Thursday 18 June 2020

Hacking my Pentax K to M4/3 lens adapter

Lens adapters that allow you to connect different format lenses to camera bodies can be very handy.

But they ideally need to be the correct length to maintain focus range.

I decided to try to hack an adapter that is just 1mm or so too short.

As described in this earlier post, I now have two lens adapters that allow me to use my old Pentax K lenses on my Olympus m4/3rds camera. Both have an effective length of 25mm, which is just over 1mm short of the ideal length of 26.21mm.

In use, this deficiency reduces the useful focus mechanical adjustment range, although it is normally still possible to focus successfully within the range of the lens. It also means that zoom lenses will not stay in focus over the full zoom range.

the hack

I decided to try using washers under the lens mounting face plate. This plate is easy to remove by slackening the 4 fixing screws and being careful not to lose the pin and spring mechanism which locks the lens in place.

 Although I had some washers that were about 1mm thick, I decided to start by using a smaller set with a thickness closer to 0.5mm. The diameter of these washers is far too large to fit in the narrow width of the face plate, so I used my Dremel with a grinder attachment to reduce the washer width.

Its a fiddly job getting the washers in position, so I used clear Vaseline to hold them in place and a small kebab stick to align each one over the associated hole.

This washer needs a bit more taken off each edge

Once reassembled, I was surprised to find the focus distance scale on my Pentax 28mm lens was almost perfectly aligned. Likewise my Pentax 80-200mm zoom, which also stayed in focus when zooming between 80 and 200.

I seem to have lost my vernier calipers, so I measured the effective length of both adapters as best I could.

This is the unmodified Gobe adapter which appears to be just over 25mm

This is the modified K&F which is just about 26mm

When my new vernier calipers arrive I'll update these dimensions as its difficult to be accurate using a steel rule graduated in 0.5mm steps. But I'd guesstimate that the K&F adapter length has been increased by about 0.8mm.

I wondered if the increase may be partly due to the washers not sitting flat under the face plate, so I did a double check by simply standing the Gobe on 4 washers...

The unmodified Gobe sitting on 4 washers to check the washers influence on adapter length

...but the length looks very similar.

stuff you might want to worry about

It should go without saying that if you attempt to modify these or any other lens adapters, you are on your own. Don't come crying to me if something bad happens ...I'm not your mummy!

In attempting to hack these adapters, I have created at least three potential problems;
  1. However much of the fixing screw threads were in contact with the adapter body, there is now about 1mm less of it. It would be tragic if the face plate came off and my old lens fell and crashed to the floor. I think I may use thread-lock to help keep the tiny screws from slackening off.
  2. If the adapter was weather proof before this mod, it certainly won't be after adding spaces between the body and the face plate.
  3. Similar to 2, you may find that light is now finding its way in behind the lens.
To check for light leaks, I set ISO to maximum, set shutter speed for longest exposure and fitted the lens cap. Then I used a Manfroto bright LED light to check for leaks.

Yep, it lets in the light.

Although light is clearly getting into my modified adapter, it doesn't appear to be a problem for me under most conditions;
  • when outside in the sun, my ISO setting will generally be low and shutter speed high
  • when lighting is less bright or even poor, although my ISO may be higher and shutter may be slower, the ambient light is generally much lower
  • it may be a problem for astro-photography due to moonlight, street lights & so on
I might use some kind of sealer/sealant between the adapter body and the face plate to block out the light. But for the moment, this hacked adapter works fine for me.

Edit 20th June 2020:

My new Moore & Wright vernier calipers have now arrived, so here are a few more accurate measurements.

The effective length of these lens adapters is the distance between the metal surface that mates with the guest lens, and the plastic surface that mates with the camera.

I now have another sample that Gobe kindly sent me. I'm referencing my original adapter as Gobe #1 and this new sample as Gobe #2. My K&F Concept adapter has been modified by adding 4 washers which I now know are 0.7mm thick.

Effective lengths:-
  • Gobe #1: 25.3mm
  • Gobe #2: 25.5mm
  • K&F (modified): 26.1mm
...and just to compare the focus setting for my Vivitar zoom length when correctly focused on a distant object (infinity);

left: using unmodified Gobe #2, right: using modified K&F Concept adapter

So these Gobe & K&F Concept adapters "as sold" are only 0.7mm too short, which doesn't sound like much, but it sure makes a difference.


  1. Thanks for the report, but just to clarify: The K&f, being 0.7mm too short, does not let you focus to infinity? Or is it not even noticeable?

    1. Hi Chris. Take a look at the last pair of photos for my Vivitar lens. The left image shows the effect of the "0.7mm too short K&F". Yes it will focus on infinity, but the scale on the lens shows the range to be 6 or 7 Feet (2m approx).
      So yes, having an adapter like the K&F which is a bit too short will allow you to focus any lens at/on infinity, but the useful range of the lens scale is cramped from shortest setting to the 'new' infinity. Beyond this point (beyond infinity) the lens wont focus on anything).
      Most users don't seem to worry about this.
      I hope this makes sense.

  2. Can I ask you how noticeable in focusing objects that are more far away is the 0.7mm too short of the K&F Concept?

    1. ...because the 'useful' focusing range is reduced, the focus ring is more difficult to set (it needs a finer touch). For example, I struggled quite a bit focusing my Pentax 200mm zoom. But this has about twice the magnification when fitted to a micro four thirds camera as compared to a Pentax dSLR.
      I think these adapters (Gobe & the K&F) are well made for a £20 accessory, and there is no guaranty that buying a more expensive one would be any better. I guess its safer to make them slightly too short and know that users will always be able to focus up to infinity, than it is too make them the right length and run the risk that some production units might just a little too long, meaning they wont allow users to focus on infinity.