Pentax – the road to insignificance

It’s a bit early to write Pentax’s obituary. But there’s no denying that the company (now a subsidiary of Ricoh) is  a mere shadow of its former self.

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The Spotmatic – a great camera in the sixties – but Pentax should have replaced it earlier than 1975

From the mid fifties to the early seventies, the Asahi Optical Corporation  was an innovator. They scored an impressive number of “first” :

  • First Japanese single lens reflex camera to enter production (Asahiflex – 1952)
  • First reflex camera with instant return mirror (Asahiflex II b – 1954)
  • First modern single lens reflex (SLR) camera,  with a pentaprism at the center of the top plate, a winder arm and shutter speed knob on the right side, and a folding rewind crank to the left (the “original” Pentax of 1957). This was to be the model for all other reflex cameras for the next 20 years. The camera was so important for Asahi that the whole corporation became later known as “Pentax Corp”.
  • First SLR with Through the Lens (TTL) metering on the market  (Pentax Spotmatic – 1964)
  • First automatic exposure SLR with an electronic shutter (Pentax Electro Spotmatic – 1971)
  • First multi layer coated lens  (or at least the first manufacturer to communicate about multi-layer coated lenses to the public at large – 1971)

As a result, Asahi Pentax was a sales leader in the sixties and early seventies: for example, it was the first Japanese camera company to sell over one million SLRs.

Pentax lost its supremacy during the first half of the seventies

  • they stuck to the Spotmatic form factor until 1975
  • they stuck with stopped down metering on their line of bread and butter Spotmatic cameras until the launch of the Spotmatic F in 1973, and to the m42 screw mount until far too late. Because they had adopted a proprietary bayonet early on, Minolta and Nikon had been able to offer full aperture metering (a major comfort improvement for the photographer) since 1966, with Canon and Olympus following in 1971.
  • As a result, Pentax was out-innovated by new entrants:  Olympus OM-1 (the first ultra-compact SLR and camera system); Fujica ST-801 and ST-901 (first use of Silicon metering cells and of LED displays in the viewfinder); Olympus OM-2 (first implementation of On The Film (OTF) real time flash metering).
The Pentax Spotmatic F (1973) with a Pentax specific version of the universal 42mm screw mount – designed  for full aperture metering.

The second half of the seventies was not better:  Pentax was in reactive mode and started progressively being pushed to the bottom of the market :

  • Changes to their lens mount are always very risky for camera manufacturers. It may not bother the beginner or the amateur who are only going to shoot with the kit lens they bought with the camera, but it’s an invitation for enthusiasts and pros to reconsider their aleigence to the brand.  Between 1971 and 1976, Pentax changed the lens mount of its cameras twice.
  • Pentax could not compete with Canon and Nikon in the “pro” market because they did not have a modular camera to offer until they launched the LX in 1980, and after they did, they lacked some of the specialized lenses and the support network that the pros required,
  • they were out-innovated in the heart of the enthusiast market: Canon with cheaper to manufacture and feature rich micro-processor driven cameras such as the AE-1 and the A-1, Minolta with multi-mode SLRs.
  • they had to face new competitors in the “amateur” segment of the market with  Nikon and Olympus successfully entering the broader consumer market with cameras such as the EM and the OM-10 in 1979.

By the end of the eighties, Pentax had been relegated to the 4th position on the photo-equipment market, behind Canon, Minolta and Nikon. They had completed the transition to auto-focus SLRs, but were primarily known for their two remaining manual focus SLRs (the K1000 and the P3) and their water-resistant point and shoot cameras.

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Pentax P3 – a camera for beginners – very successful on the market (3 million sold between 1985 and 1997)

They survived until the advent of digital photography. Konica-Minolta’s deep troubles gave them one last chance of resurgence in 2003-2004. They recovered the #3 position on the market for a while. But after early successes – their first dSLRs, the *ist D and *ist DS were good cameras, technically on par with contemporary Canon and Nikon offerings –  they did not (or could not) keep up with the pace of their competitors, and let their market share decline to the point where their presence is hardly noticeable today.


