Saturday, June 20, 2009

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.

VoilĂ , 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)






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 )






From f/11, the background will be darker.



1 comment:

  1. I have tried this but there is a problem.

    On a K-M lens the displacement of the lever is proportional to the aperture. But on a KA lens the displacement of the lever is proportional to the *square* of the aperture.

    So if you set the maximum and minimum apertures correctly with the Roberts method you will get correct exposure at f1.7 and f22. But in between these the lens is expecting a linear relationship, while the camera is assuming a squared relationship. So the aperture opens too much and the exposure is too great - it is worst in the middle of the range, around f5.6.



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