The Evolution of the Pentax K-mount

Technical Information and a Little Bit of History

By PF Staff in Articles and Tips on Dec 29, 2014
The Evolution of the Pentax K-mount

Pentax introduced the K-mount in 1975 and it's still the mount used in today's digital SLR cameras.  Over the course of its 40-year history, the K mount has gone through several enhancements to facilitate new camera features. Even then, a K-mount lens from 1975 will mount on and work on your Pentax DSLR today!  In fact, Pentax users are among the few who have access to such a wide range of legacy lenses.  In this article we will describe the K-mount in its various incarnations.

The K-mount is a bayonet lens mount (the type employed by all modern lenses) which replaced the original M42 screw mount, thus making changing lenses must faster.  While the screw mount requires three complete revolutions of the lens in order to mount tightly, the K-mount only needs a twist of about 60 degrees. The diameter of the K-mount is also larger than that of the M42 making faster lenses and more robust optical designs possible.

Exceptional backward/forward compatibility has been retained over the years meaning that K-mount lenses, old and new alike, will work on nearly all Pentax SLR/DSLR cameras. This is a significant advantage of the Pentax K-mount. There are only very few exceptions to the any lens/any camera compatibility and they are primarily related to some low-end SLR bodies from the late 1990s where certain lenses can be mounted fine but the camera will refuse to fire. Refer to our Lens/Camera compatibility table for details.

In daily speak, "K-mount" refers to all of the variants of the mount described below, not solely to the original version. We will in this article call the original mount the "K" mount and use "K-mount" as the generic term for any variant.

Overview

This table summarizes the evolution of the K-mount at a glance:

Mount
Introduced
Comments
Features
K 1975 The original
  • Av and M exposure
  • Manual focus
KF 1981 World's first autofocus mount, ahead of its time. Eventually abandoned completely
  • Av and M exposure
  • Autofocus with ME F camera
KA 1983 The original "K" mount with the addition of electrical lens contacts enabling the camera to control the aperture and offer full exposure automation
  • P, Tv, Av, and M exposure
  • Manual focus
KAF 1989 First mainstream autofocus mount, still in use today. KA functionality with autofocus facilitated by a drive shaft
  • P, Tv, Av, and M exposure
  • Autofocus (screw drive)
  • Transmits lens info
KAF2 1991 Added two power contacts, originally intended for power zoom, now used to power in-lens autofocus motors
  • P, Tv, Av, and M exposure
  • Autofocus (screw drive, and power zoom or SDM/DC)
  • Transmits lens info
KAF3 2007 In-lens autofocus only. No autofocus drive shaft
  • P, Tv, Av, and M exposure
  • Autofocus (SDM/DC)
  • Transmits lens info
KAF4 2016 Electromagnetic aperture diaphragm.  No stop-down lever.  In-lens autofocus only.
  • P, Tv, Av, and M exposure
  • Autofocus (PLM)
  • Transmits lens info

P: Program | Tv: shutter priority | Av: aperture priority | M: Manual | Lens term glossary

Lens Series and Mounts

The many different series of Pentax lenses have lens mounts as specified below:

Lens
Mount

Pentax
Pentax-M

K
Pentax-AF KF
Pentax-A KA
Pentax-F KAF
Pentax-FA/FA J (no power zoom) KAF*
Pentax-FA/FA* (power zoom) KAF2
Pentax-DA/DA-L/D FA (screw drive AF) KAF
Pentax-DA/DA* (screw drive & SDM/DC AF) KAF2
Pentax-DA/DA* (SDM/DC autofocus only) KAF3
Pentax-DA (PLM autofocus) KAF4

A complete listing of these lenses can be found in our lens database.

*originally desginated as KAF2, prior to the debut of SDM lenses in 2007

DSLR Usage

All the lens types listed above can be mounted and used on all Pentax DSLRs with no optical limitations.  No adapters are ever required.

Pentax KAF4 lenses only support the Pentax K-1, K-S1, K-S2, K-3 II, K-70, or newer. Earlier models cannot operate the aperture mechanism, and thus aperture adjustments are not possible.

