K to J:
the end of legendary Pentax compatibility
By Dario Bonazza
introduced some lenses without an aperture ring, suitable for their low-end
cameras but not fully compatible with current midrange and top level models.
This caused so much concern among Pentax users about a possible end of
acclaimed Pentax compatibility, also considering the "simplified"
bayonet introduced some years ago on a few budget models. In order to
understand what is going on, here follows a technical overview of the
variations of Pentax K bayonet from 1975 to date, and an investigation about
the limitations when using either a camera or a lens with
The original K bayonet was of
a completely mechanical type, featuring a sturdy metal bayonet with lens
release mechanism and quick alignment mark, a diaphragm simulator coupling
for full-aperture metering and a camera-to-lens stop-down actuator for when
taking the picture (actual diaphragm set on aperture ring). In this
operation, the stop-down actuator simply releases the diaphragm, while
actual taking aperture is manually set by the diaphragm ring on the lens. In
case of a few manual diaphragm lenses (stop-down operation), the diaphragm
simulator was replaced by a coupling fixed on minimum aperture position,
informing the camera circuitry that the metering aperture should also be
considered the taking value. This bayonet allows both aperture priority and
metered manual exposure. Lenses featuring the original K mount are the K
series and the M series.
We’ll just mention the Kf
bayonet mount for the ME-F here. Five electrical contacts were added to the
original K bayonet, allowing operation of meter and AF sensor in camera body
and AF motor inside the lens. Unfortunately this concept had no further
development at a time when Pentax was the technology leader, but was unable
to take advantage of that. The only lens featuring the Kf mount is the SMC
Pentax-AF 35-70mm f/2.8 for the ME-F.
An important improvement came
with the KA bayonet introduced with the Super A (Super Program in USA). Six
electrical contacts were added to the bayonet, on its lower side opposite to
that of the contacts in the Kf mount for preventing incompatibility between
the two bayonets. Those contacts transmit minimum aperture and aperture
range of each lens (coded, in half stops) to the camera. According to George
A. J. de Fockert, contacts r1, r2 and r3 transmit diaphragm range, while the
combination of m1 and m2 tells minimum aperture (f/16, f/22, f/32 or f/45).
This kind of digital coding is called "gray code" and applies the
same concept of DX coding for informing the camera meter about film speed.
A-series lenses feature an additional position on aperture ring, marked with
A (auto), beyond minimum f/stop. Provided that the lens diaphragm ring is
set to A, a ball contact (*) is raised on the lens bayonet. This way the
camera is informed that it has to read at full aperture and then operate the
diaphragm, by means of a proportional movement of the stop-down actuator. In
this operation, the camera adjusts the aperture instead of the lens itself.
To that purpose, when changing K bayonet to KA type, Pentax modified the way
the stop-down mechanism works in their lenses, from incremental (angular
movement proportional to the iris diameter) to linear (each f/stop needs the
same angle movement). This improved bayonet also allows both shutter
priority and program exposure. Lenses featuring the KA mount are the A
multi-segment metering was not implemented in any A-series camera, however,
A-series lenses were already designed to allow it. That’s the lovely
Pentax way of planned evolution.
With the introduction of the
autofocus SF-series cameras and F-series lenses, Pentax introduced the
original KAF bayonet, featuring an AF coupling between body and lens. An
additional contact transmits information from a ROM in the lens to the
camera CPU by using proprietary serial protocol. Transmitted data in the KAF
mount are: focal length (useful for preventing camera shake during handheld
photography), effective selected aperture (at any focal length of a zoom
lenses) and lens dimensions for managing possible shading of a close subject
when using built-in flash. Of course, KAF mount also retained all of the
features of the KA bayonet. Lenses featuring the KAF mount are the F
The Z-series SLR’s
(PZ-series in USA) introduced the KAF2 bayonet, featuring power zoom and
second generation electronics for digital transmission. Each lens tells the
camera more information, including its MTF curve (useful for getting the
best lens performance, when possible) and lens-to-subject distance (another
Pentax first, several years before Nikon D-series). Two more contacts were
also added inside the flange for powering the zoom motor in each lens. Full
compatibility is still guaranteed between Pentax bodies and genuine Pentax
lenses, while some incompatibility problems show up with third party lenses
and their not fully compatible electronics. Lenses featuring the KAF2 mount
are the early FA-series (power zoom).
The market didn’t like so
much the power zoom of FA-series lenses, considered rather an expensive and
little useful frippery. So, on occasion of launching their successful
MZ-series compact SLR cameras and new manual zoom FA lenses, Pentax got rid
of power zoom, introducing a second generation KAF bayonet. This KAFn
bayonet (unofficial name) featured same mechanics and different electronics
(new ROM with extended data transmission, same as KAF2 lenses). Lenses
featuring the KAFn mount are the latest FA series (manual zoom). Starting
from 1999, power zoom contacts are removed also in new bodies. Cameras
featuring the KAFn mount are the MZ-7/ZX-7 (1999) and the MZ-6/ZX-L (2001).
The manual focus MZ-M (ZX-M in
USA), features the little known KA2 mount. By removing the AF coupling from
the KAF mount (or by adding the serial contact to the old KA mount, if you
prefer), Pentax updated their manual focus bayonet with up-to-date
electronics and performance. That makes sense, since the manual focus MZ-M
couldn’t use the AF coupling in any case. There are no A2-series lenses
available, but the KA2 is compatible with all K mount variations introduced
between 1975 and 1997.
Despite so much
electronics added over the years, all of the mechanics were still there to
grant full backward compatibility. That’s one of the best reasons for
sticking to Pentax, you know. Then, all the mess began.
