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View Full Version : Do Cirkut cameras rotate around the aperture of the lens?



BetterSense
09-30-2009, 03:41 PM
I'm building such a moving-film-slot-type panoramic camera out of a 35mm camera. I figure I can then print the negatives with my 4x5 enlarger. I was going to have the camera rotate with the aperture at the center of the rotation. It seemed the obvious way to do the calculations, but looking at currently existing cameras, it doesn't look like they do. It kind of looks like the cirkut cameras, both the aperture and the film plane trace out circles. In my design, the aperture would be stationary and the film would move around it.

Also, what is a good lens focal length for this type of camera? I only have a 50mm lens for this camera. I think the vertical angle-of-view should be pretty good actually. Are lenses for these cameras of the "long", "normal", or "wide" family?

jamie young
09-30-2009, 04:20 PM
cirkuts don't move around the aperture. They do usually take into account the position of the nodal point. I don't know the science of it though. With longer lenses the rotation point is pretty far off the aperture point on a cirkut

Bruce Watson
09-30-2009, 05:23 PM
cirkuts don't move around the aperture. They do usually take into account the position of the nodal point. I don't know the science of it though. With longer lenses the rotation point is pretty far off the aperture point on a cirkut

Yes, one would think they would rotate around the rear nodal point for the lens in question.

BetterSense
09-30-2009, 10:01 PM
Yes, one would think they would rotate around the rear nodal point for the lens in question.


They do usually take into account the position of the nodal point

What is this "nodal point"?

polyglot
10-01-2009, 07:05 AM
Not knowing much about a Cirkut except that it's a panning camera, I'm assuming that the lens rotates in space and there's a film transport that runs at the same time to expose a slit continuously... which means the lens would need to rotate about its FRONT nodal point so as to not change perspective throughout the scene. Same principle as used when stitching photos - you rotate about the front nodal point so as not to introduce parallax errors. The rear nodal point is relevant only with respect to the details of the film transport.

Definition of Nodal Points (http://en.wikipedia.org/wiki/Cardinal_point_(optics)).

Mike1234
10-01-2009, 08:41 AM
BetterSense, the nodal point is where all the light converges at the smallest point... where the rays cross over each other near the center of the lens. The aperture is usually centered there.

godi
10-01-2009, 09:33 AM
you have to rotate around the entrance pupil, which is generally not identical to the nodal point:
http://en.wikipedia.org/wiki/Entrance_pupil

BetterSense
10-01-2009, 11:04 AM
Regardless of what it's called, I think I now understand that I should position the camera with respect to the pivot such that there is no perspective change when it rotates. With an SLR, this should be easy to determine by experimentation. It looks like my design will be stuck with a normal 50mm lens.

Mike1234
10-01-2009, 11:47 AM
You can often find the diagrams of lenses online. It's fine if you get pretty close.

BradS
10-01-2009, 12:06 PM
...and if you're not close, you get more interesting photos!

clay nz
10-01-2009, 07:54 PM
In theory any rotating camera should rotate around the lens nodal point but Cirkuts prove the rule can be ignored and produce acceptable results.

Clayton

godi
10-02-2009, 04:00 AM
sorry, but that's wrong. Have a look:

http://en.wikipedia.org/wiki/Cardinal_point_%28optics%29
http://doug.kerr.home.att.net/pumpkin/Pivot_Point.pdf

c.d.ewen
10-02-2009, 06:44 AM
[QUOTE=godi;871009]sorry, but that's wrong. Have a look:QUOTE]

Darn, Clay..... you, me and Goldbeck have been fools!

godi: Cirkuts revolve on a geared platform, and can't be adjusted to accomodate the nodal point of the lens.

See APUG member Jamie Young's website. (http://www.jamieyoungphoto.com/cameras1.html)


Charley

Bruce Watson
10-02-2009, 10:19 AM
Yes, one would think they would rotate around the rear nodal point for the lens in question.

I stand corrected. Entrance pupil it is.

Janos
01-06-2010, 12:55 AM
Where the light enters the lens, at the front element, is the front nodal point. Where it exits, is the rear nodal point. The centre, where the light crosses at a central point, where the aperture should be, is also a nodal point- could be called the cardinal point. I think you want to pan the camera around the cardinal point. I'm sure there are some which have been designed to do this, but I'm pretty sure that's not the case with the old Horizont which I had once. Irrelevant on this sort of camera anyway, as the image won't require stitching, unless you are operating 3 cameras back to back, as they once did at the peak of Mt. Everest, with three panon wideluxes. It was a clear day too. I don't know where you can find this image, sorry.