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SMC Pentax-M 100mm f/2.8 Lens Review

John Riley reviews the slightly rarer 100mm f/2.8 telephoto from the Pentax M Series of lenses. How will this small & light lens perform? John provides his verdict along with sample photos and test charts to help answer that question.

Posted: 21/05/2024 - 17:57

Handling and Features

Our look at the M series lenses continues with the less commonly found SMC Pentax-M 100mm f/2.8, which follows on from the highly regarded 105mm lenses of the screw thread and then early K mount ranges. The traditional second lens for beginning photographers was of course the 135mm, but there are many who find this too long and restrictive and who much prefer the smaller, lighter and arguably more versatile 100mm.

In the 1970s I had a copy of this lens that was well used until it was sadly stolen and I have remembered it as a top performer every since. Now is the moment to put that to the test and see if memory cheats or whether it is indeed something special. Time to pick up the 36MP Pentax K-1 DSLR and find out.

SMC Pentax-M 100mm f/2.8 Handling and Features

Even smaller than the 135mm, the SMC Pentax-M 100mm f/2.8 weighs in at a very light 224g, measuring a svelte 62.5mm x 55.7mm. This looks tiny on the K-1 and yet not so small that controls, such as they are, become fiddly. There is no bayonet fit for a lenshood and a hood was not provided with the lens in the box when new, although a dedicated hood was available to purchase. This plastic hood is a snap on design, every bit as convenient to use as a modern bayonet fit design. As was the style of the day, the hood is shared between the 85mm f/2 and 100mm f/2.8 and f/4 (Macro) lenses. There is a standard 49mm filter thread.

Manufactured from 1977-1984, the lens has the usual ultra-slick focusing action, and this is a tactile joy on its own right. There is a cut out for the distance scale, clearly shown in blue (feet) and yellow (metres). Focusing is down to 1m or 3.3 feet, for a maximum magnification of 0.13x. This is close enough to photograph roughly an A4 document, so a useful degree of magnification.

There is a depth of field scale that is wide enough to be actually usable. The infra-red index shows us the amount of focus correction to bring IR light to the correct point of focus. It is at the f/8 mark, so having focused the lens we then need to move the focusing ring closer to the IR index mark to ensure IR images will be sharp. This is all because these lenses are not corrected for IR light.

Optical construction is 5 elements in 5 groups, with no special glasses used. The diaphragm comprises 6 blades. The SMC (Super-Multi-Coated) coatings are highly effective and remain so to this day.

The aperture ring is nicely engineered, with a smooth action and click stops at half stop intervals, and full stop intervals between f/2.8 and f/4 and f/16 and f/22. There is no A setting and there are no electronic contacts on the mount, so M series lenses are used with stop down manual metering.

This basic mount, developed jointly with Zeiss, was introduced in 1975 with the K series of cameras and lenses, and replaced the long established and ubiquitous 42mm screw mount. At the time this caused some consternation, but there were good reasons to make the change. The lens throat was increased in diameter from 42mm to 45mm, thus enabling more ambitious lens designs, especially fast primes and new wider ranging zooms. There is also the obvious convenience of faster lens changing. In typical Pentax form, an adapter allows the use of older screw mount lenses and we still make use of this today, opening up a huge number of screw thread options going back well over 60 years.

Being a manual focus lens with no electronic communication with the camera, there are some handling pointers to think about. When the camera is switched on it will ask what the focal length is, for the purposes of setting the shake reduction system correctly. Then we need to focus the lens, and this may be a lost art for many that needs relearning, but with no visual aids in the viewfinder to help. No microprisms to shimmer when out of focus, no split image rangefinder to align to find perfect focus, just the eye judging the point of sharpness. Unfortunately most eyes are quite poor at this and the amount of variation when finding that point of focus is significant, especially with a telephoto lens where depth of field is reduced. The answer is Live View, and using LV with magnification reveals that point of focus easily and also shows up how dismal the ability of the human eye can be. This has always been known and in the past various magazines have demonstrated this in tests. At longer distances and smaller apertures we get away with it, but close up with wide apertures or where precision is necessary then the use of LV with magnification is the way forward.

Shake reduction is very welcome with even a 100mm lens. The alternative is a tripod with SR switched off. If the self timer is used SR is automatically disengaged and the time delay allows any vibration from pressing the shutter release to die away. In addition, the mirror is raised before the exposure when the self timer is used, thus removing any shutter vibration from the equation.