Pentax KAF3, KAF2, and KAF lenses support autofocus and full exposure automation on Pentax DSLRs.  Exception: the *ist D series, K100D, and K110D will not autofocus with KAF3 lenses, and cannot manually focus specifically with the HD and SMC DA 18-50mm RE.

Pentax KA lenses support full exposure automation on Pentax DSLRs.  Manual focusing is required.

Pentax K and KF lenses require stop-down metering on Pentax DSLRs and can only be used in M mode.  Manual focusing is required.  Metering instructions can be found here.

Read on for a detailed description of the K-mount.

The Original "K" Lens Mount as Introduced in 1975

The original version of the K-mount had two mechanical links between lens and camera body:

Aperture control lever (i): The camera pushes  down on this lever during exposure to stop the lens down to the aperture set on the aperture ring.

Aperture simulator (ii): Indicates to the camera  how much the lens is stopped down with the aperture ring. It moves in unison with the aperture ring.

On the original version of the K-mount the camera cannot determine the actual aperture value set, but only how much the lens is stopped down ("three stops from fully open" to take an example). That information is sufficient to allow for open aperture metering. It explains why a camera can only display F-- in the view finder and on the monitor when a lens with the original K-mount is used.

A few lenses (such as the smc Pentax 1000mm F8) do not have the aperture control lever and the aperture must be stopped down manually for each shot (open up to focus, close down to measure the exposure and shoot). Pentax reflex lenses (the smc Pentax smc Pentax 2000mm F13.5 Reflex for example)  do not even have an aperture ring (their aperture is fixed). The aperture simulator is then set to a fixed position. The mount is still considered a "K" mount.

  • Exposure modes supported: Av and M
  • Focusing modes: Manual
  • Example of a lens with the "K" mount: smc Pentax-M 50mm F1.4

The "KF" Mount (1981)

This was the first Pentax autofocus mount and it was designed for lenses with a built-in focusing motor. There is no mechanical link involved in the autofocus operation. The mount is a "K" mount with one addition (iii):

Aperture control lever (i): The camera pushes  down on this lever during exposure to stop the lens down to the aperture set on the aperture ring.

Aperture simulator (ii): Indicates to the camera  how much the lens is stopped down with the aperture ring. It moves in unison with the aperture ring.

Autofocus contacts (iii): The camera controls the focusing operation through these contacts.

The mount was ahead of its time with its all-electrical autofocus coupling, but the implementation was not successful: The motor in the lens required so much juice that four batteries were needed. The batteries were placed in the lens which therefore became quite bulky and heavy.

Only one lens, the smc Pentax-AF 35-70mm, was produced in this mount and only one camera could autofocus with it: the Pentax ME F. On all other cameras this lens works as if it just had the original "K" mount. The "KF" mount was abandoned and did not form the basis for the newer mounts.

  • Exposure modes supported: Av and M
  • Focusing modes: Autofocus (with the Pentax ME F camera), manual focus
  • The only lens with the "KF" mount: smc Pentax-AF 35-70mm

The "KA" Mount (1983)

In 1983 Pentax introduced a new line of cameras that had Tv (shutter speed priority) and Program exposure modes in addition to Av and M. This required that the camera be able to set the aperture on the lens and therefore the "KA" version of the K-mount was created with two new features: a redesigned aperture control lever (i) and lens information contacts (iv).

Aperture control lever (i): It looks the same as on the "K" mount but it is now calibrated differently and can stop the lens down very accurately by fractions of F-stops as determined by the camera's meter.

Aperture simulator (ii): Unchanged from the "K" mount. Indicates to the camera how much the lens is stopped down manually in M and Av exposure modes. It moves in unison with the aperture ring.

Lens information contacts (iv): One to six electrical contacts. The contact marked "A" is always present. It was originally a pin that would move out and establish contact to the camera when the aperture ring was set to the "A" position. It would be retracted otherwise. On teleconverters and in later versions of the mount it became an electrical contact that is either shorted or open.

