Minolta MD,MC Mount to Pentax K Mount conversion

Guide to converting the legendary Minolta Rokkor PG 58mm f/1.2
1) Introduction

The Minolta MC/MD register distance is about 43.72mm, whereas the Pentax one is 45.46mm. The difference between them is about 1.74mm.

Required things:
- A M42-PK adapter (~7$)
- Some screwdrivers with small diameter.
- A drilling machine with 2 drills: 2mm and 3mm.
- A file.

2)Disassembling the mount

There are 8 screws, we need to unscrew the 4 long ones, because the 4 short ones hold the diaphragm actuator with the mount.

File off the ridge around the outside of the aperture ring.

In this above photo, the ridge were cut off.

Continue to unscrew the 4 screws of black ring.

Rotate the aluminum ring in counterclockwise direction. Because we don't need the mount spacer which have a thickness 4mm. Therefore we must adjust 4 - 1.74 = 2.26mm in height by rotating the ring.

3)Reassembling the new mount:
After rotating, we re-screw the black ring.
Drill the 4 holes respectively on the M42-PK adapter.

Note that: the red dot on the adapter will the same position with the diamond shape

Place the aperture ring in its position with the ball ring. Beware of the ball ring. It is very small, Do not lost it.

Re-screw 4 long screws on the new PK mount.

Voilà, the conversion is done.

We can do the same way with Minolta Rokkor-X 50mm f/1.2. This is the later version with MD mount. This lens has not the spacer mount, therefore we must file the lens cover as the image.

I converted the Kiron 28mm f/2 as the same way of the Rokkor-X 50mm. Its focal length is smaller, so it needed to file more precisely in order to focus to the infinity.

Here some results of Kiron 28mm with infinity focus:

Click on the image to view it in a larger size

and close focusing:

How to convert the K-M lens to KA lens

I believe that every Pentaxian like to have KA lenses than K,M lenses because of some benefits:
- You can set the aperture on the camera with KA lenses instead of setting the aperture on the K,M lenses. That allows less manual operations when shooting.
- Multi-segment/matrix metering
- The aperture can be displayed in the EXIF of your photo
...

Mark Robert has presented how to enable matrix metering mode for the K-M lens, but this method can work only with the non-digital camera. He believes that the digital data pin transmits the aperture information, therefore it does not work with Pentax DSLR.

I will present how to convert my Pentax-M 50mm f1.7 to KA lens. There consists of two steps:
- Modifying my 50 1.7 lens to KA lens.
- Modifying my K100D. Both can undo if you don't like this

To modify my lens, I follow the principe of the Robert's method (For more details, please read the article of Mark Robert) The max and min aperture of the lens are : f/1.7 and f/22. Therefore the arrangement of insulator on my lens will be 101*11, where 0 represents the position of the insulator. In Robert's method, he drilled a small hole in order to make an insulator.
Instead of drilling, I use a corrector to paint on the "0" position which have a distance from itself to locking notch about 2.2 cm (0.87 inch)

But my camera still does not recognize the "new KA" lens. Because the "A contact" of the lens does not touch the "A contact" on the camera. So I need to modify my K100D:

- Unscrew 5 screws on the camera mount
- Put the aluminum foil under the ring mount, cover the "A contact" in order to make it conducting.
- Re-screw the camera.

Voilà, my K100D can recognize the "new KA" 50mm 1.7. From now, I can activate the matrix metering mod, set the aperture on the camera, have the aperture information in the EXIF....

With the code 101*11, the exposure is not correct. Therefore I use the corrector to change easily the code to 111*10, corresponding to the lens f/1.4-f/22.

@ f/1.8 (I cannot set f/1.7 on the camera)


@f/2.2


@f/4


@f/5.6


@f/8


@f/11


The new result are good. The aperture f/16 and f/22 are not used frequently, so I don't test them here.

An other benefit of A-lens is to get the proper exposure with PTTL flash. With M lens, the PTTL flash always fires full power. I post some other images taken with built-in PTTL flash (sorry, I have not enough money to buy a true PTTL swivel flash )

@1.8


@2.2


@4


@5.6


@8


From f/11, the background will be darker.
@11


@16


@22