Question(s) about a Cirkut Outfit #6 and its lens.

[Neanderman]

What constitutes the "constant?"

Ed

 

[DaveM]

Thanks, Jamie;

In Len's post on this he quotes 9+1/16" (9.0625") rather than 9" as the diameter of the No.6 Outfit ring-gear; Roberto says he counted 290 teeth on his, and 290 divided by 9.0625 = 32, the DP of the gear, which is exactly what you'd expect. (Roberto: there is no rounding error - you need to measure from half-way up a tooth on one side of the gear to half-way up the tooth directly opposite, ie on the other side - not between the tops or bottoms of the teeth. Why? Think about two gears in mesh.........).

Adding this 1/16" (the tooth depth) to the ring-gear dimensions given in your post we get diameters of 12+1/16" and 20+1/16", which translates to 386 and 642 teeth respectively. Jamie, could you possibly confirm this (by measurement, as above, rather than by counting teeth - which would take forever)?

And does anyone know the Constants (ie as quoted by Len for the No.6 Outfit) for the No.8 Outfit and No.10 and No.16 cameras?

Or any of the above information for the No.5 and No.6 cameras (which use 48 DP gears), ie ring-gear specs, pressure angle (likely also 14+1/2 degrees) and Constants?

The reason I'm asking for this information is that I'm compiling an Excel spreadsheet to show the pinion gears required for each camera based on the distance between the RNP and Film plane, ie if you know the exact position of the RNP for any lens, and know which camera you're using(!), by measuring the distance between the RNP and film plane for any focussed image you can simply look up the required pinion on the table.

So far this looks OK; once I've finished testing it on my No.8 I'll make the table freely available for others to test, particularly on other Cirkut models.

Of course if anyone thinks this is impossible, please let me know why - you could save me a lot of work!

Cheers to all; Dave

 

[DaveM]

Hi all;

Len has responded off-line and confirmed (from Jim Lipari's notes) the 9.1/16" dimension for the No.6 outfit ring gear; he was also able to confirm Jamie's data for the Nos 8, 10 and 16 cameras (so my sincere thanks to you both).

I plugged these data into the spreadsheet and tested it against a number of examples I was able to find, both here on the forum and elsewhere. The results are encouraging - in each case the spreadsheet is indicating either the correct gear as supplied with the camera, or the next one up or down (at this stage I'm assuming this is because the actual Rear Nodal Point to Film Plane distance falls between the 'ideal' values for two gears).

I'm still waiting for my No.8 to arrive from the US (it's a long way to Oz!) but when it does I'll test to determine the rear nodal point position (and focal length), then try it at various focussed distances to see whether the spreadsheet values hold true. If anyone else feels inclined to assist in the testing process (!) I'd welcome details of your cameras, lenses and gears to feed into the spreadsheet - the more examples we can compare the better.

Cheers; Dave

 

[frobozz]

What constitutes the "constant?"

Ed

Seemed like as good a thread as any to resurrect to answer this question.

It would appear that the "constant" and that formula that uses it ( Constant/true focal length = small gear teeth, or if you prefer Constant/gear teeth = true focal length) is simply the result of combining and simplifying all the formulas that go into that calculation, and moving all the constant things into one spot and simplifying further. The constant things are constants like pi, and various measurements for a specific camera.

So for any camera, the constant is ("number of big ring teeth" * "average film drum circumference")/(2*pi)

Average film drum circumference would be the circumference with some leader and film on it. I measured mine (#10) by simply running a piece of paper around it so it overlapped and marking it on both laps, then unwinding it and measuring the distance between the two marks. I wasn't being very precise, and got about 11.5" so I'll use that since it's easy to remember.

So, for my #10 that's 384 teeth (yes, I counted!), 11.5" circumference, which all comes out to 702.83. Note that when using inches for circumference you must also use inches for the focal length. You could do them both metric if you'd prefer, which would of course result in a metric constant instead of an inch constant.

I'm cribbing off of some copies of Jim Lipari's notes; I am not fully understanding this stuff all inherently myself yet. But I'm getting there!

Duncan

 

[frobozz]

OK I now have permission from Jim's daughter Kris to post his notes, so a big thanks to Len Robertson for the scans! This should help quite a bit with this thread, or any thread discussing Cirkut theory.

http://cirkutcamera.com/docs/jim_lipari_notes_01.pdf

Duncan