Pentax P30t & P50 - Nikon F Body Mount Conversion

Scheimpflug · Expire: 2011-11-18

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. Wink

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 · Expire: 2015-01-07

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

Regards.

Jes.

Lloydy · Expire: 2022-01-01

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

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

Scheimpflug · Expire: 2011-11-18

Scheimpflug · Expire: 2011-11-18

Ok, time to bring this back up to speed. Wink

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. Wink

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. Shocked

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. Cool

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. Cool

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. Cool

Scheimpflug · Expire: 2011-11-18

Scheimpflug · Expire: 2011-11-18

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

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. Wink

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. Shocked

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. Neutral

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. Cool

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

rkdmagic · Joined: 10 Dec 2011 Posts: 1

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 · Expire: 2014-01-04