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SMC Pentax-F 1.7x AF Adapter Review

John Riley reviews this dual purpose Pentax adapter which not only acts as a 1.7x teleconverter, but also provides auto focus for older K Mount lenses, such as the M & A Series.

Posted: 02/05/2024 - 14:40

Handling and Features

SMC Pentax-F 1.7x AF Converter - Front View

Pentax have always offered excellent backwards compatibility to their users. This started as early as the move from Asahiflex lenses to 42mm screw thread Takumar lenses, with an adapter being available to use the old lenses on the new bodies. Likewise, with the move to the K bayonet mount, an adapter was provided to enable the continued use of all the thousands of screw thread lenses in circulation. So it might come as less of a surprise than it might have been that with the advent of AF lenses, an adapter was provided to turn many manual focus lenses into AF lenses.

It is an intriguing thought indeed, so having acquired a mint example of the adapter, let's have a look and see how practical and how effective it is, not only in regards to AF performance but also in terms of optical quality.

SMC Pentax-F 1.7x AF Adapter Handling and Features

The adapter was manufactured from 1987-1997 and is an F series optic, sharing the features and limitations of the F series, the first of the AF lenses. This basically means the electronic mount does not transmit MTF data nor does it identify the focal length of the attached main lens. The first task is to enter the focal length when prompted, but to be fair this does not need to be done every time the camera is switched on, only when we change the lens. The previously set value is displayed on switch on, but can be dismissed with a light touch on the shutter release. The adapter is also a 1.7x converter, so when using a 50mm lens the actual focal length is 85mm. There is a 1.5 stop loss of light, so that would make an 85mm f/2.8 lens from a 50mm f/1.7.

SMC Pentax-F 1.7x AF Converter - Rear View

All K mount lenses can be used, from the original K lenses through to the M series and then to the A series, from 16mm to 400mm. It is stipulated that the prime lens should have an aperture of f/2.8 or faster, but it can be possible to use f/4 and even slower lenses in good light, so it is an area for experimentation. Of the A series lenses that the instruction book lists, one zoom is included, the SMC Pentax-A 35-105mm f/3.5, although the macro setting will not work. The 85mm f/2.2 Soft cannot be used either. For the purposes of this review the SMC Pentax-A 50mm f/1.7 lens was mainly used, with a few shots successfully made with the SMC Pentax-M 75-150mm f/4 zoom. Use of an A series lens is more convenient as the aperture can be set normally by the lens or camera. The adapter supports Tv, P, Av and M exposure modes. With earlier lenses, stop down metering will be needed using the green button in the way detailed in the M series article.

The adapter does not add very much bulk to whatever lens is chosen, weighing in at a modest 135g and measuring just 64mm x 26mm. It comprises 6 elements in 4 groups. There is no more to it than that, as it acts as a teleconverter, achieving focus by moving its elements. All that needs to be done is to manually focus very approximately so the limited adjustment range of the adapter can do the final fine focusing. Depending on the throw of the manual focus of the main lens, most of the time very little help needs to be provided and certainly with the 50mm lens it acts pretty much like any other AF lens, without intervention from the photographer.

SMC Pentax-F 1.7x AF Converter - With 50mm On K-1

Focusing speed and accuracy are very impressive, without any hunting or hesitation. AF systems have improved dramatically since the original issue of this adapter, so perhaps modern cameras can work even better with it than the original AF film cameras ever could. In terms of convenience this brings the A series lenses in particular into new life. Of course, it is received wisdom that teleconverters do not help much with optical quality, so let's now have a look at the performance and see if the end result is up to the mark.

SMC Pentax-F 1.7x AF Adapter Performance

It is obviously impractical to test every lens at every aperture to find out which ones excel and which are less happy, but the sharpness graph looks at how the SMC Pentax-A 50mm f/1.7 fares with and without the adapter, at three apertures, wide open, stopped down and closed down almost all the way.

How to read our MTF charts

The blue column represents readings from the centre of the picture frame at the various apertures and the green is from the edges.

The scale on the left side is an indication of actual image resolution and sharpness as LW/PH and is described in detail above. The taller the column, the better the lens performance.

For this review, the lens was tested on a Pentax K-1 using Imatest.

The lens on its own shows very good sharpness centrally, at f/1.7, f/5.6 and f/16. The edges are fair at f/1.7 and very good at f/5.6 and f/11.

Using these same settings on the lens, using the adapter shows central sharpness to be very good at f/2.8 and f/11 and fair at f/32. The edges are soft at f/2.8, very good at f/11 and good at f/32.

Bearing in mind an 85mm f/2.8 lens might make a very effective portrait lens, the soft open aperture edges might well be advantageous. When stopped down, the combination is very impressive and although the figures do drop compared to the lens alone, they do not drop by very much. This is a great performance and images shot at mid apertures are crisp and sharp.

How to read our CA charts

Chromatic aberration (CA) is the lens' inability to focus on the sensor or film all colours of visible light at the same point. Severe chromatic aberration gives a noticeable fringing or a halo effect around sharp edges within the picture. It can be cured in software.