More about Pentax’s last manual focus cameras in a few weeks with reviews of the Super-Program (Super-A),  P3 (P30) and ZX-M.

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Pentax’s last manual focus SLR – 1997

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Haleakalā volcano, Maui, Hawaii – Pentax *ist DS – Pentax 18-55mm lens.
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Hawaii – Big Island – Pentax *ist DS
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Hawaii – Big Island – Pentax *ist DS

Pentaprism, Contax, Pentax and Pentacon

Who created the first 35mm camera, or the first 35mm single lens reflex camera?

Difficult questions. First, you would have to agree on what constitutes exactly a “real” 35mm camera, or a “real” 35mm SLR, and then, you would have to determine what really counts: is it presenting a prototype at a trade show, applying for a patent, launching a limited series production, or inundating the world with tens of thousands copies of a “game changing” camera?

It is generally recognized that with the Leica, Leitz created the first commercially successful 35mm camera in the early twenties, and but it was not until 1932 when they launched the Leica II that the rangefinder camera with interchangeable lens had found its “real” final form.

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Foca *** with a Foca turret viewfinder (left) / Olympus OM-1n (right) The Foca, a French derivative of the pre-war Leica, is a good example of what a rangefinder camera with interchangeable lenses looks like. With its pointy prism housing, the Olympus illustrates the typical SLR shape.

The Contax S

1932 is also the year when Zeiss launched Leica’s most serious pre-war competitor, the Contax.

Zeiss was at that time the largest manufacturer of cameras in the world. They had a long tradition of innovation and a great team of engineers; conscious of the limitations of the rangefinder formula, they kept on working on a better solution until, after the war, they finally presented the Contax S, one of (if not the first) modern 35mm single reflex lens camera.

With its pentaprism, its horizontal curtain shutter and its 42mm screw lens mount, the Contax S was very close to the typical 35mm SLR design, and should have been commercially successful.

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Contax S – (second series from 1950). Courtesy of Pentax-SLR.com – the best source of information about early SLRs (not only Pentax)

But at that time, the Zeiss factories were in the Russian occupation zone (soon to become the German Democratic Republic) and all sorts of issues slowed down the roll-out to production: the Contax S only started to be mass produced at the very end of 1949. The launch of the Contax S also coincided with the start the Cold War – products from communist countries were not always welcome on the more affluent markets of the West – and to make the matters worse, the East German entity of Zeiss lost the rights to the Contax name in 1956. After considering multiple options (including apparently the “Pentax” name), the East Germans rebranded their cameras “Pentacon” (a portmanteau for Pentaprism and Contax) and the Contax S line of SLRs was abandonned.

Why is a pentaprism so important, that Zeiss and (later) Asahi changed the name of their cameras to include “Penta”? 

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On the ground glass of a plate camera, the image is reverse (top/down, left right) – image courtesy of http://www.michaelstricklandimages.com/

Composing a picture on a piece of ground glass located behind the lens is nothing new (plate cameras have been following that model forever), but the image is reversed top-bottom and left-right, which makes the composing process very slow and totally unsuitable to candid photography.

If a mirror inclined at 45 degrees is placed behind the lens, and the image projected on a piece of ground glass, it is not reversed top/bottom anymore, but is still reversed left/right. The photographers has to shoot from waist level, after having used a magnifying glass for focusing. It’s workable, but not the best formula for action shots,  journalism or simply spontaneous family photographs.

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Nikon F3 with the pentaprism viewfinder removed: the image formed on the focusing screen is reversed laterally.

 

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Left is right, right is left and of course the Coke and Powerade labels are also reversed

The pentaprism addresses all those issues – and as we all know from experience  with SLRs, the image is fully redressed, focusing is easy, and eye-level composition makes action photography intuitive even for beginners.

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Nikon F3 – The view of the same scene from the eye-level pentaprism viewfinder

Asahi Optical Co

Asahi started manufacturing lenses in Japan in 1919, and launched the first Japanese 35mm single lens reflex camera – the Asahiflex – in 1952. It was inspired by the pre-war German Praktiflex, but brought some improvements:  it had two finders: a waist level through the lens viewfinder (for focusing)  and a smaller eye level optical viewfinder to be used when taking candid snapshots.

