How to use K/L floating lens for RB67

AgX said:

Typically a "floating" group is displaced automatically by being interlocked to the focussing barrel, similar as groups are moved in zoom lenses.
Recently we had a thread about a floating group at an MF lens actually having to be manually set by a dedicated ring. I guess it was about your lens.

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Ian C said:

What Mamiya refers to as a floating unit is more commonly called a field flattener. Many helicoid-focused lenses, especially wide-angle lenses, have this cam-actuated inside the lens assembly.

It does essentially nothing at infinity and almost nothing at medium focusing distances (assuming that the float ring is at the infinity setting) . Nikon uses the term CRC (Close Range Correction). It’s all the same. Lenses, especially fast wide-angle lenses, generate field curvature as the subject distance becomes close. That can soften the image radially outward. The field flattener on the RB and RZ lenses use a cemented doublet that is moved independently of the main focus to flatten the field near the image plane.

The following gives the correct focusing method for use on RB67 and RZ67 lenses equipped with the independently adjusted field flattener (“Floating System”).

Mamiya-Sekor_Macro_C_140mm_f4.5.pdf (mamiyaleaf.com)

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• Floating Element: This lens has a floating element which is moved by a separate ring marked with green numbers, located in the front of the shutter speed ring. Most lenses, like this one, are designed to have the best sharpness when focused at infinity. Other lenses, like many macro lenses, are designed to have the best sharpness when focused closely. The floating element allows you to get the best sharpness at a range of focusing distances.
• Notes: Acceptable results can be had by setting the floating element ring at infinity. This will give acceptable focus from about 7 feet to infinity. Setting the ring at 3.3 feet gives acceptable result from 3.3 to about 7 feet. If focusing closer, set at 3.3 feet at stop down as much as possible.
• Using: To use the lens, first focus with the bellows focusing. Second, use the focusing scale on the side of the body or a tape measure to find the focusing distance. Third, align the floating element ring so that the green number matching the distance lines up with the red dot on the depth of field scale

Neil Grant said:

.. the instructions for setting the floating elements (link) are specific to the 140mm macro C and not necessarily the other RB lenses with floating elements. The 140mm macro can show a slight but noticeable focus shift when the FE control is adjusted. The focus will then need touching up. Compare this with the situation when adjusting the FE's on the 50mm C and 65mm C lenses - no observable focus shift occurs. Not sure if the FE's on the 90mm KL cause a focus shift - but if they do, the lens focus should be adjusted.

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peterB1966 said:

... while the fora on the RZ67 seems to indicate the DOF calculator at the front of the RZ67 lenses is just that, a calculator, the K/L actually tells [it ontrols a] floating element ...

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AgX said:

Can't one just look at the lens and see what it is doing when turning that ring?

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AgX said:

I did not mean looking through the viewfinder, but looking at the lens direcly.
(One must do the same for instance when trying to differ between a plain triplet and a Tessar type, or when to fo find out where a speck sits.)

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peterB1966 said:

I shoulda known there would be the inevitable discussions of is it reallllly a floating element etc, so should have just put it to bed with a snapshot of the lens. See below:

View attachment 284052

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MattKing said:

There must be two 90mm K/L versions, because the ones I have seen (admittedly not many) didn't have that marking.
The Mamiya RZ system brochure also doesn't include that version of the 90mm in the list of floating element lenses.
I would suggest using the procedure specified in the manual for the 140mm macro lens.

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Dirb9 said:

All of the Rb67 90/3.5 K/l lenses have a floating element. The confusion might come from not all of the K/L lenses having the green "floating system" engraving, it seems that Mamiya started that engraving in the mid 90s, a few years after the lens was introduced, however all RB67 lenses with the rubber ring have true floating elements. The lenses with a small simple plastic or metal ring to check DOF are not floating elements. The earlier Sekor C 90/3.8 did not have a floating element.
As for the RZ67 version, you are correct, it is a drastically different design, much simpler in a 6e/6g configuration with no floating element, as opposed to the 11e/9g arrangement in the RB67 K/L. I don't know why the more advanced 90 never was introduced in an RZ67 version.
The basic procedure of focus, read the distance, and set the ring works fine, and actually pretty fast once you get used to it. And, as other people have mentioned, it really makes the most difference at close focusing ranges, so you don't need to worry about it for landscapes.

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Dirb9 said:

All of the Rb67 90/3.5 K/l lenses have a floating element. The confusion might come from not all of the K/L lenses having the green "floating system" engraving, it seems that Mamiya started that engraving in the mid 90s, a few years after the lens was introduced, however all RB67 lenses with the rubber ring have true floating elements. The lenses with a small simple plastic or metal ring to check DOF are not floating elements. The earlier Sekor C 90/3.8 did not have a floating element.
As for the RZ67 version, you are correct, it is a drastically different design, much simpler in a 6e/6g configuration with no floating element, as opposed to the 11e/9g arrangement in the RB67 K/L. I don't know why the more advanced 90 never was introduced in an RZ67 version.
The basic procedure of focus, read the distance, and set the ring works fine, and actually pretty fast once you get used to it. And, as other people have mentioned, it really makes the most difference at close focusing ranges, so you don't need to worry about it for landscapes.

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MattKing said:

So the lens pictured is not a RZ67 lens, it is an RB67 lens.
That now makes much more sense. And I now realize that the OP's first reference was to the RB67 K/L version. Unfortunately, he/she immediately started referring to the RZ67 K/L lenses, and I immediately misunderstood what he was referring to. All of my earlier references in this thread are to the RZ67 lenses. My fault.
FWIW, I don't tend to get the RB67 and RZ67 lenses confused, because I never think of them as being the same, even for the few of the K/L lenses that might share the same optical design.

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Dirb9 said:

All of the Rb67 90/3.5 K/l lenses have a floating element. The confusion might come from not all of the K/L lenses having the green "floating system" engraving, it seems that Mamiya started that engraving in the mid 90s, a few years after the lens was introduced, however all RB67 lenses with the rubber ring have true floating elements. The lenses with a small simple plastic or metal ring to check DOF are not floating elements. The earlier Sekor C 90/3.8 did not have a floating element.
As for the RZ67 version, you are correct, it is a drastically different design, much simpler in a 6e/6g configuration with no floating element, as opposed to the 11e/9g arrangement in the RB67 K/L. I don't know why the more advanced 90 never was introduced in an RZ67 version.
The basic procedure of focus, read the distance, and set the ring works fine, and actually pretty fast once you get used to it. And, as other people have mentioned, it really makes the most difference at close focusing ranges, so you don't need to worry about it for landscapes.

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