as the mount orientation would be correct.



The second option would be to rotate the mount 45 degrees, and then oval out three of the holes (left, bottom, and right) to pick up the original Pentax mount points. With the way the spacing is, these screw holes would essentially all turn into slots, going all the way to the edge of the Nikon mount. I would probably just grab a small file and work inward from the outer edge, as opposed to working outward from the holes. This mount option would be more reversible, and potentially stronger, but I would lose a bit of flexibility since everything would be 45-degrees rotated.



I haven't yet decided what to do, so I'm open to suggestions.


I should also add that the Nikon mount plate is almost identical in thickness to the Pentax mount plate, so this new camera will end up as a Nikon with a 45.5mm flange-focal distance, 1mm short of the 46.5 that is standard for Nikons. Rather than fuss with a 1mm shim, I think I will just leave it this way. The lenses will focus past infinity and I will lose some close-focus ability, but that is preferable to having it the other way around.

[peterqd]

[Scheimpflug]

That's a good idea! Unfortunately, the Pentax ring is about 1.5mm, so unless I could remove 0.5mm from it that would push the mount out too far. I checked, and I would have to remove both the bayonet tongues as well as enlarge the opening to accommodate the Nikon mount.

If only I had a machine shop...

[peterqd]

Ah, that's a shame. You could easily make a ring out of 1mm sheet brass or stainless steel using an adjustable circle cutter and a drill. Cut the outer diameter first then the inner.

One other idea - I've heard that Nikon lenses will fit the Pentax mount directly. Sometimes the lens can be a loose(ish) fit and obviously it won't lock but a lot of people find it works OK. Have you tried it?

[Scheimpflug]

[Scheimpflug]

Ok, it's finished!


I decided that I didn't want to rely on epoxy, and that I wasn't going to be happy with a rotated mount. This would seem to have eliminated all of my options, and I still don't have a drill press, lathe, or milling machine, so I had to get creative.


Here's a photo of the two mounts side-by-side, to better show the differences in size, mounting, and design. They are both oriented correctly in this picture.



Next, here is my tool of choice for the modification. A thin bastard file. Yes, this is the only tool you need!



First, I marked the Pentax hole locations on the Nikon mount, and filed thin slots into the edges of the Nikon mount plate. I placed it on the camera body to check my work, and then put four of the five screws in to double-check. The fifth screw's location on the Pentax is in the same place as the latch pin on the Nikon, so I will not be able to put a screw there.




Next, I used the same edge of the file to make shallow "cuts" across the top of the mount, going all the way across the top. This is what will give the mount plate the clearance required for the screw heads.



With the screws in place, we can check the clearance. Looks good!





Time for a test fit, using what is left of the teleconverter:



Everything looks good, but the TC is a touch loose since we have not yet added a locking pin. We can temporarily work around this by spreading the springs on the back of the mount plate:



The last step is to take the flat part of the file, and gently de-burr and bevel every edge we made. This takes a while, but ensures that we won't cut our fingers or scratch the backs of any lenses. When finished, the mount is perfectly smooth to the touch.



So there we have it! In the end, I found a solution which is strong, easily reversible (for both the Pentax body and the Nikon teleconverter!), non-permanent (so I can still add a spacer if I need to), and didn't require exotic tools- all while maintaining the correct orientation for the Nikon mount!

Needless to say, I'm very pleased with the results.

[Farside]

Excelletn Penkon!

[Scheimpflug]

[peterqd]

[Scheimpflug]

The screws should be long enough to accommodate thin washers... but wouldn't that give me light leaks?

I have looked at other P-series cameras, but it is the shipping that really drives the prices up. It's the sort of situation where, if you're going to spend $25 to ship a $3 parts camera, you might as well spend an extra $10 to get a working camera shipped to you instead. And at that price, I would have tripled my investment! Can't do that!

This whole project is really just a low-budget stopgap.. something to let me shoot some film while I save up for a "real" Nikon body. Well, unless I end up liking the Pentax.

I'll definitely keep checking for a local broken P-series though. Actually, I wonder if any of the other Cosina bodies would have the same winder lever?

[peterqd]

[kuuan]

great effort Scheimpflug, and thank you for documenting it so well. I hope it will all turn out to your satisfaction.

[martyn_bannister]

[Scheimpflug]

Thanks for the kind words, Andreas and Martyn.

Since I still had the parts handy, I took a picture of the Pentax mount on the back of the Nikon teleconverter shell. Even without the modifications, I think this picture demonstrates why I wouldn't trust a lens to be mounted like this.



You can see that the Nikon mount is significantly smaller in diameter than the throat of the Pentax flange... enough so that with the lens pushed all the way to one side (as pictured here), very very little of the Nikon bayonet tabs actually hold against the Pentax tabs. It isn't rotated all the way in this picture, as that would require the modification pictured in that other forum thread... but even as it is here, it is possible to slip two of the three tabs over each other and remove the lens from the mount.

[Scheimpflug]

Also, I have determined one more useful bit of information. I had a short break the other afternoon, with some great weather, so I pulled the mount and put one quick layer of tape down to act as both a thin spacer and as a light leak prevention. It was only temporary just to do some testing, but I had forgotten that one of my lenses was an AI lens. The little peg on the side of the aperture ring snagged the tape, and really did a number on it!






So for "Round 2" of light leak prevention, I'll have to take a bit more time and ensure that no part of the tape extends beyond the perimeter of the mount. Peter's suggestion of string is another good one, and almost makes me wonder if a thin piece of soft wire insulation might also work? Or even just a rubber band?

