Sonntag, 2. Juni 2013

Conversion of a Pentax K24/2,8 to 'A'

There is a lot of great  K and M lenses around - and they would even be better if the are "A"!

Therefore I thought about a way of conversion........

Starting lens was a K 24/2,8 and to get some of the necessary spare parts I bought a cheap F35-80 at ebay.

Initially I tried to fit the F-back-ring to the K-lens. The positive surprise was: It fits! 
It is just necessary to put the right contact pins to the right positions.

To find out which are the right pins Bojidar Dimitrov's Pentax K-Mount Page is a good source. It shows that 4 pins have to be inserted and 2 positions need to be isolated (011*10). 
After recognizing that the insertion for spring loaded contact pins is much more difficult than the isolation of positions I changed my plans and decided to use the original ring as a starting point. The F-ring was used as a drilling jig only.

This should work with one contact pin and 2 holes/isolated points.

  But during the first camera trials it shows poor results:
- On Pentax P30 and Z70 the cameras recognized "A" - but apertures are  1,4...22
- On Pentax K10d and K7 the lens was not recognized at all :-(

Applying one additional hole didn't help in the expected way. After some discussions with a friend from German Pentax Forum DFN I decided that one of the holes was too much.

After soldering the excessive hole and isolated the last drilling it works: Apertures form 2.8 to 22 can be adjusted by the camera!  

The first tests on my K7 showed the expected results.

Exposure is OK at 2,8 and needs to be adjusted by minus 0,5 step for each step the aperture is closed (means F8 --> - 1,5). Without this correction it causes over-exposures the more you close the aperture. It is caused by a lack of linearity in these old lenses but I think that is something I can use.
During the tests I found that the spring loaded pin didn't like any movement of the aperture ring. Using one of the longer screws from the F-lens blocks the ring @ F22 now. 

The surprise came when I tried the lens on my Pentax K01:

Right exposure was possible only at F8 and above.
What is the reason/A hypothesis:
If you look at the K01 behavior you can see that A/F/FA/DA-Lenses have partly closed apertures during view and measurement mode depending on the light conditions. This may cause in the result that aperture values cloes to the settings during the measurement are well exposed and wide open shots are underexposed.

 If you have additional/other ideas your input is appreciated!

Update September 2013: Due to the liniarity failure and the limited advantages of the modifications I rebuild the lens now using a back ring of a defect K28/2,8. Parallel I got a manual Sigma A lens 

Therefore the K24/2.8 is on sale now!

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