Pentax 110 SLR System


Pentax 110 SLR System

Pentax 110 SLR


The Pentax Auto 110 was the world's first and only subminiature ‘SLR system'. It wasn't the first subminiature SLR – that honour goes to the Narciss, a Russian 16mm SLR that debuted in the early 1960s before Yellow Box Inc introduced the 110 cartridge. It wasn't even the first 110 SLR, that was Minolta's 110 Zoom, an aperture-priority autoexposure model of the mid-70s. The Minolta followed the horizontal arrangement of the Kodak Pocket Instamatics and fitted with a decidedly un-pocketable, non-interchangeable f/4.5 zoom lens, and metering through a sort of small telephone dial on the front panel next to the lens.

The Pentax Auto 110 System

Introduced in 1978 as a system the Pentax was the most thorough the 110 format would ever see. It's modest but pocketable 24mm f/2.8 lens was interchangeable with an 18mm wide-angle or a 50mm telephoto, both with the same f/2.8 speed; a 70mm tele and a 20-40 f/2.8 wide-to-tele zoom would follow, together with a 1.7X teleconverter. In addition to optics, the Auto 110 was designed to accept a power winder, and its lack of a standard hot shoe (there wasn't room) was made up for by the provision of a proportionately teeny weenie dedicated flash that coupled mechanically with the body to set the camera at f/2.8 and 1/30 sec. All this was available, if desired, in a miniature aluminium attache that looked like a cross between a Halliburton case and a makeup kit.

The Auto 110 focused to 14″ (10″ with the wide-angle), without resorting to an internal swing-away closeup lens as the Minolta had done. Also unlike the Minolta, its TTL metering (a first for a 110 camera) allowed for filters without compensation. Auxiliary closeup lenses were also provided for those for whom 14″ wasn't close enough.

The Auto 110 is equipped with an electronically controlled program, an automatic behind-the-lens leaf shutter and an instant-return mirror. It was a unique combination and one which Asahi engineers deftly used to their advantage.

In 35mm SLRs a leaf shutter between the lens and the mirror becomes a nightmare of complexity and the need to cross-couple it with a removable lens diaphragm would most likely have sent it far beyond any hope of redemption. By comparison, the combination in the Auto 110 resulted in an extraordinarily simple design, made all the simpler by its autoexposure control requirements.

The 110's automatic diaphragm and leaf shutter system contains neither a diaphragm nor a leaf shutter as we generally think of them. The entire system consists of two V-shaped blades which form a square opening between them, similar to the diaphragm (but not the shutter) of any number of 70s-era 35mm compact viewfinder cameras. Having closed at the beginning of the mirror-raising sequence, these travel in an opening direction for a given time (determined by the light intensity), at the end of which they close again.

If the given period is less than, say, 1/60 sec., the blades will begin closing without ever having reached their fully open (f/2.8) position; hence, for speeds from 1/30 to the max. of 1/750, both the duration and distance of travel vary together. When done in this way, there's really nothing to it, as long as you don't have to know just what aperture and shutter speed you got for any given exposure. So the logical final touch is just to let the built-in meter handle everything on its own in automatic-only operation, with no indication of what it's doing other than a yellow light that comes on at all speeds allowing full aperture (1/30 sec. or longer).

The other trick in a leaf-shutter SLR is the need for a light seal to protect the film when the shutter is open for viewing. In the Auto 110, this task is handled elegantly by the instant-return mirror. It seals against a flange in front of the film plane when in the down position.