[Scheimpflug]

[Scheimpflug]

Just a quick comment- I have edited my post above. I had initially thought that the top cover of the P50 was made of metal, based on how it had "dented" on one of the edges. After taking the top apart to repair the shutter button, however, I discovered that the cover was in fact made of plastic.



The full set of photos can be found in my shutter button repair guide:
http://forum.mflenses.com/how-to-pentax-p50-shutter-button-repair-guide-t31702.html

[Jesito]

Again an outstanding work, congrats!.
Thanks for sharing it, I'm delighted with your works

Regards.

Jes.

[Lloydy]

Interesting....................

I've got a couple of Praktica's with the bayonet fitting that might be better off with my Pentax lenses.

[Scheimpflug]

[Scheimpflug]

Ok, time to bring this back up to speed.

Note that I tried some things that didn't work. So don't necessarily think of this as a how-to guide, but rather just as a running diary of what I've done.

With the basic mount figured out, my next challenge was to solve the light leaks. In the P30*/P50, there is just one that we will have to address - a wide gap at the top of the mount casting, and a cavernous opening underneath it.



These empty spaces are provided for the assembly and operation of the diaphragm simulator. The diaphragm simulator is a system where the camera can read the aperture value from the lens via a lever, and apply this as an input to the metering circuit, all while leaving the lens aperture wide open to permit easier focusing. When you actually take the picture, the lens is stopped down to the value selected on the aperture ring.

With the spring-loaded sensing ring in place, we can see that this forms half of the light block (the sides). The original K-mount then sat down on top, against this plastic ring, and blocked the rest of the light.




Since I won't be using Pentax lenses on this camera anymore, and since the protruding lever interfered with the Nikon mount, I had removed this plastic ring. I scratched my head for quite some time figuring out how to fill this gap, and came upon a simple solution using black electrical tape.



What you see here is actually two pieces of tape. The first is a longer piece, cut to about 35mm or so in length. This piece was then rolled over itself backwards (sticky side out) to form a thin tube. The second piece was about double the length, and again rolled over itself to form a loop, sticky side out, however this time it was rolled lengthwise to form a short wide tube about the diameter of your pinky finger.

The first tube was then put into the second, and the second flattened so that you end up with a thin tube with two wide tabs coming out from the center, all sticky side out.
(My apologies for not having a picture at this step - I can make another one if anyone would like to see it.)

Now how this works - the length of the thin tube is cut such that it is a bit longer than the gap for the variable resistor portion of the diaphragm sensor - the big hole under the mount. Note that the gap is longer on one side than the other. You can then press the assembly into the gap, and it will form a banana shape, with the ends of the thin tube hitting the ends of the hole, ensuring that it won't fall out even if the tape doesn't hold.

Since the center part has two tabs on opposite ends, the bottom tab will be pressed against the variable resistor, effectively holding the top tab up against the mount. In the picture above it is sticking out from the gap slightly. When the mount is installed, it pushes this down, making a nice seal against the bottom of the mount plate.

So with just two pieces of tape, you get a nice light-tight seal of an odd shape, that will hold itself in place even if the sticky part of the tape fails, and is completely removable.

[Scheimpflug]

[Scheimpflug]

Ok, so continuing the catch-up, the cause of my metering problem was actually my fault.

I had assumed that if the aperture sensing portion of the diaphragm simulator was removed, that the camera would meter as if the lens was left wide open. The camera logic wouldn't be applying any "corrections" for stopping down, and since the lens wasn't mechanically linked, it wouldn't be stopping down for the shot. I could just use the standard stop-down metering technique of opening it up to focus, and then closing it down to meter & take the shot. It should have worked.

However, what I found was that the meter read just a bit differently with the sensing ring removed versus having the lens wide open. It was puzzling at first, but I ended up concluding that the difference was an offset to a curve, so the exposure was somewhat close at one end of the exposure scale, but way off at the other.


The solution was to put the aperture sensing ring back in, and then align it to the position that told the camera "wide open". But remember, I had removed it because it interfered with the Nikon mount... so, here folks is my very first "destructive" (non-reversible) modification.

The removable piece of the diaphragm simulator system:



... and the same piece with the lens aperture ring sensing peg clipped and filed down:



The next step was to place the ring back in the camera, but omitting the springs which had originally pulled it against the stops. This left the ring free to be positioned to any rotation that was necessary. To come up with a good reading, I used the kit zoom lens on my P30t (a Takumar-A-Zoom 28-80mm f/3.5-4.5) and my 28/2.8 Soligor on the "P50-F", adjusting the aperture sensing ring rotation until I got the two meters to agree throughout the f/4 through f/16 range. I should note that the appropriate spot was not at the end of the ring's travel... but just a bit away from one of the ends.


Once I had the meters agreeing, I fashioned another tape light seal from a sticky-side-out loop of electrical tape. This light seal now does double-duty, and also holds the aperture sensing ring in position. The ring is not really going to move without the tape, but the tape gives you a bit of extra insurance that it won't accidentally get turned when changing lenses.




With the lens mount now back in place, we can see the black plastic ring around the inside:



And of course, the final step is to re-check for light leaks. This photo is somewhat "staged", with the lights on so that I can show you the setup. But what I have found works well is to get a very bright LED keychain, and then mount an extension tube in the camera mount and shine the light down inside the tube at every angle you can. You can then turn the room lights off and look around the mount from the outside, and see where any light comes through.




No light leaks, so we're good to go!

[rkdmagic]

wow, very nice modification. I had the same idea but on K-mount lenses, not the camera.

I'd like to convert permanently (unscrew the F mount lens then put on K lens) K mount lens to use on Nikon camera. I know that I can get the adapter to try on that.

Any idea about this?

Andy

[peterqd]