Episode 8: Ultra Telephoto Lenses
– The age of chromatic aberrations and mirror lenses

The era of SLRs and ultra telephoto lenses

Founded in 1961, SIGMA will celebrate its 60th anniversary in 2021.
1961, the year SIGMA was founded, was a time when the 35mm camera market switched from rangefinder cameras to single-lens reflex (SLR) cameras.
Browsing the comprehensive camera catalogue “JAPAN CAMERA SHOW” from 1961, I learn that nine companies were already selling SLR cameras and exchangeable lenses that year. Particular interesting is the fact that many of the available lenses were telephoto lenses. SLR pioneer PENTAX, for example, had two wide-angle lenses in their lineup, two standard lenses, and eight telephoto lenses – and two of these even were ultra telephoto lenses, with focal lengths of 500mm and 1000mm respectively.
SLR cameras are far more suitable for telephoto lenses than the rangefinder camera could hope to be. The beginning of the SLR camera era is, in other words, also the advent of the telephoto lens. Finally, the dream of photography enthusiasts had become reality: it was now possible to photograph things far away in the distance. In 1961, my father’s monthly wage amounted to about 10.000 Japanese Yen (about $100, the starting salary for university graduates in Japan). At 30.000 Yen, SLR cameras were expensive luxury items – not to mention ultra telephoto lenses, at unattainable prizes of about 100.000 Yen (c. $1000).

SIGMA’s first ultra telephoto: a mirror lens

If the camera manufacturer’s ultra-telephoto lens is expensive, then it’s time for independent lens makers. And with this ultra-telephoto lens, the independent lens makers were surprisingly showing their originality.
Tokina were among the first to start development of ultra telephoto lenses. Their lineup included a 400mm F6.3, a 450mm F6.5, a 600mm F8 and a 800mm F8, four prime lenses, and a 180-410mm F5.6 zoom lens.
TAMRON has a high development capability of zoom lens since that time. In 1965 they released a 200-400mm F6.9, and demonstrated their technical prowess with a 200-500mm F6.9 lens in 1969, breaking the 500mm barrier.

All of these lens were normal lenses with a refractor-type design. SIGMA, however, decided to create a lens with a reflector lens (mirror lens). Despite the unavoidable issues such as “the aperture mechanism cannot be designed optically” and “bokeh becomes ring-shaped”, there is a reason that we chose the reflective type for the ultra telephoto lens. Mirror lenses have no chromatic aberrations caused by refraction.
Strictly speaking, chromatic aberrations still exist, as the lens is still built with lenses and a mirror. But compared to normal, refractor-type lenses, the chromatic aberrations are almost nonexistent. Roughly speaking, a 500mm lens has ten times the focal length of a 50mm lens, and ten times the chromatic aberrations. SIGMA did not want to accept lower resolutions caused by chromatic aberrations, and so they decided to go ahead with the mirror lens design.

The result was, the ultra telephoto lens MIRROR-ULTRATELEPHOTO 500mm F8. With a compact size, a low price and taking advantage of the strengths of the mirror-lens design, the lens was received very favorably by the market.

SIGMA had one more ultra telephoto lens to its lineup, the large MIRROR-ULTRATELEPHOTO 500mm F4. A 500mm F4 lens. In the first half of the 1970s, there was no other mirror-lens ultra telephoto lens on the market with such brightness. Even the maximum aperture of normal refractor-type lenses was only F4.5. The lens came bundled with a 2x teleconverter, which meant it could also be as a 1000mm F8 lens. The lens worked very well with teleconverters, partly because of its low chromatic aberrations as a result of the mirror-lens design. In a way, the lens may be called the predecessor of the beloved SIGMA APO 200-500 F2.8 / 400-1000mm EX DG lenses. SIGMA has already been a defiant manufacturer in those early days.