The other contacts tell the camera what the minimum aperture of the lens is (F22 for example) and how many F-stops the lens can be opened up (six for example). Usually on "KA" mount lenses only those contacts that are open are present as is evident on the image of the "KA" mount above. Besides the "A" contact we only see two other contacts (the left most one and the rightmost one) - click image to enlarge. The contacts on most "KA" mount lens are effectively just a drilled hole filled with a grey mass of insulating material. The metal mount acts as a short circuit in the places where the contact should otherwise be a short.

The image to the right is of the "KA" mount of a teleconverter and it has all contacts present and is therefore a better reference for understanding the contact layout.

The encoding of this information is described in the table at the end of this article.

  • Exposure modes supported: P, Tv, Av, and M
  • Focusing modes: Manual
  • Example of a lens with the "KA" mount: smc Pentax-A 50mm F1.7

The "KAF" Mount (1989)

In 1989 Pentax re-introduced autofocus, now based on the "KAF" mount, which was a "KA" mount with the addition of a digital information contact (v) and an autofocus drive shaft (vi).

Aperture control lever (i): Works the same as on the "KA" mount. It stops the lens down very accurately by fractions of F-stops as determined by the camera's meter.

Aperture simulator (ii): Unchanged from the "K" mount. Indicates to the camera how much the lens is stopped down manually in M and Av exposure modes. It moves in unison with the aperture ring if the lens has one. Otherwise it is fixed in the position of the "A" setting of the aperture ring had there been one.

Lens information contacts (iv): Up to six electrical contacts. Refer the "KA" mount description for details.

Digital information contact (v): A seventh contact was added to the mount.

Autofocus drive shaft (vi): A motor in the camera can autofocus the lens by turning this drive shaft.

With the introduction of budget priced DA L lenses in 2008 Pentax introduced the "KAF" mount is plastic as shown to the right. One of the contact pins is replaced by a metal "tongue" to ensure solid ground contact between lens and camera since a mount of plastic in itself cannot provide ground contact.

From the "KAF" mount onward one or more of the "KA" lens information contacts are always "real" contacts, not just a drilled hole filled with with epoxy or a short. Some of our users speculate that these contacts carry more information when the lens is mounted on an autofocus camera. Data is not available from Pentax regarding this.

The bottom line is that the "KAF" mount allows the camera to read various types of information from the lens: the current focal length (will vary for zoom lenses), a code identifying the lens model, distance to subject, current maximum aperture (may vary as a zoom lens is zoomed), and possibly additional information.

Some of today's lenses with the "KAF" mount have no aperture ring. In that case the aperture simulator lever (ii) is fixed in a position corresponding to the "A" setting of the aperture ring had there been one. This ensures compatibility with older camera bodies.

  • Exposure modes supported: P, Tv, Av, and M
  • Focusing modes: Autofocus (screw drive), Manual focus
  • Example of a lens with the "KAF" mount: smc Pentax-F 50mm F1.7

The "KAF2" Mount (1991)

The "KAF2" mount was introduced in 1991 and added provision for power zoom to the lens mount by augmenting the mount with two power contacts (vii). On today's lenses with the KAF2 mount these contacts are for powering the in-lens autofocus motor (SDM/DC autofocusing). 

The mount originally came in two flavors: With power contacts and without power contacts. To reduce confusion Pentax has recently renamed the "KAF2" mount without power contacts to "KAF". As an example, the smc Pentax-FA 31mm F1.8 Limited was said to have a "KAF2" mount when it was introduced. Today Pentax calls it a "KAF" mount even though its mount hasn't changed! The new terminology is used inconsistently, but should it prevail it would be a less confusing way of naming the mounts.

A "KAF2" mount with power contacts is shown to the right. It is physically a "KAF" mount with the addition of the power contacts (vii). For power zoom lenses additional information about the lens is transmitted to the camera, hereunder the aperture where the lens is physically at its shortest. This allows the camera to retract the zoom on power down.

Aperture control lever (i): Works the same as on the "KA" mount. It stops the lens down very accurately by  fractions of F-stops as determined by the camera's meter.

Aperture simulator (ii): Unchanged from the "K" mount. Indicates to the camera how much the lens is stopped down manually in M and Av exposure modes. It moves in unison with the aperture ring if there is one. On lenses with no aperture ring the tab is fixed in the position of the "A" setting of the aperture ring had there been one.