In two critical areas, the Asahiflex was not as advanced as the Contax S:  it did not have a pentaprism viewfinder, and it used a narrower 37mm screw mount.

Asahi’s first major innovation came two years later with the introduction of the instant return mirror on the Asahiflex IIb (1954). The IIb was without equivalent for a while, but the step forward it represented was nothing compared to Asahi’s next giant leap, with the “Pentax” of 1957. The first (mass produced) Japanese camera with a pentaprism, it combined for the first time in a compact, elegant and well made camera the instant return mirror, the film advance lever, easy film loading with a hinged back, and the 42mm screw mount.

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The original Asahi Pentax camera from 1957 (source: official Pentax Web site).

The Pentax line of cameras sold by the millions and became the model that all other manufacturers would copy in the subsequent years. The Pentax name became so well known that the Asahi Optical Co. decided to sell all its products (including its line of medical equipment) under the name Pentax, before it finally changed its own name to Pentax Corporation in 2002.

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Pentax Spotmatic F from 1973 (left) and Nikon FM from 1977 (right). More than 15 years after its launch, the design of the “original” Pentax was still the model that all camera manufacturers were following

It is widely assumed that  “Pentax” is also a portmanteau for Pentaprism and Contax. According to Wikipedia, the name was purchased  by Asahi from the East German Zeiss  company just before the launch of the original Pentax SLR in 1957.

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Asahi Pentax – the top plate of the original model (1957) – source: eBay
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The Pentax Spotmatic F on 1973 – still true to the model defined by the “original” Pentax of 1957

Today, the single lens reflex formula is on its last breath – superseded by mirrorless cameras where the pentaprism has been replaced with a high resolution LCD – the Contax brand is dormant, and Pentax, as a subsidiary of Ricoh, is in life support with a line of three rather old dSLRs and no plan to launch a mirrorless system.

Sic transit…


More about

By far the most comprehensive source about Pentax cameras, as well as early SLRs of all makes: Pentax-slr.com

Asahi Optical Historical Club 

The official corporate history of Pentax (the Ricoh-imaging-co Web site)

The Asahi Pentax original (AP) from 1957 – CameraQuest


From a Pentax to another Pentax…

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Big Birds – Mableton, GA – Pentax Spotmatic F. Lens Pentax Super-Takumar 55mm f/2
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Cumberland Island National Seashore (GA) –  Some of the islands of the national park are limited to 50 visitors/day and have to be vacated before sunset. Pentax *ist DS – Lens Pentax 18-55mm
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Cumberland Island National Seashore (GA) – The Cumberland forts were built by the British in 1733 to protect their most southern colonies from the Spaniards. The forts were abandoned after the final defeat of the Spaniards at the Battle of Bloody March in 1742. Pentax *ist DS. Lens Pentax 18-55mm

The Pentax m42 lenses – meet the Takumars

The Asahi Optical Corporation (known for its Asahi Pentax and Pentax cameras) was founded by a gentleman named Kumao Kajiwara. The brother of the founder was a painter of some fame named Takuma Kajiwara, and in his honor, Asahi named its lenses “Takumar”. We’ve seen stranger things in the past: in the thirties, Leica had named a line of lenses “Hektor”, for Oskar Barnack’s dog, and in 1901, Daimler cars had been re-branded  “Mercedes” after the daughter of their main car dealer on the Cote d’Azur.

Takumar lenses still enjoy a very good reputation, and some of them are highly sought after and sell for hundreds of dollars.

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Pentax Spotmatic SP with a 35mm f/2 Super-Takumar lens

With most vendors (Canon or Minolta, for instance), the different generations of lenses are named for their mount (a Canon FL lens has a so called FL mount and works stopped down, while a Canon FD lens has the so called FD mount that enables full aperture metering).  No such thing with Pentax. The name of the lens (Super-Takumar as opposed to Super-Multi-Coated Takumar or SMC Takumar) relates to the coating of the lens. The most recent lenses (Super-Multi-Coated or SMC) are generally the ones with the updated lens mount supporting Full Aperture metering, but there are exceptions both ways. The only way to determine for sure that a Pentax screw mount lens can meter at full-aperture is to have a good look at the mount.

