Macro, apertures, sharpness and diffraction questions in Medium Format

[Overread]

So I've got a Mamiya RB67 Pro-S somewhere in the post on its way to me and I'm keen to experiment with its use within macro areas. However I'd like to see if I can save myself a roll of film and pick some peoples minds with regard to how the optics behave with this camera with regard to the limitations on sharpness with regard to diffraction.

In short if anyone has any ideas as to how far I can stop the lenses down before softening starts to affect the final image, both with and without extension tubes, I'd be very grateful.

For reference these are the lenses in the post on their way to me:
Lenses 90mm f/3.8, 50mm f/4.5, 180mm f/4.5,
tubes: Mamiya No.1 and No.2

With some light reading and research it seems that the 90mm and 180mm will be the better choices since the 50mm might well show distortion/blurring/softening of the corners with the extension tubes added.

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[maris]

I shoot The Mamiya RB 67 lenses from the 37mm fisheye to the 360mm telephoto and at the risk of a generalisation I would say maximum sharpness in the centre of the image is available 1 stop down from wide open. Image corners come good about 2 to 3 stops down from wide open. Importantly, corner sharpness requires that the lenses with a floating element are adjusted correctly. The softening effects of diffraction can be seen even with moderate stopping down and at minimum aperture, f32 or f45, diffraction cancels the finest details. But macro is a different game and diffraction is rarely a consideration. Limited depth of field is the big sharpness killer.

For macro work with subjects that have front to back depth I nearly always use flash illumination and the lens at minimum aperture to maximise depth of field. Shooting macro 1 stop down to minimise diffraction gives so little depth of field that 99% of the image is not just out of focus but mostly missing entirely. It is an irony that the maximum amount of detail comes with the lens stopped all the way down and maximum fineness of detail comes with the lens nearly wide open. Unfortunately optical law has not been repealed.

The 90mm and 180mm lenses work well with tubes No.1 and No.2. The 50mm lens doesn't work because it is a retrofocus design that runs out of space between it and the subject. Even without tubes the 50mm is only 4.9cm from the subject at closest focus and it's delivering 0.88x which is nearly lifesize.

[Overread]

Thanks for the input maris and yes I agree these kinds of discussions always get a little hazy around the specific limit points as personal standards and output mediums tend to result in differences of opinion. You're points regarding maximum detail and sharpness are interesting to read and it sounds like sitting part way through is the ideal; however depending how tricky it prove to use in the field I might well even try to aim for getting maximum detail and sharpness together with focus stacking after scanning the film. Course that might prove a little impractical if my tripod can't keep the setup very still.