Floating element(s)?

Alan Edward Klein · Jan 9, 2021

138S said:
hmmm, it has to be quite interesting for critical focus.... now that you say it....

Sometimes, with the RB67 on tripod , I have used the x8 loupe for the view camera, after removing the chimney, but I had to also use a dark cloth to see...

It focuses on the center of the image. I don't know its magnification. Maybe 4x. But definitely helpful for critical focus. Here's one for $82 on ebay.
https://www.ebay.com/itm/3733787170...9Cevjm6lkgflUFmHFK4xJw1GnT3pBSwhoCmugQAvD_BwE

Mamiya_Repair · Jan 9, 2021

Mamiya made three floating element designs on the original RB67 lenses (without regard to the k/l series). The 50mm, 65mm and 140mm macro were the three floating element lenses. However, there where five versions of the 65mm and only the last two versions, "NB" and "C", have the floating element.

Alan Edward Klein · Jan 9, 2021

Lachlan Young said:
I think the CoC on the scale may be pretty relaxed - it looks like about a 2.4x enlargement or so, at the minimum paper resolution requirement. It's a bit like the seemingly perplexing max reproduction scale on the Pentax 6x7 135mm macro - till you realise that it's intended to intended to deliver a life-size image when enlarged to 8x10...

It seems their COC is around .085mm. Is that good or bad for 6x7 120 format?

Lachlan Young · Jan 9, 2021

Alan Edward Klein said:
It seems their COC is around .085mm. Is that good or bad for 6x7 120 format?
View attachment 263380

Depends how close you want to view it - at 10"/ 25cm, 0.200mm is often regarded as an adequate COC for a contact print display - thus 0.085 would roughly equate to a 2.35x enlargement. 0.055 might be more appropriate for an 8x10-ish print under the same standards. If you want to go to a 16x20, 0.029mm might be a better starting point - on the other hand, do you stick your nose into 16x20's? I think Mamiya's numbers result from a 30cm viewing distance, a 25% lower print resolution requirement and a 3.5x enlargement to an 8x10.

Alan Edward Klein · Jan 9, 2021

Just a reminder that not all lenses work on all RB67 models. Here's a good reference source on their various lenses.
https://rb67.helluin.org/lenses/

Alan Edward Klein · Jan 9, 2021

Lachlan Young said:
Depends how close you want to view it - at 10"/ 25cm, 0.200mm is often regarded as an adequate COC for a contact print display - thus 0.085 would roughly equate to a 2.35x enlargement. 0.055 might be more appropriate for an 8x10-ish print under the same standards. If you want to go to a 16x20, 0.029mm might be a better starting point. I think Mamiya's numbers result from a 30cm viewing distance, a 25% lower print resolution requirement and a 3.5x enlargement to an 8x10.

My standard procedure was to use the DOF tables furnished by Mamiya which appear to be .085mm COC to calculate the aperture. Then, stop down one additional stop for good measure.

However, based on your comments of .035mmCOC for 16x20", I would need to add three stops. The chart on the right approximates the Mamiya furnished charts at .085. The one on the left is you recommended .035MM COC. It appears three stops smaller is required. Is that how you read it?

wiltw · Jan 9, 2021

Cambridge Color web site has a good discussion of DOF...it points out that the 'manufacturer standard' CofC size and lens DOF scales assume a viewer's substandard visual acuity, which is inferior to the optometrist's goal of attaining 20/20 vision standard of acuity. Their DOF calculator advanced version allows the user to select between 'manufacturer standard' and 20/20 vision, and it also gives the DOF field limits for a specified final print size and a specified viewing distance for that print.

That is why the DOF tables and DOF scale on the lens seems optomistic, and why so many photographers consult tables or the DOF scale and use an aperture (or two) larger than the chosen aperture for exposure.

Using their advance DOF calculator in assuming 90mm f/8 on 6x7 with lens focused at 10m...

Manufacturer standard DOF zone is 24m deep
20/20 vision DOF zone is 4.7m deep

You would have to use f/2.8, using 'manufacturer standard' calculator or DOF scale to achieve the narrow 4.9m DOF zone...
3 f/stops larger!

Alan Edward Klein · Jan 9, 2021

wiltw said:
Cambridge Color web site has a good discussion of DOF...it points out that the 'manufacturer standard' CofC size and lens DOF scales assume a viewer's substandard visual acuity, which is inferior to the optometrist's goal of attaining 20/20 vision standard of acuity. Their DOF calculator advanced version allows the user to select between 'manufacturer standard' and 20/20 vision, and it also gives the DOF field limits for a specified final print size and a specified viewing distance for that print.

That is why the DOF tables and DOF scale on the lens seems optomistic, and why so many photographers consult tables or the DOF scale and use an aperture (or two) larger than the chosen aperture for exposure.

Using their advance DOF calculator in assuming 90mm f/8 on 6x7 with lens focused at 10m...

Manufacturer standard DOF zone is 24m deep

20/20 vision DOF zone is 4.7m deep

You would have to use f/2.8, using 'manufacturer standard' calculator or DOF scale to achieve the narrow 4.9m DOF zone...
3 f/stops larger!

Manufacturers use different standards. So you'd have to know where to start in any case for each chart.

One thing I checked is to see if Mamiya's lens chart for DOF actually matches what they imprinted on their lenses for DOF ranges. I checked 90mm lens and they seem to match. Not exact but it's all approximate anyway.

Alan Edward Klein · Jan 9, 2021

Lens compatibility chart for RB67 lenses and other accessories.

MattKing · Jan 9, 2021

Alan Edward Klein said:
Lens compatibility chart for RB67 lenses and other accessories.
View attachment 263398

That is the Reznitsky table, and is excellent.

Donald Qualls · Jan 11, 2021

FWIW, I started treating DOF as largely fictitious after I got my first scanner and could look at what was actually happening in the negatives (never looked that close at my prints back in the day). DOF is just a way of quantifying how much defocus you're willing to accept. Want the shot to look like an RB67 image, or a Holga one?

Floating element(s)?

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