Coating and Multi-Coating – what is it about?

When it comes to the optical lenses used with cameras, flare is the enemy. And reducing light reflections also improves the contrast (the images look sharper). That’s why lens coatings were developed.

A coating treatment is engineered to block the reflections in a given wavelength. Multi-coating treatments block reflections in a wider range of wavelengths.

Lens coating was a process unknown to the public until Pentax and Fuji started using it as a differentiator in their advertising campaigns in the early seventies (it had been invented before WWII in Germany and had long been considered a military secret).

The 42mm Pentax lens series  – an over-simplified summary…

Auto-Takumar: 42mm lens mount, with aperture pre-set: the photographer has to cock the spring loaded aperture mechanism of the lens after each shot, and will compose and focus at full aperture. The lens will automatically stop down to the pre-set aperture when the shutter release is pressed. The pre-Spotmatic cameras of the late fifties-early sixties (Model K, Model S) came with Auto-Takumar lenses.

Super-Takumar: 42mm lens mount. Automatic pre-selection lens for stopped down metering cameras. They were released in the early sixties and their long sales run more or less corresponds to the Spotmatic’s. The aperture pre-set mechanism does not need to be cocked by the photographer anymore. And the lenses benefit from some form of single layer coating.

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The mount on two Super-Takumar lenses: on the left, the Pentax ES variant of the m42 screw mount with the prong transmitting the aperture value to the camera (generally found on S-M-C and SMC lenses) , on the right a Super-Takumar with the conventional m42 mount.

Super-Multi-Coated Takumar: Introduced with the Electro-Spotmatic of 1971, they provide full aperture metering on the ES, the ES II and the Spotmatic F cameras – the lens mount was modified and now transmits the pre-selected aperture value to the camera via a prong (because it made its first appearance with the Pentax Electro-Spotmatic, this variant of the m42 Universal mount is sometimes named Pentax ES mount).

Super-Multi-Coated Takumars remain compatible with the cameras with stopped down metering like the original Spotmatic and the  Spotmatic II (although compatibility issues arise when mounted on cameras of other brands and with some modern lens mount adapters). Obviously, they get their “Super-Multi-Coated” name from Pentax’s multi-coating.

SMC Takumar: Minor cosmetic differences with the “Super-Multi-Coated” Takumar. Same full aperture metering capabilities and same Pentax ES mount. Introduced with the Pentax ES in 1972.

To the despair of Zeiss and Nikon who had been manufacturing multi-coated lenses for years without letting it known, Pentax  decided to use “multi-coating” as a marketing differentiator – and using a short acronym such as SMC probably helped convey the message to the consumers.

In any case, Pentax’s SMC multi-coating was more than a marketing ploy: when Popular Photography tested the multi-coated lenses of Pentax against their competitors, the SMC coating proved to be the best by a wide margin.

The bayonet mount lenses launched with the KM, KX and K2 bodies of 1975 are simply named SMC Pentax.

Are they radio-active?

Some of the high-end (F/1.2, F/1.4) Super-Takumar  are radio-active- as are other ultra-luminous lenses from other vendors like Canon. Because the optical glass contained Thorium. The use of Thorium was banned at a later stage because of the harm it could do to the workers in the glass foundries.

I’m not an expert in this field – what I’m reading is that the lenses are not very radio-active (they would veil the film if they were), and that unless you grind the lens, and ingest or inhale the dust, you should be safe. (more about the issue: http://www.orau.org/ptp/collection/consumer%20products/cameralens.htm).

What to buy?

The Super-Takumar are probably the most common of all (thanks to their long production run), but even the shorter lived S-M-C and SMC Takumars are easy to find. Lenses compatible with the “universal” m42 mount abound, but there are very few third party lenses compatible with the Pentax ES variant. If you want a lens that does not exist in the Pentax SMC line-up (a trans-standard zoom, for instance), Tamron Adaptall lenses are the best option.

