Episode 9: Ultra Telephoto Lenses
– The Era of Auto-focus and Special Low-Dispersion Glass

In Episode 8 of this column, “Ultra Telephoto Lenses – The age of chromatic aberrations and mirror lenses”, I talked about the beginnings of SIGMA’s ultra telephoto lens development. The early days of developing ultra telephoto lenses was dominated by the fight against chromatic aberrations. Then, one day, SIGMA received a batch of experimental special low-dispersion glass from a glass manufacturer.

Dawn of a new era: special low-dispersion glass

The year was 1983. Finally, a glass manufacturer had succeeded in mass-producing special low-dispersion glass, allowing for its use in exchangeable lens designs. While the glass remained an expensive material, its stellar optical characteristics made it indispensable for the manufacture of high-performance ultra telephoto lenses.

SIGMA was involved in the development of this material since the early experimental stage, with efforts focused on establishing lens polishing technologies, as it was the polishing and handling, in particular, that proved most difficult. The SEIN column “GROUNDBREAKING VOL. 6: Bringing Special Low Dispersion Glass to Life” provides more details on this issue.

APO ZOOM 50-200mm F3.5-4.5

While the hard work continued, the first lens to use special low-dispersion glass went on sale in 1984 – the SIGMA APO ZOOM 50-200mm F3.5-4.5.
SIGMA named this special low-dispersion glass “SLD (Super Low Dispersion)”, and the chromatic aberration was greatly improved by this glass, that is, the lens that became an apochromat was given the logo “APO”. The appearance was painted white as if to boast of it, but there is an interesting history of this appearance color, so I will explain on it later.

SLD glass is highly effective when used with ultra telephoto lenses, but there is a reason why the lens’ zoom range ended with the 200mm medium telephoto range: there were limits to the diameter and quantity at which glass manufacturers could produce SLD glass. Also, the know-how required to polish large-diameter SLD glass simply did not yet exist.
It was in 1986, two years after the release of the APO ZOOM 50-200mm F3.5-4.5, that SIGMA succeeded in establishing polishing technologies for large-diameter SLD glass. Thanks to this breakthrough, SLD glass was finally available for use in ultra telephoto lenses. However, it remained an expensive and difficult-to-use material, putting it out of reach for low-priced ultra telephoto lenses. The first SIGMA ultra telephoto lenses with the new material were the SIGMA APO ZOOMτ 100-500mm F5.6-8 and the SIGMA APO ZOOMω 350-1200mm F11. Both lenses featured exceptional optical performance (as well as a high price tag). The 350-1200mm F11 even left its mark on lens history as the ultra telephoto lens with the largest zoom range – a record that still stands unbroken today. These two lenses became the leaders of the APO series that demonstrated SIGMA’s technological prowess.

SIGMA APO ZOOMτ 100-500mm F5.6-8
SIGMA APO ZOOMω 350-1200mm F11

The autofocus shock

1985, the year before the release of the two SLD ultra telephoto lenses, was the year of the “α shock”. Following MINOLTA with their release of the α-7000, one after another Nikon and Canon began adding autofocus function to their cameras and lenses. SIGMA also actively embraced the new autofocus technology (“Ohsone’s Anecdotes, Episode 5” for more detail), which meant that SIGMA was tackling two important challenges at the same time: trying to increase production of SLD glass, and adopting autofocus for their interchangeable lenses. Both of these goals were very difficult to achieve with ultra telephoto lenses.

As mentioned before, large-diameter SLD glass was not yet available for mass-manufacture in 1985, which proved a difficult obstacle for the production of ultra telephoto lenses.
Adding autofocus to ultra telephoto lenses was another task with many hurdles to overcome. MINOLTA’s autofocus system relied on camera-internal DC motors, couplers and gears to move the focusing lens element within the lens barrel. However, in this case, an essential technique was required but lacking , as the large, heavy focusing lens element could not be moved at sufficient speeds. While darking the F-number would reduce the weight of the focusing lens element, it would have impacted the precision and quality of the AF as not enough light would reach the sensor at F-number of 8 and higher. SIGMA considered the focal length limits for suitable AF speeds to be around 300mm for zoom lenses and 400mm for prime lenses.
Other manufacturers were faced with the same problems and issues, and AF-enabled ultra telephoto zoom lens over 300mm were not available on the market at reasonable prices. With their development of ring-type ultrasonic motors, Canon managed to increase the autofocus performance of their ultra telephoto lenses at once, but any lens that used both ultrasonic motors and special low-dispersion glass was, of course, going to be expensive.

