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Apparently multi-focal lenses such as the Tri-Elmar 16-18-21mm went out of fashion but I was wondering if the reasons for that were technical or commercial.

  • Are multi-focal harder to build compared to zooms?

  • Would it be economically viable to build a bifocal or trifocal lens with say 35[/50]/85mm steps and a (preferably fixed) aperture such as 2.8 or even 2.0?

  • While a set of primes in the same focal steps would probably have the upper hand, would such a multi-focal lens be able to provide enough image quality in order to compete with a 35-85mm zoom?

  • Have the manufacturers found out that a zoom in the same range would be a better deal for them and simply started selling only primes and zooms?

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Not entirely sure but I think multi-focal are the predecessors of zooms. Zooms did not work before because there was no way to see the framing at any focal-length adjust until DSLRs and zoom-optical viewfinders were invented. With a multi-focal, you had lines in a fixed OVF or accessory finders to swap for each focal-length. – Itai Feb 23 '13 at 17:54
up vote 5 down vote accepted

That lens (the 16-18-21mm Tri-Elmar) is a bit of an odd duck. It's an ordinary parfocal zoom lens in every way except one: it only has three strongly-detented focal lengths available to the user. It can be argued that this makes it easier to optimize the relationship between elements at the three available positions, but there is noless opportunity for wear and misalignment than there would be with a continuous zoom. The real reason for the lens being a Tri-Elmar rather than a Vario-Elmar is entirely to do with framing on a rangefinder camera. Without the detents, it would be necessary to have a viewfinder that zooms with the lens, probably an auxiliary finder. Whether you use an aux (hot shoe) finder with etched frames or the normal frame line sets in the rangefinder's viewfinder (which would be automatically selected by varying the position of the frame coupler on the lens mount) a multifocal lens becomes usable on a rangefinder without requiring a zooming finder as well.

Most discrete multifocal lenses, historically, were quite different. They were single, well-corrected focal length lens made in more than one part, and you could choose to leave one or more of the parts (lens groups) out of the assembly to get somewhat less-well-corrected lenses of different focal lengths. The original Symmar lens design is a good example of the breed—by removing the rear half of the lens from the shutter, you got a longer, slower lens (you still had the same iris, so the longer focal length meant a smaller relative aperture). It wasn't ideal, but the smaller aperture made up for some of the lost correction, and it was certainly cheaper than buying a separate, longer lens. Other multifocals came as kits, often with three groups, giving you three to six different focal lengths (depending on the design and how you assembled the lens). That's really only a practical (?) approach with large-format lenses where you have access to parts of the lens both in front of and behind the iris and shutter. And it's not entirely dissimilar in approach to the auxiliary lenses sold for cameras with fixed lenses.

With SLRs and other camera types having through-the-lens viewing/framing, there is no real advantage to not making a zoom. The mechanical couplings to adjust the relationships between the lens groups is not significantly more complex than it would be for a detented lens like the Tri-Elmar. And if the zoom range is kept as small as it is on the Tri-Elmar (just over a 1:1.3 range), then maintaining as high a degree of correction and recilinearity over the entire range would be just as easy.

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That's my understanding, too. A zoom lens doesn't work well with a rangefinder. If you're using a what-you-see-is-what-you-get system like with a mirror or EVF, then a zoom lens is more flexible. – Eric Feb 23 '13 at 22:03

I think the evolution of zoom lens design has reached the point that multi-focal lenses are no longer needed. With the ongoing evolution of processing capacity, modern supercomputers have enabled design and production of lenses that the worlds best lens designers could only have dreamed about in the past. Development of a lens used to be a slow, laborious process where new ideas had to be manufactured into prototypes and tested. Changes to a design that once took weeks or months to evaluate can now be virtually assessed in mere hours, and the optimum design of an element made of a particular material can be ascertained much more quickly and cheaper.

One of the first lenses that seriously utilized this process was the Canon EF 70-200mm f/2.8L IS II. It approaches and sometimes equals the capability of many prime lenses at comparable apertures and focal lengths. Not only at the "sweet spot" as we expected zooms to behave in the past, but across the entire focal range. The recent EF 24-40mm f/2.8L II is even better in this regard.

The zoom lens designs of several decades ago sacrificed a lot of optical quality compared to prime lenses for the convenience of multiple focal lengths in one lens. There is still a gap there to be sure, especially at the consumer grade level. But that gap is much narrower than even 5-10 years ago.

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