More about the Pentax 42mm lenses: http://www.klassik-cameras.de/Pentax_Takumar_e.html


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Horace – French bulldog – shot with Pentax 35mm f/2 lens (attached to a Fujifilm X-T1)

Pentax Spotmatic SP – why was it a disappointment? (for me)

Some cameras are a source of disappointment. Because they carry a famous brand name, had the privilege of being “the first camera to do this or that”, and because they still look cool, you feel compelled to buy one, and you don’t like it. Or don’t trust it. You don’t use it, and you sell it.

Pentax Spotmatic SP – Pentax Super-Takumar 35mmf/2

I had all the reasons to like the Asahi Pentax Spotmatic. I’m sympathetic to the brand – my first reflex camera was a Pentax MX that I kept for 15 years and my first digital SLR was a Pentax *ist DS (what a name!). Pentax also tends to make relatively small cameras,  and I tend to prefer small cameras to large ones. And I had bought a very nice Super Takumar 35mm f/2 lens a few months before, to use on a mirrorless digital camera,  and wanted to see how it would behave on the camera it had originally been designed for.

Historically, the Spotmatic is important. It was the first SLR from one of the 4 major vendors to offer Through the lens (TTL) stopped down metering as early as 1964. (Topcon had launched the RE Super with TTL metering at full aperture in 1963, but it did not have the installed base and the market presence of Asahi Pentax and did not make the same impact).

The Spotmatic was not an automatic camera (it just offered semi-automatic exposure determination with a matching needle setup) and although spot metering was implemented on the prototypes presented initially to the press, the models sold to the public determined the exposure with an average metering system.

The Spotmatic was so successful on the marketplace that Pentax did not feel the need to mess with it – the model remained virtually unchanged until the Spotmatic SP II was launched in 1971. Which only brought cosmetic improvements.

The first significant evolution was the Spotmatic F (in 1973), the first model of the series to support full aperture metering, but it required new lenses with a specific mount (a proprietary evolution of the universal 42mm screw mount that Pentax had been championing since the fifties), that it shared with Asahi’s first automatic SLR, the Pentax ES. The Spotmatic F was short lived: in 1975, Pentax introduced the K series (KM, KX, K2) and the K bayonet mount, effectively retiring the Spotmatic line and the M42 lenses.

Why the disappointment? 

Maybe I’ve been spoiled. Or lucky. Or maybe Nikon cameras of the manual focus era were really superiorly built and exceptionally solid. But none of the Nikon SLRs have bought so far have shown any reliability issue, or any marked weakness.

Pentax Spotmatic SP – the cloth shutter – one of the weak points of the camera

The first Spotmatic I bought was a SP500. It looked very nice on the pictures of the auction site, but when I received it, the shutter proved defective. Spotmatics have a textile horizontal shutter, and after the first curtain opens, the second curtain is pulled by two very narrow bands of textile. One was broken. Once you include shipping, the cost of the repair is probably in the $100.00 range. Much more than what the camera is worth. So it’s collecting dust.

The seller of the second Spotmatic I bought (the SP shown here) promised me it would work, and it does. It makes the right moves. The shutter fires at all speeds, the metering system seems relatively accurate with modern silver oxide batteries (good enough for print film, maybe not for slides), but it often takes two or three actions on the wind lever to arm the shutter, and the lever you have to lift to activate the metering (at the left of the lens flange) is very stiff and does not always come back into position after a picture has been shot (it did not on the SP500 either, so it’s probably a design feature).

Pentax Spotmatic SP. The base plate with the battery door.

I believe that those issues are related to the fact that the Spotmatic, like most of the cameras of its generation, is designed to let you compose at full aperture, but requires that you determine the exposure with the iris of the lens closed at the pre-selected value (you measure the exposure “stopped down”). I’ve yet to see a good implementation of stopped down metering (maybe Praktica cameras, I’ve never used them) . More often than not, it’s an ergonomics disaster: in the case of the Spotmatic, you have to hold firmly the camera with the right hand, use your left thumb to lift the metering lever (it’s stiff, you have to push hard and the upwards movement is not very natural), and use your remaining left hand fingers to adjust the aperture (stretch your fingers, you can do it) or the shutter speed (no, you can’t unless your fingers are as long as ET’s).