Pentax Unique Mechanical Arrangements

Many of the mechanical arrangements which make the Auto 110 work could not be scaled up to 35mm size. They work only in the 110 format, for several reasons: first. At the same time, the Auto 110 is unquestionably the smallest SLR ever successfully marketed and among the smallest ever built, it is quite bulky in proportion to its film size, compared to larger format cameras of conventional design. If doubled in size to bring it up to a 35mm full-frame, it would be 7 3/4″ long, 4 1/4″ high and 2 1/2″ thick without a lens. Second, the mechanisms themselves are quite massive. In 110 size the blade shutter, mirror and light baffle work very quietly, but doubling their dimensions would increase their weight by 800% for every moving part, while to maintain the shutter speed range these massive parts would have to move twice as fast. The result would hardly be likely to meet customer acceptance. Another limitation imposed on the Auto 110 by its mechanical layout comes in the area of lens design. You may have noticed that all Auto 110 lenses have the same maximum aperture, from the standard lens (for which f/2.8 may be a tad slow) to the zoom (for which it's quite fast). This is because the diaphragm is built into the body and must work with all lenses. Another problem associated with this arrangement is that the diaphragm is located behind, rather than within the lens. This has an impact on the optical design of the lens, especially in the longer focal lengths. The front element of the 50mm lens, for instance, measures 30mm across; much larger than one would find on a 50mm f/2.8 standard lens for a 35mm camera. This is necessary to allow light to reach the corners of the frame through the rear-mounted diaphragm.

But it's more than just a matter of size: enlarging the front element of a f/2.8 Tessar wouldn't get you anything but extra glass. The design of the lens would need to be changed so that light converges near the rear rather than near the centre of the lens. Furthermore, the point of convergence should be about the same for all focal lengths. A lesson in how to qo about this had been given in the 1950s, with the interchangeable-front-cell lenses of the Retina and Contaflex cameras. The ancestor of these was the supplementary wide-angle and tele attachments which went on the Contaflex I and II and the Rollei TLRs. Despite obvious differences, the basic problem, keeping the diaphragm in the same place, exists in all of these, and the basic optical approach is similar. Essentially, the rear portion of the wide-angle and tele lenses resembles the normal lens. The front portion comprises a Galilean telescope with some degree of magnifying (in the tele) or reducing (in the wide-angle) power.

As in any Galilean optic the field of view becomes a function of the objective diameter; hence the increase in size as the lens varies in either direction from the normal 24mm. Even the zoom can utilize this approach, with a moving element in the forward section changing it from a magnifying to a reducing optic as it moves. Again, this succeeds only because of the small format: on a 35mm camera, a 100mm f/2.8 tele with a front element nearly 2 1/2″ in diameter would surely be found unacceptable, as indeed it was in the Contaflex series and the Canon EX.

The question of optical attributes which are a function of this particular format size brings us to one of the Auto 110's most intriguing and significant features, and one which seems to have escaped the notice of the camera's designers. The Auto 110's 24mm standard lens, particularly in combination with the camera's reflex viewing and through-the-lens autoexposure systems, is about as near an approximation of a human eye as you're ever likely to (or would ever care to) find in a photographic camera. It approximates both the focal length and the maximum aperture of the human eye, and its field of view corresponds to that typically provided in the eyepiece of optical instruments such as binoculars and microscopes. The net result of this is that the Auto 110 can photograph through the eyepiece of such instruments far more satisfactorily than any other camera ever made. Even the diameter of Its filter thread, at 25.5mm, allows the lens to nest snugly over a standard 23mm microscope eyepiece for a quick but steady shot. Proper exposure is nicely taken care of by the TTL meter, and the camera's small size and lightweight don't jeopardize the steadiness of the instrument.

Pentax 110 SLR Lenses

Another happy coincidence relates to lens speed. In the 1950s it was a common trick to provide an otherwise limited SLR (such as a Contaflex, or an Exa) with long-telephoto capability by attaching a monocular to the front of the lens. The major problem was always that the diameter of the monocular objective was small relative to the resulting focal length of the lens. Typically, binoculars and monoculars are designed to provide a 5mm diameter exit pupil: the most common of these are 7×35, 8×40 and 10x50mm styles. These, when attached to a 50mm lens, provide an effective focal length of 350, 400 and 500mm, but in all cases, the maximum effective aperture is f/10. That's pretty slow, and it shows in the dim viewfinder as well as the long shutter speed.

