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Pentax K Series - The Original K Mount

John Riley looks at the original Pentax K mount cameras and lenses from 1975, including the classic film cameras KM, KX, K2 and the K1000 along with listing all K Series lenses with their basic specifications.

Posted: 10/07/2024 - 07:46

The K Bayonet

By 1975 the Pentax screw thread cameras and lenses were still at the top of their game, but progress still needed to be made. The cameras were fine in themselves, and the lenses second to none, but the screw thread was slower to use that the bayonets used by most other top of the line manufacturers. This slowed down lens changing, especially for professionals working at speed and under pressure. It also restricted some of the more adventurous optical designs that were being made possible by new manufacturing techniques and computer assisted design.

So 1975 saw the release of the Pentax K bayonet, a totally new design developed in conjunction with Zeiss, having an increased throat diameter of 45mm, an improvement of 3mm over the 42mm diameter screw thread mount. The new bayonet also ensured positive locking into position, giving accurate register for the open aperture metering links. The stainless steel mount was simple but rugged, with no less that six fixing screws and probably the largest contact area of any of the bayonet mounts.

Absolute vindication for the design is given in that it is still in use today, modified for electronics and now in version KAF4, but essentially compatible with almost all Pentax bayonet camera bodies and lenses. This is a remarkable achievement – the Pentax K mount has survived the electronic, autofocus and digital revolutions, showing that the original concept was extremely sound.

Backwards compatibility was not forgotten, and the Pentax Adapter K enables use of screw thread lenses on the new bayonet fit bodies, albeit it for stop down metering and fully manual aperture control.

The K Series Cameras

1975 saw the simultaneous release of three new Pentaxes – the KM, KX and the flagship K2. Almost a year later, the lower cost K1000 arrived, probably the longest running Pentax ever.

These original K bodies are very interesting in that they form a sort of bridge of technologies between the mechanical Spotmatics and the later more electronic M series. The bodies themselves are firmly Spotmatic era in terms of size and weight, but some controls and the advent of Silicon Cell Metering point firmly towards the future. The K2 in particular puts the slightly variable auto exposure of the ES and ESII behind us and gives us for the first time a really reliable electronic auto exposure system.

The Pentax KM

The KM is actually a very, very nice camera. It is essentially a Spotmatic F with a bayonet mount, and it shares all the advatages and disadvantages of that model. In terms of handling, this is the pinnacle of manual exposure cameras.

The specification is still basic, with a horizontally running rubberised silk  shutter giving the usual speed range of 1 sec to 1/1000 sec. The CdS meter opertates automatically from EV3 via a photo-switch in the electronics, and a simple meter needle is centred in the viewfinder. This metering circuit cannot be switched off without the lens cap being put into place or the camera being placed in a bag. The circuit drain may be small, but the life of the single 1.5v silver oxide G13 cell may only be six months or so as a result. However, it does do away with the relatively troublesome Spotmatic meter switch...

This basic camera never the less still has open aperture metering, depth of field preview and a self timer.

What the specification does not reveal is that ergonomically, the design is superb. The KM is an absolute delight to use, and this in itself was I suspect a dilemma for the Pentax designers. Whatever they changed was unlikely to be an improvement, and marketing requirements are always pushing to demand change...

The Pentax KX

The KX is also based on the same mechanical shutter as the KM, but small details of the MX start to creep in, such as the on/off switch around the shutter release, the memo holder on the camera back and of course the use of Silicon cells for the metering. A small window in the pentaprism shows the aperture in the viewfinder (reading directly from the lens aperture ring) and the shutter speed is visible as a scale on the right hand side of the view.

A transparent blue flag indicates the shutter speed in use and this is aligned with a swinging needle to show the recommended exposure. This new system works well, responds quickly and accurately, but ultimately is nowhere near as easy to use as the simple LED readout of the later MX.

The KX is an excellent, reliable camera, but difficult to find in good condition, and in my opinion at least, arguably not as good to use as the smaller MX.

The Pentax K2

The flagship model of the range, the K2 takes the same metering system as the KX and adds aperture priority auto exposure and a metal vertically running shutter with a much wider range of 8 secs to 1/1000 sec.