Pentax Spotmatic SP – the camera is compact compared to the monsters sold by Nikon or Canon during the same period, with a clean lines and a toned down design.

In the end, I did not trust the camera enough to bring it with me for a vacation in the  mountains. I don’t take pictures of brick walls and don’t shoot the same studio scene over and over. I use my cameras in the real life. At the risk of coming back without a picture if the camera decides it has enough.  I did not want to take the risk of missing a whole week of good picture opportunities because the camera had decided to misbehave. And I had no backup camera that could use the same lenses. So at the last minute, I removed the Spotmatic from my photo bag and replaced it with a Nikon FM.

Pentax Spotmatic SP with Asahi’s Super-Takumar 35mm f/2 lens

I like the Super Takumar 35mm lens very much though. Like most of the large aperture lenses of its generation, it tends to be a bit soft, but what a wonderful bookeh. It seems to work particularly well when mounted on an APS-C digital camera (where it becomes a 50mm equivalent).


What was the competition doing when Asahi Pentax was selling the Spotmatic? 

The Canon FT/QL and the Pentax Spotmatic SP both offer Stopped Down Metering. To determine the exposure, the photographer has to push the big switch to the left (Canon) or to lift the switch in the red circle (Pentax) – which is not a very natural movement. You wish you had three hands.

Asahi had a head start. When they launched the Pentax Spotmatic in 1964, none of the other big vendors had anything comparable: most of them were offering cameras with an external cell, sometimes optional and removable (Nikon Nikkorex), sometimes integrated, with its own little lens on the left side of the camera body (Minolta S7). The Spotmatic would remain the sole camera from a major vendor with through the lens metering for two years.

  • Canon launched FT QL in 1966 (stop down TTL) with the FL mount. Canon would only adopt Full Aperture metering with the FTb and the FD lens series in 1971.
  • Nikon and Minolta implemented full aperture through the lens metering (Nikon without changing its bayonet mount, and Minolta with a new  version of its SR bayonet, introduced on the MC Rokkor lenses). The Photomic T viewfinder for the Nikon F and the Nikkormat FT were Nikon’s first implementations of TTL metering (launched at the end of 1965).  Minolta’s SR-T 101, released in 1966, had an interesting arrangement of two CdS cells in the viewfinder, that were used to provide some form of weighted average metering (Minolta called it “Contrast Light Compensation system”, or CLC).

More about the differences between stopped down and full aperture metering in another page of this site.


Horace, French Bulldog – Shot with the Pentax Super Takumar 35mm f/2 mounted on a Fujifilm X-T1.

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Stopped down or full aperture metering – why it still matters for users of mirrorless cameras today

For a single lens reflex camera or a lens manufactured after 1975, full aperture vs stopped down metering is a non issue. But it was a key differentiator between 1965 and 1975. And if you’re considering mounting an old lens (manufactured before 1975) on a mirrorless camera, it may still impact you.

On a single lens reflex camera (SLR), the photographer composes the picture on a mat focusing screen, where the image formed in the lens is projected. This layout has all sorts of advantages, but the viewfinder tends to get too dark for focusing when the lens aperture exceeds F/8, and at smaller apertures (F/11, F/16), even composition becomes impossible.

Therefore, the best practice in the 50’s was to open the lens at the widest aperture, focus carefully, and then rotate the aperture ring to stop down the lens at the aperture needed to expose the picture optimally. It worked, but it was slow.  The process was easy to automate, and that’s what aperture pre-selection systems do.

Aperture pre-selection mechanism

Their goal : let the photographer compose and focus at full aperture, and then stop down at the last fraction of a second, when he/she presses the shutter release. Practically, the diaphragm stays wide open, until the shutter release mechanism  (through various cogs, springs and levers) activates a rod in the lens which closes the diaphragm to the aperture pre-selected by the user.

Two implementations

  • manual pre-selection : the lens stays stopped down after the picture has been taken. The pre-selection mechanism has to be re-armed by the photographer if he/she wants to return to full aperture; it’s a slow process (shoot, rearm the shutter, rearm the lens).