Pentax 110 SLR

One of the most fascinating ways to use an Auto 110 is to perch it on each of the eyepieces of a stereo microscope, as shown here. The camera's through-lens focusing and autoexposure systems give fool-proof results; an example is shown below.

Mount the same device on the 24mm lens of the Auto 110, however, and it's a different story. The view through the finder looks much the same, but it's brighter – the 7×35 becomes a 168mm f/4.8, just 1 1/2 stops slower than the normal f/2.8 and over two stops faster than it could provide on a 35mm camera. That translates into shutter speeds more than four times faster, which in turn means sharper pictures. Again the TTL meter compensates nicely for the 1 1/2 stop reduction in speed. But in this case, there is a catch (albeit one that can be lived with) with a handheld monocular.

Unlike a telescope or microscope, there is a pretty good chance of encountering enough light to make the lens begin to stop down. At these higher exposure values, the camera will tend to overexpose slightly since the meter doesn't know that the light it's reading is all coming through the central 5mm (the exit pupil) of the 8 1/2 mm (lens aperture). So if it sees enough light to require a reduction of, say, one f/stop to 6mm diameter, it only winds up closing off part of the unused edge area and allows the full 5mm beam through unaffected. The result: a 1-stop overexposure, well within the capacity of most 110 films.

One thing to watch for when selecting a monocular for this purpose is its angular coverage. The 24mm lens on the Auto 110 covers about 48 degrees across its 21.5mm diagonal; to prevent vignetting, the eyepiece of the monocular must cover at least as much. This means a 7x monocular must cover 48/7 or about 7 degrees minimum; an 8x, 6 degrees. Most modern glasses meet this requirement easily, but some older ones don't. Mine, a 7x dating from the early 50s, covers 6.5 degrees; vignetting is not visible with it at large apertures, but begins as the lens stops down in bright light. It is also important to get the camera lens as tight into the eyepiece as possible, just as it is with your eye.

Unique Applications

Other unique applications come to mind: it can be used to photograph the viewfinder display of another camera, through the eyepiece. Its lightweight and low vibration level permit use with lightweight telescopes and instruments that could not support a larger film camera, as well as those whose eyepieces cannot be detached. The Pentax can even be used to measure the field of view of an instrument. The is done by photographing through the eyepiece and measuring the diameter of the image on the negative. Finally, the apparent distance of a viewfinder or instrument display can be accurately measured by bringing the display to focus in the Auto 110 viewfinder, and then simply reading the distance from the footage scale on the lens.

A Classic Design

As price trends have indicated since the model was discontinued the Auto 110 is a classic design which is destined to be (indeed, which already is) a sought after collectors' item. More even than that, however, it is a uniquely practical machine, as useful as it is adorable, and one which, despite Pentax's gallant efforts to exhaust all possibilities, still has considerable room for future expansion at the hands of an adventurous amateur optician.

In recent years the Pentax 110 lenses have been used on mirrorless cameras such as those from Olympus and Panasonic.

The Pentax Auto 110 System Components

  • Auto 110 Super camera
  • 18mm wide-angle lens
  • 18mm Pan-focus w.a. lens
  • 24mm normal lens
  • 50mm telephoto lens
  • 70mm telephoto lens
  • 20mm – 40mm zoom lens
  • 1.7x teleconverter (by Soligor)
  • Dedicated flash units
  • Power winder
  • Close-up lens sets
  • Filter sets
  • Various component and system cases

Where to find a Pentax 110 SLR

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Image 1: Here's another one I'd like to see you take with your 35mm AF point&shoot: a herd of aphids on a stem, taken by placing the Auto 110 first on one, then the other eyepiece of an American Optical stereo microscope… it's as easy as click, wind, click (aphids don't move very fast; they all register okay, though one was wiggling his little derriere in the left frame and blurred a little). To see this in 3 dimensions, print it out and place it in an antique stereoptican viewer (the right-hand image should be about 3 1/2 inches wide for proper spacing). Each of these bugs is about half the size of a small ant. 

