Mamiya RB-67: The Soft-Focus 150mm ƒ4.0 Sekor C Lens

The soft focus lens has designed-in spherical aberration which provides excellent out of focus highlights (bokeh).  The diffusion disks are thin metal pieces that look like small metal bottlecaps.  They are fitted one at a time only, though are not required to use the lens.  They have a larger central hole with smaller holes surrounding.  The size of the central hole and the amount of light that passes the disk in total changes the apparent aperture for exposure calculation. While the smaller apparent base-aperture given by the disks does not appreciably increase depth of field, it does require compensation.  I wonder if the DoF is increased but is not apparent due to the increased softness of focus. 

The disks are, #1 (ƒ5.0), #2 (ƒ5.6) and #3 (ƒ6.3).  They are supplied in a clear plastic "filter case" style carrier with a fitted, molded soft plastic piece to hold them.  To use, unscrew the front lens cell (group of lenses held together, screwed into the shutter,) and simply slip the diffusion disk on to the back end of the cell, then return the cell to the shutter.

Using the the lens's built in aperture, the diffusion of the disks is apparent between ƒ4.0 and ƒ6.3.  Apertures ƒ8.0 and smaller shoot through the center of the lens and center hole of any of the disks, reducing or eliminating both the spherical aberration and diffusion from a diffusion disk if used, showing this to be one of the sharpest lenses built for the RB system.

Shooting Macro on the Mamiya RB System

RB and Fuji Instant Film:

The RB system will accept instant film backs. There are several made including some that require a system adapter, "P" and some that attach directly to the body in place of the rotating adapter. I've used the latter and it works quite well with stocks of Fuji Instant film. This type film tends not to be as sharp IME as slide and negative films, but renders pleasant images. You can also bleach a negative or positive image from the backing which could potentially be sharper than the print.

Tubes and Extenders:

For macro work there are two extension tubes in the system made by Mamiya and several tele-extenders made by Mamiya or others. The #1 tube adds 45mm of length (367gr), the #2 tube adds 87mm (473gr), lengths as marked on the tubes. At 840grams combined, the two tubes weigh almost 2 pounds. This is a huge amount of weight to stick on the front standard of the camera. Then put a lens, say, the 150/4.0 (799gr) and an extender such as the Kenko 2x Teleplus MC6 (657gr), and you have a long lever mounted to the camera that weighs 2,296gr (5.21 pounds!)

Light Falloff and Measuring Exposure:

Now rack the bellows all the way out and you have cut the light showing in the viewfinder by a huge amount, making it almost impossible to focus. And you have to figure a starting exposure: ƒ(exp) == (1/d^2)--T* of the extender; i.e. run a bellows-factor calculation and subtract the filter factor of the extender. This should give a ballpark exposure; I added about 3 1/2 to 5 stops to get close one time. But the lenses and extenders were so heavy that I had to find a way to support them separately from the camera.

So it's really going to depend on how macro you want to shoot as to how possible it will be to assemble, carry and use such a system with system components. The maximum rig above is probably best used in a vertical orientation with a huge amount of light to focus the subject. Longer exposures are fine if well supported, but they can be in the minutes at smaller apertures. By ƒ/32 you may be in the tens of minutes.

Alternatives to Using All RB-System Components:

An alternative to using all system components is to find a lens or chain of lenses that can deliver higher magnification and sufficiently flat field and sharpness at higher magnification while also projecting a sufficient image circle to cover the film. You would probably also want to be able to set the aperture, but at longer exposures, a shutter really isn't necessary. Most lenses extended further away from the film will enlarge the subject, so you could use anything from a 'hand' lens up to a microscope objective or eyepiece. Chances are the smaller the original imaging field design, the more image falloff or even mechanical vignetting when used for this application.