    This big lever on this Nikkorex lens has to be pushed down to re-arm the pre-selection system after each shot
    This big lever on this Nikkorex lens has to be pushed down to re-arm the pre-selection system after each shot
  • auto pre-selection: the pre-selection mechanism does not need to be re-armed after each shot. The lens returns automatically to full aperture after each shot (that’s why lenses from the 1960-1975 period are often labeled “Auto”). It’s transparent for the user, who can operate faster and with a better chance of catching the decisive moment.

    M42 Lens mount - this lens is designed for "auto" preselection. It stays at full aperture until the pin is pushed to stop down position.
    M42 Lens mount – this lens is designed for “auto” preselection. It stays at full aperture until the pin is pushed to force the lens to a stop down position.

Through the Lens (TTL) metering

Aperture preselection solved the problem of composing and focusing at slow apertures, but the introduction of CdS cell meters to evaluate the illumination of a scene Through The Lens (TTL) brought a new set of challenges: the camera needed to know how open the diaphragm was going to be when the picture is finally taken. There were two ways to do it:

  • after the photographer had set the aperture, he had to press a dedicated lever to stop down the lens, and only then would the camera evaluate the illumination of the scene. It’s stopped down metering.
    Technically, it’s the quick and dirty answer:  the metering system of the camera does not need to know the value of the aperture pre-selected on the lens. It just measures the light going through the lens when stopped down. The pre-selection lenses don’t need to be modified – they simply work. But it’s cumbersome for the user:

    • it’s a step back – aperture preselection had removed the need for the photographer to stop down the aperture before pressing the shutter release. Now it needs to be done again.
    • the viewfinder is darker during metering (the photographer loses contact with the action, he can’t adjust the focus, and it’s difficult to see needle of the meter) – you cannot compose or focus and adjust the exposure at the same time.
    • it’s a disaster from an ergonomics point of view. Even in the best implementations, the photographer has to maintain the lens stopped down by pressing or lifting a dedicated lever on the camera’s body, while trying to turn the aperture ring or the shutter speed knob to adjust the exposure. You need three hands for this type of gymnastics.

      The Canon FT/QL and the Pentaxx Spotmatic SP both offer Stopped Down Metering. To determine the exposure, the photographer has to push the big switch to the left (Canon) or to lift the switch in the red circle (Pentax) - which is not a very natural movement. You wish you had three hands.
      The Canon FT/QL and the Pentaxx Spotmatic SP both offer Stopped Down Metering. To determine the exposure, the photographer has to push the big switch to the left (Canon) or to lift the switch in the red circle (Pentax) – which is not a very natural movement. You wish you had three hands.
  • full aperture metering is transparent for the user. The diaphragm is stopped down a fraction of a second before the shutter curtains open and the picture is actually taken. The lens stays at full aperture all the time, including during the exposure determination.
    But for full aperture metering to be possible, the lens has to communicate the aperture pre-selected by the user to the metering system in the camera body, so that it can determine the right shutter speed/aperture combination.
    Most vendors chose to add a new dedicated lever inside the lens mount (this solution was chosen by Canon, Minolta, Olympus and Pentax).

    Pentax K mount: Aperture control lever (i); Aperture simulator (ii): Source:pentaxforums.com
    Pentax K mount: Aperture control lever (i);
    Aperture simulator (ii):
    Source:pentaxforums.com

    A few other vendors chose to simply modify the design of the aperture ring of the lens, and use it to transmit the aperture value to the camera’s metering system. At the beginning, Nikon used an external fork (the “rabbit ears”) screwed at the periphery of the aperture ring to communicate the pre-selected aperture to a pin connected to the metering system in the body.

    Before the adoption of Auto-Indexing, Nikon lenses used a metallic fork ("the rabbit ears") to transmit the preselected aperture to the metering system of the camera.
    Before the adoption of Auto-Indexing, Nikon lenses used a metallic fork (“the rabbit ears”) to transmit the preselected aperture to the metering system of the camera.