Image 2: This one is hardly a fair example: it was taken with a 3 diopter closeup lens AND a 1.7x Soligor teleconverter on the normal lens, at a distance of less than 12 inches from the moving subject. Still, it's an example of a photo that could never have been taken with any other pocketable camera.

Author's Footnote:

It's not my usual practice to get worked up enough to write a story about a camera that's younger than I am, that has electronic controls, & that I don't know how to fix if it breaks. Throw in a plastic body, program shutter and the fact that the manufacturer is still In business….gee – I shouldn't even own one! But in the case of the Pentax Auto 110, I've made an exception because this camera has characteristics that make it truly unique and give it capabilities beyond other 110 or even larger cameras.Rick Oleson

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This post was created with information kindly contributed by Rick Oleson

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Pentax Auto 110 camera

In 1979, as the 110 format was dying, Pentax inexplicably launched the Auto 110 - the most competent 110 format camera ever, and one of the few that looked like a real SLR. It arrived after the the original version of the Minolta Auto 110 Zoom, but unlike these Minolta cameras, the Pentax is a true camera system with interchangeable lenses and a full range of accessories, including two dedicated electronic flashes, the Pentax AF 130P and the AF 100P, as well as an electric Pentax 110 Winder.

The camera is unusual in having the imaging aperture built into the camera body rather than in the interchangeable lenses. The in-body aperture covers a range of f/2.8 to f/13.5; therefore, all lenses built for the camera include a maximum aperture of f/2.8. The aperture is a simple square scissor type aperture. While this system limits the range of lenses that might potentially be offered with the camera, it greatly simplifies the camera system by eliminating the need for any functional connections between the camera and lenses, and eliminating the complexities of in-lens apertures. For this camera system there was little to lose by limiting the lenses to a maximum aperture of f/2.8.

The camera has a bright viewfinder with a split image focusing screen and TTL light metering. The automatic program exposure system is quite sophisticated, working from a fast 1/750 sec. at f/13.5 to 1 sec. at f/2.8, with automatic film speed setting from 100 to 400ASA, programmed by the cassette. A wide choice of close-up lenses, filters, rubber lens-shades, an ever-ready case and more, complements the system. The camera was made available in black, brown and a transparent working demo version. A 1.7x teleconverter was made by Soligor.

An upgrade Auto 110 Super came on the market in 1982 featuring brighter viewfinder, low light warning, an electronic self-timer and a single stroke advance lever. A 70mm telephoto lens and a 2 × Zoom lens were also added.

The Pentax Auto 110 is considered to be the smallest consumer SLR reflex camera ever made, its contender would be the Russian Narciss launched in 1961, at first only sold at the home market, but later made available in an export version. However, it is larger in all dimensions and is much heavier.

Origins of the Auto 110

It appears that the Auto 110 was based on an evolved prototype of the Minimax Pocket 110 EE. According to correspondence with the designer of the Minimax Pocket 110 EE [1], the manufacturers of the Minimax developed a more advanced SLR prototype of this camera, but their interest waned and the prototype and the blueprints were sold to Pentax.

Pentax Auto 110 lenses

All Pentax 110 lenses are marked as f/2.8, matching the widest aperture available in the camera body.

  • Pentax-110 18mm Wide-angle lens, 6 elements in 6 groups, filter Ø30.5mm
  • Pentax-110 Pan Focus 18mm Wide-angle, fixed-focus lens, 6 elements in 6 groups, filter Ø30.5mm
  • Pentax-110 24mm Standard lens of 6 elements in 5 groups, filter Ø25.5mm
  • Pentax-110 50mm Telephoto lens of 5 elements in 5 groups, filter Ø37.5mm
  • Pentax-110 70mm Telephoto lens of 6 elements in5 groups, filter Ø49mm
  • Pentax-110 20mm—40 mm Zoom lens of 8 separate elements, filter Ø49mm


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