This model is very much the forerunner of the professional LX, and indeed it lasted in the catalogues until the appearance of the LX in 1980. A K2 DMD motor drive capable version was also available, unlike the KX motorised version which was intended for market but as far as I can determine never was actually released.

Despite being available for much longer than the 12 months or so of the KM and KX, it is still relatively difficult to find good examples of the K2. If one could be found, then it would be an excellent camera to use. The one tiny design detail I d not like is the placement of the ASA ring around the lens mount, which I find to be very fiddly indeed. Otherwise, a splendid professional tool.

The Pentax K1000

The K1000 was released 12 months after the first three bodies, and is basically a KM without the self timer/stop down mechanism. As such it is a lower specified camera, but its design is as perfect as the KM that it is based on.

This is a thoroughly recommended basic, rugged and reliable camera. Unfortunately, everyone knows this and the price can be surprisingly high. (Even higher for the brown leatherette and gold finished versions....) The K1000SE is a rare variation where the microprism focusing screen is replaced with a split image. It is worth noting that all the Pentaxes in this range could be ordered with a split image screen if desired.

The K1000 is probably one of the last SLRs to be manufactured using the more traditional techniques, and presumably cost eventually meant its demise. For many years it was the workhorse of colleges everywhere, and may be the camera that many  professionals of the period cut their teeth on.

The K Series Lenses - Primes

Initially, many of the SMC Takumar screw-thread lenses were introduced in bayonet mounts, along with the new cameras, with their optical construction unchanged. These are indicated on the table of lenses with an asterisk. This was no bad thing – these lenses are superb examples of optical engineering and will all give outstanding results.

Some other changes were made at the same time. The use of 49mm filters was largely replaced by a 52mm filter size. The minimum aperture was generally extended from f/16 to f/22, and even smaller apertures for the telephoto lenses. Although resolution suffers at these small apertures because of diffraction effects, the increase in depth of field can be worthwhile and it is good to be given the choice.

Some new designs had already been introduced right at the end of the screw-thread era. Lenses such as the incredible 15mm f/3.5 remained unchanged as they were; remounted eventually from the K version straight to the later A version, without a compact M version being developed along the way.

Others, such as the 24mm f/2.8 were introduced late in the K series run, and are very compact without ever being designated “M”. These lenses too went straight into A versions later.

Finally, some lenses never needed to be changed and the new K series lens is exactly the same configuration as the previous screw thread one. There is no such thing as a compact conventionally designed 1000mm f/8, whichever way you look at it!

So as regards prime lenses, the K series, like the SMC Takumars before them, are at the peak of the lens makers’ art. Buy them and use them with confidence.

However, equally well be aware that the design parameters of lenses can change. For example, the SMC Pentax 55mm f/1.8 is a gorgeous lens, with impeccable “bokeh”, long before the word was invented. (Bokeh being the quality of the gradation of any out-of-focus areas). These lenses are very, very nice in terms of their optical qualities, but these qualities are subtly different from those of, say, the SMC Pentax-M 50mm f1.7. This means that K lenses are well worth trying. You may be very pleasantly surprised by their pictorial style, and may well be persuaded to try others in the range.

The only downside of this is that those “in the know” compete quite actively for these lenses on eBay and elsewhere, so prices are high and unfortunately mint examples are becoming increasingly difficult to find. But they do exist, so persevere and you will find some exciting lenses.

