Pentax 50mm Standard Lenses - Part Two

In part two of our Pentax 50mm lens round up, John Riley covers the emergence of the electronic lens era through to the present day. John provides a 50mm history, explains the K mount variants along with guide to their typical use.

Posted: 25/03/2024 - 15:06

The Electronic Era

By the early 1980s AF and therefore AF lenses were on the horizon. After a few clunky starts manufacturers started to produce viable SLR lenses, with AF motors either in the camera body, or in the lenses themselves. Development was motivated by improvements in micro-motors and also the advent of Lithium batteries that are able to provide sufficent power to make all this possible.

Co-operation between Zeiss and Pentax produced the K mount in 1975 and was first seen in the Pentax KM, KX and K2 cameras. These use the basic, plain steel mount and this was continued into the M series, starting with the hugely successful ME and MX cameras. It is a credit to Zeiss and Pentax that the basic K mount was right from the start and has been suitable for the addition of various stages of electronic contacts, surviving as a perfectly viable AF mount right to this day.

Auto aperture setting on the lens to the right

The 1980s saw a need for the camera to be able to control the aperture. Up to this point, auto exposure has been either manual (shutter speed on camera, aperture on lens) or Aperture priority (aperture set on lens, auto shutter setting appropriate shutter speed). This is the Pentax way of operation, and it is long forgotten by now that initially even this was a major change in design that was resisted by many. Most other marques were using a trap needle system of mechanical linkages, where the shutter speed was set on the camera and the lens stopped down automatically depending on the position of a needle that indicated correct exposure. Reversing this was unthinkable for many, but Pentax proved that the system not only worked with a high degree of accuracy but it also meant that whatever was in front of the camera to influence exposure, such as perhaps bellows or extension tubes, resulted in the correct shutter speed being chosen by the camera.

Adding the electronics to the mount however gives the ability to have the camera also set the aperture, so now we have Program and Shutter priority modes available as well as Aperture priority and Manual. This is the KA mount, with an “A” position on the aperture ring. Set to “A” the aperture is controlled from the camera, set to the regular scale it can be used manually instead.

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K Mount Variants

We then move to variations and development as the AF range of lenses also grows and develops and new requirements arise.

K Mount (left), KA Mount (centre), KAF Mount (right)

K mount is the basic stainless steel mount that has no communication with the camera. Lenses are K and M series.

KA mount adds electronic contacts so the camera can set the aperture. Lenses are A series for program cameras such as the Super A/Program A.

KAF mount adds a drive shaft to enable the camera to drive the AF system. Cameras are the SFX/SF1 onwards, lenses are Pentax-F.

KAF2 mount adds two power contacts and enables Power Zoom lenses to be used. In addition, MTF data can be exchanged with the camera. Later, the power contacts can be used to drive SDM and DC AF motors in the lenses. Up to this point the AF motors have been in the camera body. Lenses are FA, FA-J and DA series. FA-J lenses are the first to have no aperture rings and were introduced with the *ist film camera. DA lenses are crop sensor lenses introduced with the APS-C crop sensor DSLRs.

KAF3 mount has no drive shaft for AF, so with these lenses the motors in the lenses are the only method of AF. Non-compatible cameras without power contacts render these lenses manual focus only.

KAF4 mount lacks any aperture control linkage and these lenses use PLM pulse motors that drive the AF but also drive the operation of the aperture stop down.

Ricoh K/R mount is a variation that on some Ricoh lenses adds a pin to the K bayonet. Check if this pin is there because if used on an AF camera body with a drive shaft the pin can lock into the shaft and jam the lens on the camera. Removing the lens from the camera can then need a serious bit of surgery. The solution is to remove the pin, which avoids the issue.

All this development is to make lenses faster and more accurate. AF speed and accuracy is an obvious benefit, but also electronic aperture control as this is much more accurate and stops down very precisely every time, even with high speed shooting.

The amount of compatibility is excellent and almost all Pentax lenses can be used on almost all Pentax camera bodies. Where there are exceptions, the lenses can still be used, but within perhaps some limitations. For example, an A series lens on an AF body will still be manual focus. The exception to this is the rather neat SMC Pentax-F 1.7x AF adapter, which enables AF with faster manual focus lenses. Yet another example of how Pentax have always tried to have as much compatibility as possible throughout their range.