A decision

At this point, SIGMA made a surprising decision. It was the decision to create a 400mm F5.6 lens that featured autofocus but no SLD glass. Of course, such a lens was technically possible. As I wrote earlier, with specs like 400mm and F5.6, the focusing lens element that needs to be moved for the autofocus to work would neither be too big nor too heavy. However, without the right glass materials it would not be possible to suppress chromatic aberrations sufficiently. Was this the right lens after the many years and hard battles fought to reduce chromatic aberrations in ultra telephoto lenses, this was not a lens in tune with SIGMA’s in tune with the vision of SIGMA for superior chrome aberration correction. But the market lacked a reasonably priced AF-enabled ultra telephoto lens, despite the high demand by camera stores and photography enthusiasts. Without hesitation, late SIGMA president Michihiro Yamaki initiated the development of an SLD-less, modern and compact 400mm F5.6 AF lens. It was a moment when the passion on autofocus took precedence over decreasing chromatic aberrations.
The lens, called AF-TELE 400mm F5.6, came out in late 1986. It featured an enticing price tag of ¥53,500 (about $500), a compact size and a NATO-green exterior. It sold exceptionally well – not just in its narrow category of ultra telephoto lenses, but as a lens in general.
Back then, I viewed Michihiro Yamaki as a figure not unlike Colin Chapman (founder of Lotus Cars). He possessed the technical knowledge of an engineer as well as the instincts required as a business manager.

AF-TELE 400mm F5.6

Color options

Allow me a short excursion into the appearance colors and a variety of color options of SIGMA’s ultra telephoto lenses that still cause some confusion on the second-hand market today.
Beginning with the APO ZOOM 50-200mm F3.5-4.5 in 1984, SIGMA’s APO ultra telephoto lenses initially featured a white appearance (“Pearl White”, to be precise). This changed in 1986 with the switch to NATO Green, and in 1987 with the introduction of three color options, limited to ultra telephoto lenses and lenses that featured SLD glass:
1) Pearl White
2) NATO Green
3) Gun Metallic
The choice between three color versions was another idea by the late Michihiro Yamaki that proved popular with SIGMA’s customers. However, I want you to consider the following: SIGMA’s APO lenses were available as autofocus versions for three lens mounts (Nikon F, Canon EF, MINOLTA A) and with manual focus for five lens mounts (Nikon F, Canon FD, MINOLTA MD, Pentax K, Olympus OM), each in the three color options mentioned above. This means that each lens was manufactured for eight different mounts and in three color versions – 3 x 8 = 24 versions per lens. With prime and zoom lenses, SIGMA’s active lens catalogue always included at least five to six different lens models. It was chaos at the manufacturing sites, and problems like customers ordering NATO Green lenses but being delivered Pearl White versions were a common occurrence.
By the way, it is not known why these three color versions in particular were chosen. I wish I could have asked Michihiro Yamaki about the reasons behind this decision.

Left: SIGMA APO ZOOMτ 100-500mm F5.6-8|Pearl White; Middle: SIGMA APO ZOOMτ 100-500mm F5.6-8|NATO Green; Right: SIGMA APO ZOOMτ 100-500mm F5.6-8|Gun Metallic

The era of ultrasonic motors

Starting in 1987, SIGMA releasing ultra telephoto lenses with SLD glass in sequence – first the AF APO 500mm F4.5, followed by the AF APO300mmF2.8 in 1988, and then the APO version of the 400mm F5.6 in 1989. Large-diameter SLD glass was finally available at the required quantities, and the lineup of AF-enabled APO lenses continued to grow, with lenses like the APO 500mm F7.2, the APO 800mm F5.6 and the APO 1000mm F8.

Left: AF APO 500mm F4.5;Middle: AF APO 300mm F2.8;Right: APO 400mm F5.6
Left: APO 500mm F7.2 ZEN;Middle: APO 800mm F5.6 ZEN;Right: APO 1000mm F8 ZEN

But the difficult fight for higher autofocus precision and speed still continued. As mentioned above, the AF system at that time was lacking power for lenses with a focal length longer than 400mm F5.6 or for lenses with a brighter F-number than 400mm F5.6.
The only key available to solve this problem at the time was Canon’s ring-type ultrasonic motor.
SIGMA began adopting the ring-type ultrasonic motor for its own lenses in 1997. Thanks to this technology, SIGMA’s AF-enabled ultra telephoto lenses entered their second generation. I will tell their story in my next column.

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