    Later, Nikon redesigned the aperture ring to add  a small protruding tab at its back, and this tab moved a sensor on the circumference of the body’s lens mount (Nikon Auto Indexing or “AI” lenses). Nikon’s system is similar (in its principle) to Fuji’s implementation of full aperture metering on the m42 universal mount of the ST801 (pictures below).

Fujinon lens - the aperture ring is designed with a small tab which transmits the aperture pre-selected by the photographer to a rotating ring on the camera's body.
Fujinon lens – the aperture ring is designed with a small tab which transmits the aperture pre-selected by the photographer to a rotating ring on the camera’s body.
Fujica ST 801: Fuji's version of the m42 lens mount has a ring at the periphery - the little pin in the red circle is pushed by the tab protruding from the aperture ring of the lens. That's how the preselected aperture is transmitted.
Fujica ST 801: Fuji’s version of the m42 lens mount has a recessed, spring loaded rotating ring at the periphery – the little pin in the red circle is pushed by the tab protruding from the aperture ring of the lens. Any change to the pre-selected aperture on the lens will be transmitted to the camera.

Mounting an old lens on a mirrorless camera

When the photographer is using an old lens through a lens mount adapter, the cameras  needs to work with the lens stopped down (only semi-auto and  aperture priority automatic exposure modes are supported). There are none of the inconveniences associated with stopped down aperture on a reflex camera: on a mirrorless camera, the viewfinder always shows the image as it will be exposed, and if the exposure parameters (aperture, shutter speed and ISO) are correctly set, the image will be perfectly legible in the viewfinder, even if the lens is set a f/11.

But the challenge is to force an old lens to operate stopped down:

  • lenses designed for full aperture operations and stopped down metering (typically the m42 lenses with auto-pre-selection and the Canon FL) have a slider to switch off auto-preselection and operate permanently at stopped down aperture, in a manual mode. When mounted on a mirrorless camera through a lens mount adapter, they need to be switched to “manual”.
     Lenses of the 1965-1975 era often had an auto/manual switch - by default the operated at full aperture but could revert to manual if mounted on an older reflex camera.
    Lenses of the 1965-1975 era often had an auto/manual switch – by default they operated at full aperture but could revert to manual if mounted on an older reflex camera.

    The "manual" mode has to be switched on when used on a mirrorless camera.
    The “manual” mode has to be switched on when used on a mirrorless camera.
  • Nikon lenses  – the diaphragm of the Nikon lenses is opened at full aperture when the camera is mounted on a Nikon camera (the camera side of the mount has a lever which forces the lens open), but is stopped down when the lens is removed from the camera, or  mounted on an adapter deprived of the full aperture lever.  Which is perfect if you’re mounting the lens on a mirrorless camera.
  • Canon FD – when the lens is removed from a Canon camera, the diaphragm command is decoupled (the lens stays at whatever aperture it was pre-set the last time it was on a Canon FD camera). The adapter needs to be designed with a pin that will force the lens to stop down  when mounted on the adaptor.
Lens mount adapter for Canon FL/FD lens - the pin in the red circle pushes a lever on the lens and will force it to stop down.
Lens mount adapter for Canon FL/FD lens – the pin in the red circle pushes a lever on the lens and will force it to stop down.
  • Fuji’s EBC-Fujinon lenses are highly regarded, but the brand’s implementation of full aperture metering on the m42 mount presents two problems for modern mirrorless camera users:
    • most of the lens mount adapters receiving m42 lenses do not leave room for the aperture ring’s protruding tab of Fuji’s lenses. The lenses cannot be fully screwed down on the adapter and as a consequence may not focus to the infinite,
    • Fuji’s lenses don’t have a “manual” position and cannot be forced to operate stopped down on their own (that function was provided by the Fujica camera itself, not by the lens). There are work arounds to both issues, some nice, some ugly, but a lens mount adapter designed specifically for Fujica m42 lenses still has to be developed.

New-York City - Central Park - Fuji XT-1 - Canon 35-105 f/3.5 lens with Fotasy adapter
New-York City – Central Park – Fuji XT-1 – Canon 35-105 f/3.5 lens with Fotasy adapter