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General Features of 50mm Lenses

As the 50mm standard lens was the one usually supplied with film cameras, there are huge numbers of them out there to be bought second hand, many in mint condition. They are high quality, versatile and able to be optically abused whilst still maintaining quality. For all these reasons and their “normal” view of the world, similar to the human eye, they survive to this day, despite the universal standard zoom lenses that have replaced them as the first purchase.

The different lenses have different characteristics and this has not changed much over the years. Obviously there are exceptions to the norm but some general indications as to what we might expect can be given.

50mm f/1.2 lenses are specially corrected to render point light sources well, as they will probably be used in low light condtions. They may not be the best option for close up shooting, nor for copying as the image field may not be flat enough, so centre and edge wiill not be in focus at the same time. For rendering of superb bokeh and for miniscule depth of field these could be a good choice, despite being heavy and pricey.

50mm f/1.4 lenses are the classic reportage/street lens, almost as fast but keenly priced, compact and with high contrast. This will be reflected in high MTF10 figures but perhaps not the highest at MTF50. This results in lower fine detail but images will look sharp because of the high contrast.

50mm f/1.7 or f/1.8 lenses are the traditional first lens and have better fine detail and probably a flatter field. They are an excellent compromise between quality, speed and cost.

50mm f/2 lenses are generally the low cost option, but in some cases can be as good at the faster options. The design is likely to show fine detail well and have a flatter field. In some cases, such as the later screw thread lenses, the f/2 and f/1.8 versions were actually the same lens, but the f/2 version had a baffle inserted to reduce the maximum aperture so it could be sold as the low cost option. More recently, the optical design has been different so the lower cost is reflected in that.

50mm Macro lenses are the ones to look for for the highest resolution, the greatest fine detail, and also for a flat field so images of documents will be sharp centre and edge even at wide apertures. Although designed for close up work, Pentax macro lenses will also perform well at longer distances, so they can be multi-purpose.

New designs of 50mm lenses have appeared in recent years and one fine example is the HD Pentax-D FA* 50mm f/1.4 SDM AW, which is a superb 50mm lens and the first in a new ultra-high quality line meant to rival Zeiss. The cost is two fold – the price of course but also the weight and the huge dimensions.

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Timeline of Pentax Standard Lenses

We now move to the timeline of Pentax lenses with electronic communication between lens and camera, the last of the manual focus lenses in the A series and the arrival of autofocus, digital crop sensor cameras and eventually digital full frame cameras.

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Full Frame and Crop Sensor

Apart from the DA lenses all the lenses listed are “full frame”, that is, suitable for use on 35mm format film SLRs and also digital DSLRs. In practice, that means for digital cameras, the Pentax K-1 and K-1 II. The lenses can still be used on the other cameras in the Pentax DSLR range, all using APS-C (crop sensor) format. For a 50mm lens, the modified field of view on an APS-C camera is similar to 75mm. So, a 50mm standard lens is approximately equivalent to a 75mm short telephoto and a 40mm lens equivalent to 60mm.

It is also possible to use a DA crop sensor lens on a full frame camera, either switching in the crop system on the camera or even using it as if full frame. This latter action will result in vignetting, darkening of the outer areas of the image as the coverage of the DA lens is not wide enough for the larger format. It is worth checking out a lens though, as some DA lenses might be usable on full frame, especially if stopped down.

Summary

In general, 50mm lenses are small and light, have few elements compared to zoom lenses so enjoy low flare, high contrast and give highly detailed images. This is as true of the older lenses in part One of this article as it is of the current lenses still being manufactured. Coupled with low price, plentiful suppy and fast apertures for low light they are a valuable addition to any photographers' kit. To those who have only ever used fairly slow zoom lenses they could well be a revelation.

If you missed Part One of the 50mm roundup - Pentax 50mm Standard Lenses - Part One

Members gallery photos using: smc PENTAX-DA 50mm F1.8

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