Lens information contacts (iv): Up to six contacts. Refer the "KA" mount description for details.

Digital information contact (v): The seventh contact that was introduced with the "KAF" mount.

Autofocus drive shaft (vi): A motor in the camera can autofocus the lens by turning this drive shaft.

Power contacts (vii): When originally introduced these contacts provided power for power zoom lenses (how these lenses work is explained here). Today the power contacts provide juice for in-lens focus motors (SDM or DC autofocus). High end Pentax DSLRs support some power zoom functions in addition to the SDM/DC autofocus support.

The "KAF2" mount retains the drive shaft (vi) for screw drive autofocus even if the lens has a built-in autofocus motor. All "KAF2" mount lenses will thus autofocus even on autofocus enabled cameras that lack power contacts.

Most of the current lenses with a "KAF2" mount have no aperture ring. In that case the aperture simulator lever is fixed to a position corresponding to the "A" setting of an aperture ring had there been one.

  • Exposure modes supported: P, Av, Tv, M
  • Focus modes: Autofocus (all lenses have screw-drive, some also SDM/DC), Manual focus
  • Example of a lens with a "KAF2" mount: smc Pentax-DA* 300mm F4

The "KAF3" Mount (2007)

The "KAF3" mount is the second most recently introduced variant. It has no autofocus drive shaft, but is otherwise like a "KAF2" mount with power contacts. This means that lenses with the "KAF3" mount all have a built-in autofocus motor and that autofocus is not available with cameras predating the Pentax K10D/K100D Super.

Aperture control lever (i): Fixed in the position corresponding to the aperture ring being in the "A" position had there been one.

Aperture simulator (ii): Fixed in the position of the "A" setting of the aperture ring had there been one.

Lens information contacts (iv): Up to six contacts. Refer the "KA" mount description for details.

Digital information contact (v): The seventh data contact introduced with the "KAF" mount.

Power contacts (vii): These contacts provide juice for in-lens focus motors (SDM or DC autofocus).

  • Exposure modes supported: P, Av, Tv, M
  • Focus modes: Autofocus (no screw drive), Manual focus
  • Example of a lens with a "KAF3" mount: smc Pentax-DA 18-135mm F3.5-5.6

The "KAF4" Mount (2016)

The KAF4 mount made its debut in mid-2016 with the HD Pentax-DA 55-300mm F4.5-6.3 ED PLM WR RE lens.

This mount is identical to KAF3, except that the aperture control level (i, above) has been removed.  Instead, the aperture diaphragm is electronically controlled by an electromagnetic actuator in the lens itself.  The camera gives the command to stop down the lens.

As a result, this mount only supports cameras whose firmware has been updated to control the aperture.  As of its launch, the KAF4 is only compatible with the Pentax K-70, K-S2, K-S1, K-1, K-3 II, and newer.  On other cameras, the lens will always shoot wide-open, and aperture control will not be possible.

KAF4 also debuts a new type of autofocus: PLM, or the Pulse Motor, optimized for continuous video autofocus.

  • Exposure modes supported: P, Av, Tv, M
  • Focus modes: Autofocus (no screw drive), Manual focus
  • Smoother aperture adjustments thanks to electromagnetic actuator
  • Example of a lens with a "KAF4" mount: HD Pentax-DA 55-300mm F4.5-6.3 ED PLM WR

Weather Sealing

Many current K-mount lenses have weather sealing. Seals are placed within the lens so that dust and moisture cannot get inside the lens, and there is also a rubber gasket on the mount itself.

The mount seal is black on star lenses (i.e. DA*) and red on other lenses.

