Episode 5: The SIGMA U-AF 55-200mm F4.5

Hello, my name is Yasuhiro Ohsone. My “Anecdotes,” in which I talk about the development of our products and what makes them stand out, as well as the background background information regarding the market (and how it changed), has already reached its fifth issue. In the following text, I would like to talk about a lesser known episode from my more than 30 years at SIGMA that is now safe to share – the development of the SIGMA U-AF 55-200mm F4.5 lens.

AF sensor in the lens

So far, I have mostly focused on SIGMA’s newer products in my column. This time I’d like to talk about one of our failures. The product in question is the SIGMA U-AF 55-200mm F4.5 (the “U-AF” is for “Universal Auto Focus,” a system where AF sensor, AF motor and batteries are put inside the lens barrel), released in 1989.

The bold push towards AF lenses

While SLR cameras have entered into the Age of Autofocus with MINOLTA’s release of the α-7000 in 1985, the camera market was at first rather hesitant regarding SLR cameras’ switch to autofocus. I believe this was partly due to the desire to “decide the point of focus with my own hands”, wide-spread especially among professionals and high-amateur photographers. Nonetheless, MINOLTA dedicated itself to the pursuit of autofocus SLR cameras, but there was one more company that threw itself at the development of autofocus capabilities. That company was SIGMA.

After the α-7000 had been released, SIGMA immediately began its research on order of late company president Michihiro Yamaki. A year later, SIGMA had already released six lenses for the α-7000. As other camera manufacturers joined the SLR autofocus market, SIGMA developed lenses for their systems, as well, and by 1989, SIGMA’s catalogue included lenses for six different AF mounts (MINOLTA, Nikon, Canon, PENTAX, KYOCERA, OLYMPUS). But we worked on developing something of our own in parallel – the Universal Autofocus Lens. (You see that SIGMA was very dedicated to the AF revolution.)

A different path towards autofocus

Since a few years before MINOLTA’s release of the α-7000, numerous products attempted to realize autofocus while keeping their existing mounts for manual-focus cameras by equipping the lenses themselves with necessary technology. Examples that come to mind are the PENTAX 35-70mm F2.8 AF, Canon’s FD35-70mm F4 AF, and the RICOH’s AF RIKENON 50mm F2. These lenses have autofocus motors, batteries and so on packed into the lens barrel and it’s bulging to the outside, giving them a characteristic shape.
In 1985, COSINA released a revolutionary autofocus lens – the COSINA 200mm AF F3.5. The lens had AF line sensors (the “TCL module” by American company Honeywell) on its inside, a motor and a battery holder (for three AAA-sized batteries) to supply them with electricity, and as a result this AF lens was fully usable on MF camera mounts. In other words, thanks to this lens any MF camera could be turned into an AF camera. However, the 200mm AF F3.5 failed to become a hit product, partly because of its high price-tag of ¥88,000 (about $800). In 1987, COSINA followed up with the 75-200mm F4.5 AF – a slimmer model with the same features – which became a huge success.

Influenced by these developments, SIGMA heavily interested in the AF lens market at the time, began work on a lens with AF capabilities built into its barrel.

Left: COSINA 200mm AF F3.5; Middle: SIGMA U-AF 55-200mm F4.5; Right: COSINA 75-200mm F4.5 AF

Cylindrical design

SIGMA quickly closed a licensing deal with Honeywell to use their TCL modules and began developing the lens. The optical design featured the classic variator-compensator four-group zoom lens setup. The stationary lens group (the one closest to the mount) had a half-mirror (the “beam splitter”) inserted at a 45° angle, which reflected about 30% of the light entering the lens, passing through AF optical system towards the TCL module.. (fig.1 )
It is almost like taking the mirror and distance measuring units from an AF SLR camera and inserting them as-is into the inside of a lens.


We developed the lens with the specs “58-200mm F4.5” and brought a prototype to the 1988 PMA (an American camera show). However, following late company president Michihiro Yamaki’s request that “the wide end of the focal length should be closer to a standard lens (50mm)” and the remark that “in the past, non-cylindrical lenses didn’t sell well. We need to change the barrel to a cylindrical design,” We changed the optical design to allow for a 55-200mm focal length (at F4.5) and adjusted the barrel design, slimming the lens down even more. In charge of these design changes was none other than I, who had just joined the company.

New problems, new solutions

The first change I implemented was regarding the layout of the zoom cam parts. We relocated the moving parts (e.g. the cam barrels) to the inside when possible and opened a hole in the stationary outer parts in which we hid the AF sensor unit. Then we simplified the optical position-adjustment mechanism of the AF unit and made it smaller. Further, working together with the Die and Mold Tooling Division we were able to reduce the thickness of the outer, visible parts while maintaining the strength, and achieved a lens that was both slim (compared to the COSINA 75-200mm F4.5) and housed inside a cylindrical barrel.

Countless other aspects of the lens – the position of the large circuit board, the AF-MF clutch, how and where to add the AF function button, securing the beam splitter precisely etc. – gave us quite a headache. This was also when I learned that standards for AAA-sized batteries, both regarding size and shape, were rather loose, and that such batteries came in much greater variation than expected. As a result, designing the battery holder took a lot of work.

The most difficult part, however, was the adjustment equipment used during the manufacturing process. The lens required a precise construction in which the line sensor’s focal point was finely aligned with the optical axis. But SIGMA did not have the necessary equipment to realize such precise adjustments during manufacturing. Also, for the focusing to work the camera, lens and optical metering need to be in alignment, requiring further fine-adjustment processes. Furthermore, since it was impossible to use an oscilloscope at the assembly line, we had to create a device which allowed us to see the output of the AF sensor on a computer. The budget spent on developing just a single lens was absolutely out of scale.

The age of autofocus SLRs

After strenuous efforts, we were finally able to launch the SIGMA U-AF 55-200mm F4.5 in 1989 – to almost no commercial success. The market had already moved on and AF SLR cameras were wide-spread at that point. With the release of cameras like the EOS 650/620 or the MINOLTA α-7700i, the possibilities of AF technology had made significant progress and the AF SLR camera market was vibrant. Also, the line-up of lenses for MINOLTA’s AF mount had already grown to more than 30 lenses, and customers looking to buy AF lenses for the old MF cameras were rare. U-AF 55-200mm F4.5 disappeared silently.

The U-AF’s legacy

The U-AF 55-200mm F4.5 was a giant failure. While we have not made precise calculations, I doubt that the lens’ sales managed to recoup its development costs at all – especially considering that Honeywell’s TCL module (for distance measuring) could not be used in any other of our products, meaning the entirety of the licensing costs had to be recouped with this lens alone.
But the lens had a positive impact, too. SIGMA was now able to acquire the technique to incorporate phase detection AF with line sensors. Our software and electrical engineers had learned to control autofocus using the waveform output from a line sensor, and our mechanical designers had learned how to adjust the X, Y and Z position of AF units. Also, our factory was now able to handle the necessary fine-adjustment operations, with the members of our manufacturing and assembly departments trained and experienced in the processes involved. And I, too, have learned much about autofocusing technology (as well as the perceptions, questions and assumptions involved) thanks to this lens.

The U-AF 55-200 F4.5 was a commercial failure, but it enriched SIGMA with a wealth of experience and technology.

Yasuhiro Ohsone

Head of product planning at SIGMA

Entered SIGMA in 1987. Has worked in the development of optics and mechanisms as well as collaborating with many companies. Became head of product planning in 2013.

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