Rollei SL66 Pinhole Calculation Help Please

[TimVermont]

I've acquired a 0.3mm pinhole machined into an SL66 mount taken from a 120/5,6 S-Planar. It came with a card that has conflicting information. On one side the card reads: "40mm EQ, f225"; the other side reads: "98mm EQ, f343, meter ei250 at iso 8, f64"

The SL66 flange focal length is 102.8mm and the mounting ring is 9mm thick. Given these dimensions I don't think the information on either side of the card is correct. I would appreciate it if someone who is familiar with pinhole calculations would confirm the equivalent focal length (or angle of view) and the f-stop of this pinhole.

 

[DWThomas]

I'm not sure I fully understand what you're describing, but you can make it a bit of a lab exercise. The pinhole to film distance divided by the pinhole diameter gives the f-stop. The film frame dimensions, the pinhole to film distance, and a little trig can provide the angle of view.
If the "9mm thick" mounting ring places the actual plane of the pinhole plate 9mm farther from the film, I'd expect that should add directly to that flange focal distance.

I happen to prefer Pinhole Designer, a utility ("app") that only runs on Windows, but on the web there is Mr. Pinhole which uses scripts and runs via a web browser. A typical "6x6" frame generally runs about 56mm square, with that, the pinhole to film distance, and the pinhole diameter, all the numbers should fall out. Note that a millimeter or two out of 60 or more will not provoke a photographic crisis -- most pinhole camera designs can be characterized as loose!

I'm thinking if the pinhole plate is mounted in a portion of the barrel from an interchangeable lens, the plate could be closer to the film than the mount flange -- or it could be farther away. You should be able to measure and sort it out. 0.3 at the 102.8 flange focal length does indeed work out to f/343 but I've no clue what the "98mm EQ" is going on about. A pinhole to film distance of 102.8 and frame 56 mm wide should be about a 30º angle of view. I usually prefer something wider, but you may have no way to place the pinhole closer to the film without endangering a mirror or creating a mechanical nightmare.

That's my 1.3 (after tax) cents ...

 

[RalphLambrecht]

I've acquired a 0.3mm pinhole machined into an SL66 mount taken from a 120/5,6 S-Planar. It came with a card that has conflicting information. On one side the card reads: "40mm EQ, f225"; the other side reads: "98mm EQ, f343, meter ei250 at iso 8, f64"

The SL66 flange focal length is 102.8mm and the mounting ring is 9mm thick. Given these dimensions I don't think the information on either side of the card is correct. I would appreciate it if someone who is familiar with pinhole calculations would confirm the equivalent focal length (or angle of view) and the f-stop of this pinhole.

I also get f/343

 

[TimVermont]

Many thanks to you both. The pinhole sits above the flange in the mount, so I added that distance to flange to film plane distance, put the numbers into the Mr. pinhole site. I'm going to read Ralph's PDF, print an exposure table based on f/377 and go play!

 

[DWThomas]

You are welcome. Also bear in mind (I still wrestle with this myself) going from f/343 to f/377 is only a tiny fraction of one stop! In addition, with long exposures (5 or 10 seconds and beyond) reciprocity begins to rear its ugly head, although that is wildly variable depending on the film. What I'm trying to say is that in my view, pinhole is not an exact science! There is a whole forum (and a group even) here on APUG dedicated to pinhole work. Don't know if you've explored that, but lots of good stuff passes through there. It's definitely an exercise for experimentation.

I'm sort of a once a year pinholer, unleashing something for Worldwide Pinhole Photography Day, the last Sunday in April each year. And I have to admit, my "thing" tends to be making an engineering project out of the cameras. (My lensless activities)