Apochromatic lenses have special lens elements (aspheric, extra-low dispersion etc) to minimize the problem, hence they usually cost more.

For this review, the lens was tested on a Pentax K-1 using Imatest.

CA (Chromatic Aberration) is low in the prime lens, and only compromised at the edges with the adapter. Even so, software can help to sort that, and in normal use the CA is not particularly intrusive.

Distortion measures -0.83% barrel for the lens alone and -0.22% barrel with the adapter, actually an improvement. This is very close to being rectilinear.

Bokeh is the smoothness of the out of focus areas in an image and no doubt that will change depending on the lens used. With the 50mm f/1.7 it is pleasantly enhanced, especially at the wider apertures where the edges are naturally softer.

Flare depends very much on the prime lens, and the SMC lenses are generally excellent in this respect. The lens plus adapter are no exception to this, and although it is possible in extreme lighting to create some mild artefacts, contrast is well maintained and flare is not obtrusive.

Vignetting is reasonable for the lens on its own, especially when stopped down, and reduced to very small amounts with the adapter.

Aperture Vignetting
f/1.7 lens only -2.1
f/5.6 lens only -1.2
f/16 lens only -1.2
f/2.8 lens + adapter -1
f/11 lens + adapter -0.3
f/32 lens + adapter -0.3

Value for Money

For somewhere between £120 and £190 a great example of this adapter might be found, and given that it breathes new life into all our manual focus lenses, that could well be thought of as excellent value for money.

SMC Pentax-F 1.7x AF Adapter Verdict

The SMC Pentax-F 1.7x AF Adapter is a unique bit of kit that performs an incredibly useful function. It does revitalise our MF lenses, especially so with the A series that carry electronic contacts. This means an end to stop down metering. Handling is absolutely straightforward, with very few requirements beyond the use of f/2.8 or faster lenses for best results. Even this can be stretched a little, depending on the lenses and the light levels. The 1.7x crop does mean we end up with different focal lengths, but this can work both ways and could be an advantage for telephoto lenses, albeit a disadvantage for wide angles.

The real forte is the fast prime lens, and the SMC Pentax-A 50mm f/1.7 was perfect for this review. In summary, the SMC Pentax-F 1.7x AF Converter works, it works well and does not overly impact on optical quality. Highly Recommended.

SMC Pentax-F 1.7x AF Adapter Pros

  • Revitalises use of MF lenses
  • AF is fast and accurate
  • Electronic contacts, as other F series lenses
  • Modest impact on sharpness
  • Low vignetting
  • Very low distortion
  • Light and compact

SMC Pentax-F 1.7x AF Adapter Cons

  • Some edge CA
  • Wide open edges soft

Features: 4.5/5
Handling: 5/5
Performance: 4.5/5
Value: 4.5/5
Overall Verdict: 4.5/5

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.

Biography Profile John Riley Photography

Pentax SMC Pentax-F 1.7x AF Adapter Specifications

Manufacturer Pentax
Effective Magnification 1.7x
Fitting Pentax K Mount
Box Contents
Box Contents No Data
Weight 135g
Width 64mm
Height 26mm

Posted 07/05/2024 - 22:57 Link
A very interesting read John, thanks for your informative insight

I guess the 1.7 converter is a very rare product in the market place today, but I did purchase a 1.4 converter several years back, that is a valued possession

regards, Alan
Posted 21/05/2024 - 17:24 Link
Believe it or not, but you can change a manual focus Takumar lens into an autofocus Takumar lens. What you need to achieve this is an SMC Pentax-F 1.7x AF Adapter, a Mount Adapter K (maybe also an Asahiflex Adapter) and a piece of aluminum foil!

According to the instruction manual, the AF Adapter does not work in conjunction with M42 lenses and a Mount Adapter K. I have discovered that this is due to the fact that the backside of Takumar lenses does not short-circuit the electrical contacts of the AF Adapter. If you put a strip of aluminum foil on the electrical contacts of the AF Adapter, you get a real AF Takumar!

I have tried the AF Adapter on an AF Pentax with various Takumar lenses from 24 to 200 mm. The camera can focus perfectly, even with lenses that have a maximum aperture of f/4. As long as there is sufficient light and subject contrast, focusing is fast and accurate. Under a cloudy sky and with subjects that have little contrast, focusing fails!

As the focal length increases, the shortest focusing distance becomes progressively longer. With a 24 mm lens you can still focus to the same distance as with the “bare” lens. With a 50 mm lens, however, you get to 1 meter (instead of 45 cm), with an 85 mm lens you get to 2.5 m (instead of 80 cm), with a 120 mm lens you get to 5 m (instead of 1.2 m) and with a 200 mm lens you get to 12 m (instead of 4 m).

The best way to use the AF Adapter is to focus manually to roughly the setting you need for a specific subject and let the camera use the AF Adapter to take over the actual final focusing. After getting used to this method, it works very comfortably.

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