As there is no communication with the camera, exposure is via stop down. Focus the lens, set the aperture on the lens, press the green button and the lens will briefly stop down, exposure will be measured and the camera will set the shutter speed. The lens opens up again. Press the shutter release and the lens stops down, the exposure is made and the lens opens up again. With practice, the process is simple and quick.

We can also use an APS-C crop sensor DSLR of course, in which case the “35mm equivalent” field of view for a 100mm lens will be around 150mm. The real benefit lies in its use on a full frame DSLR such as the K-1. The lens is ideal for portraits, landscapes, close up sports and all close range telephoto applications.

Now let's find out how the lens performs in the technical tests.

SMC Pentax-M 100mm f/2.8 Performance

Central sharpness is excellent from f/2.8 through to f/5.6 and very good from f/8 to f/22. The edges are excellent from f/2.8 to f/5.6, very good from f/8 to f/16 and still good at f/22. This a very strong result and makes the lens fully usable on current DSLRs.

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.

CA (Chromatic Aberration) is kept under 1 pixel all through the range, and although some slight fringing might be seen on very demanding subjects it is not obtrusive.

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.

Distortion measures +0.86% pincushion which is very satisfactory and it is unlikely that further correction will be needed. Software is the answer if necessary.

Bokeh is the smoothness of the out of focus areas in an image, and in its day was not something often discussed, not having aquired the name yet. Here it proves to be very smooth and certainly helps to give that magical something to the lens that we can recognise, even if we can't actually technically measure it.

The SMC coating has improved over the years, but is very satisfactory and in the 1970s was ahead of many of its competitors. Whilst it is possible to generate some slight artefacts, if we try hard to do so, flare resistance is generally excellent.

Vignetting starts off at -1.5 stops, but by f/5.6 is a very mild -0.6 stops. Impressive, especially for such a compact lens.

Aperture Vignetting
f/2.8 -1.5
f/4 -0.8
f/5.6 -0.6
f/8 -0.6
f/11 -0.6
f/16 -0.6
f/22 -0.6

Value for Money

This example of the lens is in excellent condition and was picked up in a private sale for just £80. Whilst this is more expensive than the cost of a 135mm, it still remains excellent VFM.

SMC Pentax-M 100mm f/2.8 Verdict

Clinging on to the tradition of 135mm lenses means that SLR, and now DSLR, users are hung up on using what is basically a lens that is too long to be convenient. A 135mm lens might focus down to 4 feet, some only 5 feet, but this 100mm lets us get to within 3.3 feet. We are thus closer to our subject and if this is a portrait communication becomes easier. The 100mm lens is lighter, smaller, often faster at f/2.8 as opposed to maybe f/3.5 or even f/4 for a contemporary macro lens. The biggest competitor may well be the SMC Pentax-M 100mm f/4 Macro, but this is more expensive, heavier, much larger and very much slower to focus having a long focusing throw.

So the SMC Pentax-M 100mm f/2.8 overcomes all these points and is a convenient, fast and gorgeous to use lens. A question was posed at the start of this review – does memory cheat? No, the lens totally justifies its reputation, delivering excellent images that have that magic ingredient, the wow factor, the “Pixie dust”, call it what we will, and the lens is Highly Recommended.

Further reading for the M series of lenses:

SMC Pentax-M 100mm f/2.8 Pros

  • Excellent sharpness
  • Compact and light
  • Full frame format
  • Excellent value
  • Beautifully made
  • Very pleasing imaging characteristics
  • Excellent flare resistance
  • Low central CA
  • Modest distortion
  • Lovely tactile handling

SMC Pentax-M 100mm f/2.8 Cons

  • Manual focus
  • Stop down metering

Features: 4/5
Handling: 4.5/5
Performance: 4.5/5
Value: 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-M 100mm f/2.8 Specifications

Manufacturer Pentax
Lens Mounts Pentax K
Focal Length 100mm
Angle of View No Data
Max Aperture f/2.8
Min Aperture f/22
Filter Size 49mm
Stabilised No
35mm equivalent 100mm
Internal focusing No
Maximum magnification 0.13x
Min Focus 100cm
Blades 6
Elements 5
Groups 5
Box Contents
Box Contents No Data
Weight 224g
Height 55.7mm

Posted 07/06/2024 - 08:48 Link
This is one of my favourite lenses on digital (crop sensor) or film. I found the plastic hood can get knocked off easily so I often use the metal hood designed for the Tak 105.
Kris Lockyear
It is an illusion that photos are made with the camera… they are made with the eye, heart and head. Henri Cartier-Bresson
Lots of film bodies, a couple of digital ones, too many lenses (mainly older glass) and a Horseman LE 5x4.

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