Third Party Variants of the K-mount

Pentax made the original "K" mount available for other camera manufacturers to adopt. The goal was to create a universal standard bayonet mount much like the M42 screw mount had been. Ricoh, Chinon, and others produced cameras and lenses with the "K" mount. Unfortunately, with the introduction of the "KA" mount, Pentax reversed the decision with the result that the other manufacturers either abandoned the "K" mount or developed their own, somewhat incompatible, enhancements. Several third party lens manufacturers produce or has produced lenses with a K-mount. There are no issues with current third party lenses in K-mount, but for older lenses there are two traps to watch out for:

The infamous "Ricoh pin". Where Pentax created the "KA" mount in order to support Program and Tv exposure automation, Ricoh had to create their own variant, which has a protruding pin placed in a somewhat unfortunate location, namely in the exact spot where Pentax years later placed the autofocus drive shaft. Such a Ricoh lens will get stuck when mounted on a Pentax autofocus body - the Ricoh pin will get lodged in the drive shaft and prevent the lens from being removed without deployment of finesse, patience and tools.

The other trap is the shield that protects the aperture stop down lever. The image to the left shows what the shield should look like and does look like on a genuine Pentax lens. Some older third party lenses have a much larger shield which prevents mounting the lens on a Pentax DSLR. Current third party lenses (Sigma, Tamron, Rokinon, etc.) have correctly shaped shields (or no shields) and pose no problem.

 

The "Crippled" K-mount

This is not a lens characteristic, but a camera "feature", or rather a lack thereof: all Pentax DSLR bodies lack the tab that would otherwise read the position of the aperture simulator (lever (ii) in the pictures of the mounts above). Certain low-end film SLR bodies lack this tab as well. As a result, original "K" mount lenses must be used in manual exposure mode with the stop-down metering method ("green button" metering) on these cameras.

Flange Distance (Register)

The distance form the film plane or sensor to the mount flange on the camera body is 45.46 mm. This is the same distance as was used with the M42 screw mount. Pentax (Takumar) screw mount lenses dating back to 1957 can therefore be used on a K-mount camera with a simple adapter that sits flush with the K-mount.

This distance is shorter than what Nikon uses (46.5 mm) and longer than what Canon uses (44 mm). This means that K-mount lenses can be adapted to a Canon camera without loss of infinity focus, but not to a Nikon.

Decoding of the Lens Information Contacts

The "KAF" and later mounts have 7 lens information contacts. The first six (numbered 1 through 6 on the image to the right) are also found on the "KA" mount and will be explained below. The 7th contact is a digital data/control contact where exact way of working hasn't been published by Pentax, but we know it facilitates communication of detailed information about the lens to the camera (like identity of the lens, focal length, distance to subject, aperture as it varies during zooming, MTF lens quality data, etc.).

The functioning of the first six contacts in connection with a "KA" mount like the one below to he right is well understood, however:

Pins 1, 5, and 6: Conveys the aperture range of the lens in half F-stops.

Pins 2 and 4: Conveys the minimum aperture of the lens.

Pin 3: Shorted if the aperture ring is set to "A", or if there is no aperture ring. Open if the aperture ring is set manually to some F-stop.

Example: an smc Pentax-A 50mm F2.8 lens has a range of 6 stops (from F2.8 to F22). This is represented as follows: Pin 1: open, pin 5: shorted, pin 6: open, and the minimum aperture of F22 is represented as pin 2 open and pin 4 open.

The possible settings are as follows, where "o" means open contact (or an insulated spot) and "s" means a shorted contact (or blank mount).

Aperture Range

F-stops
Pin 1
Pin 5
Pin 6

5

o o o

5.5

s o o

6

o s o

6.5

s s o

7

o o s

7.5

s o s

8

o s s

8.5

s s s

Minimum Aperture

Pin 2
Pin 4

F16

o o

F22

s s

F32

s o

F45

o s

Aperture Ring Status

 
Pin 3

No aperture ring

s

Set to 'A'

s

Set to some F-stop

o

Conclusion

It is remarkable that so many Pentax lenses, from the original "K" lineup to the latest "KAF4" releases, can be used on modern Pentax SLRs without the need for an adapter.  With only a handful of exceptions, Pentax engineers have chosen to keep their iconic lens mount backward-and-forward-compatible by improving the mount incrementally without ever redesigning it.

We hope that this article has given you valuable insight into the various features of K-mount lenses and how those features have changed over time.  If you'd like to learn more, we recommend the following resources:

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