Pentax K Series Prime Lenses
Specification Elements/Groups Min Focus Filter Size Weight Comments
17mm f/4 11/7 0.2m BI 234g *
15mm f/3.5 13/12 0.30m BI 550g *
18mm f/3.5 12/11 0.25m BI 328g  
20mm f/4 12/10 0.25m 58mm 300g  
24mm f/3.5 9/8 0.25m 58mm 248g *
24mm f/2.8 9/8 0.25m 52mm 195g  
Wide Angle
28mm f/2 9/8 0.25m 52mm 423g  
28mm f/3.5 8/7 0.30m 52mm 261g  
28mm f/3.5 Shift 12/11 0.30m BI 611g Shift Lens
30mm f/2.8 7/7 0.30m 52mm 215g  
35mm f/2 8/7 0.35m 52mm 295g *
35mm f/3.5 5/4 0.35m 52mm 161g *
50mm f/1.2 7/6 0.45m 52mm 391g  
50mm f/1.4 7/6 0.45m 52mm 265g *
55mm f/1.8 6/5 0.45m 52mm 221g *
55mm f/2 6/5 0.45m 52mm 221g *
85mm f/1.8 6/6 0.85m 52mm 331g *
85mm f/2.2 Soft 2/1 0.57m 49mm 235g Soft Focus
105mm f/2.8 5/4 1.2m 52mm 304g *
120mm f/2.8 5/4 1.20m 52mm 355g *
135mm f/2.5 6/6 1.50m 58mm 483g *
135mm f/3.5 4/4 1.50m 52mm 365g *
150mm f/4 5/5 1.8m 52mm 373g *
Long Telephoto
200mm f/2.5 6/6 2m 77mm 950g  
200mm f/4 5/5 2m 58mm 535g *
300mm f/4 7/5 4m 77mm 1020g  
400mm f/5.6 5/5 8m 77mm 1285g *
500mm f/4.5 4/4 10m 52mm 3366g *#
1000mm f/8 5/5 30m 52mm 5294g *#
1000mm f/11 Reflex 6/4 8m BI/52mm 2300g # Mirror Lens
2000mm f/13.5 6/4 20m BI/52mm 8000g # Mirror Lens
50mm f/4 4/3 0.23m 52mm 241g *
100mm f/4 5/3 0.45m 52mm 370g *
100mm f/4 Bellows 5/3   52mm 186g *
* Optically identical to the last SMC Takumar versions
# Filters are attached to rear of lens mount

The K Series Lenses – Zooms

The zoom revolution was starting to gain traction, but was not yet totally under way in 1975, although new glass types were becoming available and, combined with the SMC coating, new designs were just starting to appear.

The first of the “new” zoom designs was probably the 28-50mm, which just makes it into a K version before the other revolution of the M series sweeps it all away...

The zooms in the K range are capable of excellent results, but they are very big, very heavy and do not focus particularly closely. A close focusing distance of 3.5m (12 feet) for a 85-210mm lens is not useful in modern zoom terms.

Some of these zooms have 58mm filter threads, but they are intended for use with a special intermediate filter ring that enables the use of 67mm filters. The reason for this is that smaller filters may cause vignetting.

Pentax K Series Zoom Lenses
Specification Elements/Groups Min Focus Filter Size Weight Comments
28-50mm f/3.5-4.5 10/10 0.55m 52mm 310g  
45-125mm f/4 14/11 1.5m 67mm 612g *
80-200mm f/4.5 15/12 1.6m 52mm 555g  
85-210mm f/3.5 12/11 3.5m 67mm 1050g  
85-210mm f/4.5 11/10 3.5m 58mm 729g *
135-600mm f/6.7 15/12 6m 52mm 4070g *#
400-600mm f/8-12 12/7 3m 67mm/40.5mm 730g # Mirror Lens
* Optically identical to the last SMC Takumar versions
# Filters are attached to rear of lens mount

Using M42 Screw Thread Lenses

The Pentax Adapter K is a simple steel adapter that enables screw thread lenses to be used on the K bayonet fit cameras, and is yet another example of Pentax supporting backwards compatibility. All the fine screw thread lenses do not have to be discarded but can continue to this day.

The technique is to attach the adapter to the lens and then bayonet the lens onto the camera body. When wishing to change lenses, the lens can be unscrewed and another lens screwed in. This effectively converts the camera body into a screw thread body.

To remove the Pentax Adapter K release the small spring catch with a small pointed objects such as a small screwdriver, turn the adapter and let it fall out into the palm of your hand.

Cheap third party adapters can be fine, but also can release bits of chrome plating into the camera as they wear, so perhaps are best avoided altogether. The genuine Pentax adapters may seem expensive, but they are sure to give excellent service without risk.

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John Riley

My specialised interest in Pentax started from the first moment I looked through the viewfinder of my first Spotmatic, the SP1000. That gorgeous clarity, sharply defined within a pure black frame is my definitive way to view the world and make images. Pentax is a superb example of a range of manufactured tools that is both the path to creativity and also a gem of engineering elegance and excellence in its own right.

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