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05-16-2015, 08:03 AM   #1

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The K mount

I just have to know if there is any specific reason why Pentax put the lens contacts poking directly through the mount surface of the K mount? I can't think of any other mount that has this, as the lens contacts seem to be inside the mount on Canon, Nikon, Sony, Olympus, Fuji, even on the Pentax 645 cameras! This design seems to work ok for the most part, but lens changing even with the lens release button held down is noisy and gritty feeling with all those contacts bouncing around as you slide the lens on, as opposed to the Minolta/ Sony A mount I once used which was smooth as butter. So, does anyone know the history or reason behind the K mount contact placement?

05-16-2015, 08:25 AM   #2
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The electrical contacts in the lens mount pre-date even the KA mount. Pentax put the contacts there in 1981 with the KF mount. Of the other mounts you mention, only Nikon F had electrical contacts added onto an older mount.

Summary of the K-Mount Evolution, Names, and Features

Features and Operation of the Kf Mount

Features and Operation of the Ka Mount
05-16-2015, 08:56 AM   #3
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I think putting the contacts on the mounting surface simplified manufacturing. Putting the contacts on the mounting surface eliminated another part to make and align on both the lens and body. This also freed up room in the mirror box for the aperture arm. Remember, the K-mount is pushing 40 this year and everything was much more mechanical back then. There was no computer control aperture motor or OS system.

Take a look at the Q mount. That's the modern day version of the K-mount if Pentax could dream it all up again. The contacts are inside the mount. There is no mirror box and no aperture arm to compete with.
05-16-2015, 09:03 AM   #4
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Lens registration distance and mirror box size relative to lens diameter may have played a part too. Remember, Pentax was one of the originals and sorting out what worked best and avoided patent issues was in progress. Since then, maintaining mount compatibility, even so far as accommodating the requirements of the legacy M42 design, has been a much appreciated bonus in the Pentax design history for most of us. Especially those of us still using lenses bought in the '60's!

For me, the glass is way more than 1/2 full -- say thank you, Pentax.

05-16-2015, 09:33 AM   #5
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The aperture control linkages are on the inside of the K-mount. The same for the Canon FD mount. This doesn't leave much room to add electrical contacts on the inside. Nikon lenses (as well as the Minolta SR mount) these linkages are on the outside with the aperture setting ring. In the auto-focus era Canon and Minolta abandoned their old bayonet mounts and opted for new ones with electrical contacts on the inside. It's easier add features when designing new equipment from scratch rather than to keep some backwards compatibility and shoe-horn them into existing designs. Olympus got out of the interchangeable lens SLR game and went for P&S and bridge cameras.

As noted above the electrical contacts on the K mount predate auto-focus. Moving the contacts inside the mount probably would mean giving up the aperture control linkage to make room. Pentax opted for maximum compatibility between the systems.
05-16-2015, 09:40 AM   #6

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Wow quick responses! The insight provided has been helpful. When changing lenses in the future, I will think of all the history behind the bouncing lens contacts during the swap and will be proud to own a piece of historical engineering. I just want to say I enjoy my Pentax system regardless of how lens changing lenses feels, and it now makes sense that the current K mount is the result of the best engineering that could be done to maintain compatibility over the years. I do appreciate the fact that Pentax did not abandon all their users like Canon did with the FD mount, and that I can use my K, M, A, F, FA, as well as my DA lenses on my K30. Thank you Pentax!

Last edited by stillshot2; 05-16-2015 at 09:45 AM.
05-16-2015, 09:42 AM   #7
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The K-mount can still grow. Communication between lens and body can be handled by a 2 or 3 pin contact similar to USB. Power for AF can be transmitted over the digital lines, even some of the pre-existing contacts.
05-16-2015, 12:13 PM   #8
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Pentax wasn't the first

I agree that Pentax's electronic contacts seem a little less elegant than what others did later.

However, there were a couple of other mounts that used similar, on the mount-face contacts.

M42 lens fans often come across the Pentacon electric lenses that sold for a couple of Praktica models. The contacts were used for communicating aperture information to the light meter while keeping the aperture wide open (something not possible on original M42 designs - Pentax, Fuji, and Mamiya all had different approaches).

When Pentacon/Praktica switched over to a bayonet mount (which is eerily similar to the K-mount, but not quite the same), they put electronic contacts on the face. Of course, Praktica closed up about 10 minutes after the Berlin Wall came down.

Photo shows the M42 Pentacon electric on the left. On the right is a Praktica bayonet.

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PENTAX K-01  Photo 
06-11-2015, 01:28 PM   #9
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'Elegant' and 'smooth' are no design targets for something invisible when in place.
Important is reliable electrical contact, life time of the system, ease of application, and cost.

The rotative motion cleans the contacts that on the camera side are not gold plated, which saves cost and brings lifetime.
The exerted pressure is exactly determined as the fit of the bayonet is accurately controlled and thereby the distance over which the spring action works in each contact.
Being embedded in stainless steel makes the positions well defined and allows for small pitch between the contacts.

In essence a very future proof design approach.
06-11-2015, 03:51 PM   #10
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QuoteOriginally posted by pacerr Quote
Lens registration distance and mirror box size relative to lens diameter may have played a part too.
It is good to note that the same is true for Nikon as well. The F mount registration distance is very close to and diameter is same as Pentax K. Placing the contacts on the mount face may have something to do with legacy lens support. The aperture actuator pin is a potential source of interference with contacts set into the mount, a risk that has been noted by some users on this site in reference to the power contacts on AF bodies. Yes, some pins do strike the power contacts.


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