With the introduction of the
MZ-50 (ZX-50 in USA), Pentax introduced a simplified KAF bayonet, but for
commercial reasons they chose not to adopt a new designation for such a
partially compatible mount. In order to reduce manufacturing costs, the
simplified KAF lacks the diaphragm simulator, so the camera cannot know how
much the lens will be stopped down during exposure (unless the camera itself
is operating the diaphragm). Again missing an official naming, this new
mount was dubbed the "crippled" KAF mount by Pentax users in the
internet. However, I’d suggest to call it the JAF mount (name explanation
in box). Cameras featuring the JAF bayonet are the MZ-50/ZX-50, the
MZ-30/ZX-30, the MZ-60/ZX-60 and the new *ist, soon to be on sale. Dedicated
lenses for the JAF mount only saw light at PMA 2003, and are called FAJ.
any possibility to call with the same name two different performing
bayonets, I’d suggest to name the so far called "crippled
KAF" as the JAF mount, for the following reasons:
– It couldn’t be K-anything, since it lacks one of the important
features of all K-bayonet mounts and is not fully compatible with
– It is especially designed for the FAJ lenses. Since the lenses are
identified by J, why not using the same designation for the mount?
Then, A and F in JAF are welcome, since these letters indicate
specifications which are retained by the new bayonet.
– JAF is mirror writing of FAJ, and gives a good idea of two
matching parts: JAF on camera and FAJ on lens.
– In alphabetical order, J comes before K, and can indicate
something lesser than it. Can you deny the JAF to be less than the
KAF? Sure it cannot be more, as the only difference is something
– J can mean Junior, and is as good for camera mount as it is for
lens mount. Being aimed to the young people, who could have neither
older lenses nor habits to save, it can be good.
Hopefully, sooner or later Pentax should name it, and they could adopt
the established unofficial name, like it happened with the AP and the
Super S2 unofficial designations, now official. I don’t know if they
will ever adopt a name chosen by users, but sure they won’t use
"crippled KAF". On the contrary, JAF is a possible serious
designation and doesn’t sound defamatory.
JAF-mount cameras and
how they behave
With JAF-mount cameras, when
setting a A-/F-/FA-series lens aperture ring on "A" position (or
using FAJ lenses), diaphragm is adjusted by the camera and everything works.
The only drawback is you have to change your way of setting aperture on
lens. If you set aperture ring other than A (or if your lens has no A
position, like K- and M-series), the different JAF bodies behave
The MZ-50/ZX-50 shoots assuming the lens is set to its full aperture. This
can cause either underexposure (if the lens aperture is set other than full
aperture) or shooting with low optical performance and reduced depth of
field (in case the lens is set to full aperture).
With the MZ-30/ZX-30 and MZ-60/ZX-60 bodies, in case of non-A position lens,
the shutter doesn’t fire at all. With the new *ist, Pentax gives you the
option to customize the camera, so that you can choose between shooting as
if the lens is wide open or not shooting. According to the latest
information, it is believed that the new *ist D digital SLR will follow the
same behaviour of the film *ist.
when using Pentax lenses with Pentax JAF bayonet cameras
Not allowed *
|K/M manual or
fixed diaphragm (e.g. mirror lenses)
Not allowed *
|AF 35-70 f/2.8
Not allowed *
|FA (power zoom)
|FA (manual zoom)
depending on F17 custom function
FAJ lenses and how
With no aperture ring, in
practice the FAJ series only feature the A position. Attaching a FAJ lens on
any Pentax camera featuring diaphragm control on body, you can still achieve
full operation. Unfortunately, just two out of six models in current Pentax
SLR line do that: MZ-6/ZX-L and MZ-60/ZX-60. Old models in the Z/PZ-series
having such a capability are available in the secondhand market. By mounting
FAJ lenses on cameras with no diaphragm control on body (most MZ models,
including the flagship MZ-S), you miss the capability to shot in both
aperture priority and manual exposure modes. For all Pentax classic models
(K-series, M-series and the LX), that means you cannot use them at all!
when using Pentax FAJ lenses on Pentax bayonet cameras
|K-series, M-series, LX
|Program A/Program Plus,
|Super A/Super Program,
SF-series, MZ-3, MZ-5/ZX-5, MZ-5n/ZX-5n, MZ-10/ZX-10, MZ-M/ZX-M,
& shutter priority only
MZ-6/ZX-L, MZ-7/Zx-7, MZ-30/Zx-30, MZ-50/ZX-50, MZ-60/ZX-60, *ist
In order to make the
MZ-S compatible with the FAJ-series lenses, Pentax should release an
updated firmware for that camera, allowing on-camera aperture
selection. By pressing the green dot (reset) button, the camera
could cycle from P through M, Av and Tv modes. Then, in both M and
Av modes, the main dial could set aperture. We do hope that Pentax
will do something like that.
The end of Pentax
Can you call it compatibility?
In my opinion, cameras featuring the JAF mount are only compatible with A-,
F-, FA- and FAJ-series lenses. Even worse, FAJ lenses are fully compatible
with just Z- and a few MZ-series cameras, while they are partially
compatible with A-, P-, SF and most MZ-series cameras and they are not
compatible at all with cameras only offering aperture priority and manual
exposure. JAF-mount cameras and lenses cause serious compatibility problems
within the Pentax system, not only with old cameras and lenses, but also
with equipment currently on sale, and that’s truly a sad first for Pentax.
You find published here a
comprehensive schematic of K mount evolution, highlighting its working
parts. K, KA and KAF are on the left, KAF2, KA2 and JAF are on the right.
thumbnails to enlarge.
For further in-deep info about K mount development, variations and
compatibility, visit Bojidar Dimitrov’s excellent "Pentax K-mount
Page" at: http://www.bdimitrov.de/kmp/