malykh.com notes: Riconar/Rikenon 55/2.2 (K): It's not a triplet

2023-06-21

Riconar/Rikenon 55/2.2 (K): It's not a triplet

The Riconar aka XR Rikenon 1:2.2 55mm (from Ricoh) is a somewhat strange Pentax K-mount manual focus lens.

Basically it is a budget [kit] lens, made of plastic to reduce the price (but the mount is metal), 55 mm focal length instead of 50 mm, and not a very fast aperture of f/2.2.

52mm filter thread and only 5 aperture blades:

A good copy is acceptable at infinity, but the closer you focus, the more softness is seen. This behavior has led to claims that this is such a "boosted" triplet (a three-element lens).

But no, f/2.2 is very fast for a triplet, and the quality at infinity would be worse. This lens has 4 elements in 4 groups, as stated in the manuals:

(source)

This is also confirmed by disassembling the lens. The lens is most likely a Cooke triplet derivative with an additional element.

And the strange behavior is caused by the cheap focusing mechanism. Focusing is done by moving only the front optical element (which rotates). At infinity, the optical design is normal, but at close focusing, the displaced front element "ruins" the optical design, causing softness.

For this reason, apparently, the minimum focusing distance is quite long for an SLR lens (0.8m):

Reducing it would make the image completely unacceptable.

Compare this image at 0.8m at f/2.2 (difficult to focus due to many aberrations):

Soft focus is observed.

At f/8, the image is just fine:

But when I set the focus on the lens to infinity, and use the lens with an extension tube (15 mm), the image no longer has soft focus at f/2.2, even though the focusing distance is less than 0.8m:

This is an extremely strange decision made when designing the lens. A proper focusing by moving the whole optical unit would have resulted in a normal lens, something like the Pentacon Prakticar 50/2.4 (Praktica B-mount, also 4 elements in 4 groups, see 1 and 2). On the other hand, you got a "hybrid" soft-focus lens, which might be even more interesting today.

Also, sometimes this Riconar/Rikenon 55/2.2 is considered to be a version of the more expensive Fujinon 55/2.2 (also 4 elements in 4 groups). No, that's not true. They are different lenses that produce different images.

See also related notes: