Mamiya RB67 90mm lense questions

[RPippin]

I have a 90mm and I think a 65mm with floating element. My question is outside of the obvious, what is the best way to use this feature? It's (the 90mm) a new lense to me and any advice would be appreciated. Thanks

 

[archphoto]

The 90 is the shorter of the 2 standard lenses for the RB, the other one is the 127mm.
Just use the lens as you would with other lenses: put it onto the RB and have a look what you can do with it, experiment a bit.

As for the 65: it is a wide-angle as you know and using the floating elements goes: focus, look at the distance set and set that distance on your lens.
The 65 (and 50) has a general setting for its floating elements: a red dot that can be used for most distances, it is in the lens-manual.
For best optical quality it is best to adjust the floating elements to the correct position.

Peter

 

[xtolsniffer]

The newer 90mm KL and 65mm KL both have floating elements. The older 90mm Sekor C doesn't but the 65mm Sekor C does. The floating element is there to correct for flatness of field as you focus closer to your subject. As you get closer to your subject, the light rays diverge at a greater angle, so to compensate for this, a floating element will correct for this (I say this as a rank amateur in terms of optical theory, so this is my take on it). The floating element should correct for resolution at the edges of the image, but should have negliable effect in the centre. To use it, set the floating element ring to align the red dot with the infinity mark on the ring (note that on some lenses there are two rings, one for the floating element and one for the depth of field - don't confuse them, some lenses only have the depth of field ring, such as most of the Sekor C lenses this is NOT a floating element ring. The exceptions are the Sekor C 50mm, 65mm and 140mm macro which do have a floating element). Either read off the subject distance from the scale on the side of the bellows or estimate it, then rotate the floating element ring so that this distance aligns with the red dot, and that's it. To be fair, I have tested the effect of the floating element on a 65mm Sekor C on some flat printed targets and it actually makes damn-all difference to my eyes printed up to 12x16", so if in doubt, leave it set to infinity.

 

[philosomatographer]

To be fair, I have tested the effect of the floating element on a 65mm Sekor C on some flat printed targets and it actually makes damn-all difference to my eyes printed up to 12x16", so if in doubt, leave it set to infinity.

I agree with this! And though I have never tested this, I suspect the only lens this feature makes sense on is the 140mm Macro, because this is the only RB lens (as far as I know) for which performance is not optimised at infinity, but rather at close subject distance. Like other Macro lenses with floating elements (e.g. Olympus OM Zuiko 90mm f/2.0 Macro) the floating elements allow for good performance at infinity also, and perhaps at the very closest distances (1:2 or 1:1 magnification).

 

[m_allard]

I basically do the same - set the floating element distance to your focus distance. Not sure if it makes a difference, but that's how it was intended to be used. It's supposed to help with edge sharpness and resolution. I was actually wondering about the theory behind how it actually works - if anyone has a good reference.

 

[philosomatographer]

I was actually wondering about the theory behind how it actually works

In a nutshell (without pictures, sorry):

Most lenses are optimised to photograph objects at infinity, i.e. when the object is so far away, that the light rays reflecting off it and entering the lens enter the lens as parallel rays of light. The lens elements are then designed to bend these rays of light in all sorts of weird and wonderful ways until they exit and hit the film, such that each *point* on the object being photographed is reproduced again as a *point* on the film.

When you photograph something very close to the lens, the light rays entering the lens come in at an angle (no longer parallel) and thus do not play nice with the optical path in the lens, i.e. one may see less resolution, more abberations such as chromatic aberration, and less flatness of field (i.e. sharp in the centre of the frame, soft in the corners, even though the objects are at the ame distance).

A 'floating element' is simply one of the lens elements which is designed to, at close focusing distances, correct some of these abberations. Of course, the opposite would then happen with light rays coming in from a distant source, hence the need to adjust it based on focusing distance.

Most lenses with floating elements adjust it automatically (mechanically) because the lens knows how far it is focused, by virtue of turning a focusing ring. Since Mamiya RB lenses (like large format lenses) do not know how close they are focused, because the whole lens moves backwards and forwards on a lens board + bellows, they had to put a ring on the lens for you to turn yourself to dial in close-focus correction via the floating element.

As it so happens, the mamiya RB lenses are generally amazingly good, and a 6x7cm neg so big, that nobody has really been able to show the effect of the floating element, but if you are an absolutely perfectionist it's probably good to use it in anyway.

Hope that helps!