SIGMA’s refractor-type ultra telephoto lenses

While the first ultra telephoto lens featured a mirror-lens design, SIGMA was simultaneously developing ultra telephoto lenses with the orthodox refractor-type design. SIGMA demonstrated its unique personality here as well and became pioneers when, in 1976, they released a lens with internal focus: the SIGMA XQ 400mm F5.6. Unfortunately, the SLR era’s first product of inner focus was Canon’s FD 400mm f / 4.5. But I am simply amazed by the courage and hard work of the lens designers who realized an internal focus with a lens design consisting of only four groups with four elements. The internal focusing mechanism allowed for smooth and easy focusing, and together with its reasonable price the lens became a hit product for SIGMA. With minimal changes like an improved minimum focusing distance and the new name ULTRA-TELE 400mm F5.6, the lens remained available in stores up until 1980.
The 400mm F5.6 was intentionally lengthened to mitigate the power of the first lens group and keep the chromatic aberrations at an absolute minimum. However, one has to admit that the chromatic aberrations were not suppressed sufficiently. SIGMA did not release another refractor-type lens with a focal length higher than 400mm until they had found a way to reduce chromatic aberrations sufficiently. That’s how much SIGMA disliked losing resolution to chromatic aberrations.

SIGMA XQ 400mm F5.6
APO SIGMA456 400mm F5.6

The fight against the chromatic aberrations and the apochromat lens

How do we get rid of chromatic aberrations? This is a problem lens designers are still grappling with today. One method to correct chromatic aberrations is to use a so-called dense-flint apochromat, a technique often used in astronomical telescopes. An apochromat comprises three lens elements – two of them made of dense-flint glass – that bring three different wavelengths into focus. While effective, the method poses several problems. The required curvature of the lenses is quite large, resulting in thick and heavy lenses. The large curvature also makes it difficult to correct any aberrations but chromatic aberrations. Further, the process of combining three lenses with large apertures into a single unit is quite difficult. For these reasons, although some manufacturers tried to implement apochromats in their exchangeable lens designs, the technique did not become mainstream.
Companies like Canon and Nikon developed the know-how to reduce chromatic aberrations by using low-dispersion glass or artificial fluorite, and in the 1970s, high-performance lenses with reduced chromatic aberrations began to replace the previous ultra telephoto lenses. SIGMA had no choice but to keep up with these developments. However, at the time SIGMA lacked the funds and the technologically to begin development of the necessary materials.
Instead, SIGMA decided to enter the fray by employing dense-flint apochromats. In 1981, SIGMA released the APO SIGMA345 300mm F4.5 and the APO SIGMA456 400mm F5.6, both of which made use of the apochromat technique. The two lenses were widely discussed and became a huge boost to the company’s brand image. However, the manufacturing factory had problems accurately polishing the large-aperture lenses and glueing the three elements together into a single apochromatic lens. The yield rate was poor and the prices jumped high enough to equal lenses of camera manufacturers.

Mirror lenses as affordable ultra telephotos

SIGMA decided to expand their lineup once again with mirror-type ultra telephoto lenses, in response to the rising price of the refractor-type 400mm F5.6 lens. In 1982, SIGMA revived their lineup of affordable ultra telephoto lenses with the release of the Mirror 400mm F5.6 at a price tag of 50.000 Yen (c. $500). Later the same year, SIGMA released the Mirror 1000mm F13.5 – this compact 1000mm lens with decent enough optical performance and a price of 75.000 Yen (c. $750) sent shockwaves through the camera industry.
While the price may seem dramatically low, the 1000mm F13.5 was actually nearly identical to the 600mm F8 (roughly calculated: 1000mm/13.5=74mm; 600mm/8=75mm). You could almost say that everything was the same except for the curvature of the mirror and the length of the lens barrel. Business-wise, it was quite a good lens.

Left: Mirror 400mm F5.6; Middle: Mirror 600mm F8; Right: Mirror 1000mm F13.5

The era of special low-dispersion glass

In 1983, a glass manufacturer sent us a sample of their new special low-dispersion glass. SIGMA would finally be able to join its competitors, using glass whose performance was on par with Nikon’s ED glass or Canon’s UD glass. Everyone at the company was ecstatic at the news. The special low dispersion glass became used in a large number of lenses, and it enabled SIGMA to make chromatic aberrations a problem of the past.
However, this glass also came with its very own difficult problems to solve…
I will talk about the special low-dispersion glass in the next episode of Ohsone’s Anecdotes.

Share on social media