question regarding Roundshot cameras
I am designing a 360-degree 120 film rotational panoramic camera (for my own use, not commercially) and I need some help regarding film transport. I am using two motors, one for camera rotation and one for film advance, but I am not sure how to get a consistent film advance speed. If I drive the take up spool then I assume the film transport speed will slow as the empty spool winds up the film. I have seen images of the Roundshot 220VR and I see it has a drum that the film wraps around, does the drum actually drive the film or just tension it? And is the take up spool (also) driven, and if so does the speed vary as the film winds up? Does it actually make a difference in reality? Thanks!
If you drive the take up spool at constant speed the film speed will increase linearly with time (because of the increasing diameter of spool). This means the exposure will decrease and there will be a horizontal distortion of the image. Both effects will be considerable (factor >2 comparing film-start and film-end).
You have different possbilities to minimize/avoid this effect:
(a) Increase the size of the take up spool
Use of own construction instead of a standard spool. The larger the diameter, the less the diameter will change (less film layers), the film-speed will be more constant.
It will not avoid the effect but reduce it so much it is not recognizable by eye on the print. I have constructed a medium format 360° Panoramic camera this way. I can post pictures if you want.
(b) Control the rotating speed of the take up spool
Most probably not easy to achieve, but doable using stepper motors and digital circuits/sensors.
This will avoid the effect if adjusted precisely.
(c) Drive the film itself
Drive the film instead of driving the spool(s). I guess this is rather complicated for 120 film, but for sure possible - It will fully solve the problem.
(d) Use a lens with small focal distance
The length of the negative for a 360° shot will be shorter, thus the effect less dramatic. However it will not avoid the problem completely.
I guess the simplest approach is (a) in combination with a coupling of the take up spool and the camera rotation (without having a second motor). This way you do not need to align the speeds of the two motors and you can use the rotating speed and the slit size to control exposure.
I quickly googled pics for the Roundshot 220VR I assume the drum is moving the film, so it will be of type (c); most probably driven by a dedicated motor with constant speed depending on the focal length.
Last edited by Zishe Breitbart; 04-26-2013 at 10:15 AM. Click to view previous post history.
Thanks for your reply. I think driving the film directly with a drum (with rubber rings to press against the film) would be the best solution for me. From what I can tell from pictures, it seems that the Roundshot 220VR and also the Voyageur cameras use this method. What I am not sure of is if they also drive the take up spool -I wouldn't think the film would wind up tightly any other way but I may be wrong. Which brings me to driving the spool at a suitable speed so that the film will wind up smoothly, but not so much as to force the film to slip past the transport drum, or to cause the drum to rotate faster.
Incidentally, I am using two motors in order to separate the camera rotation from the film transport, with the camera body mounted on a short rail to allow variable positioning relative to the rotation axis, so I can use different lenses and compensate for the varying entrance pupil location and focus distance.
The take-up spool can have a tensioning motor. You basically want to have something which holds a tension, and as that tension is relieved (i.e. the drum advances film), the take-up mechanism brings the tension back to par. This can be done using a clockwork mechanism. Perhaps it may be easier just to have a torque sensor which automagically (dis)engages the take-up motor.
Originally Posted by plungefrog
Roundshot and Lookaround
Hello - I have a roundshot 65/220/70 and a Lookarund for testing
complete different stuff.
If i were you i would use the lookaorund-book-free for download now- to construct the cameras. alan has multiple versions there.
one can also use 35mm unperforated(better)-he shows also motor-constructions.
Roundshot 70/220 is working like this..
Yes film is pressed against a flat rubber-ring below. Pressure plate has one stiffer o-ring below and a loose thinner one above to press film against a second surface which is standing still against the film. Just talked to seitz about replacement of that main film-rubber-surface. is must be adjusted infactory to get precise distance to lens. Its a simple but delicate mechanisme. if the tensions are not right(worn-out o-ring) film-transport is not ok. One must work very precise.
film-takeup. its a special mechanisme which is connected to the main-o-ring.
except for the film-contact surface, all other rings are o-ring. two of them are holding the spool in place to avoid lateral movement.
vertically there is a spring-takeup for using both 220 and 70mm-spool.
btw: the pressure-plate has two positions for 220 and 70mm. the surface where the film is slipping over can be moved for 70m and 220 width. 60 and 54mm(53mm) images.
most interesting is how distances are set:
1. lens is shifted about 2-3degr. to increase DOF.
2. slit is moving when distance are changed. It will change the position towards the film/lens.
3. sharpness of slit is most important. there are instructions on the net how to do this.
If you dont mind carrying around large camera or weight i would get one of the roundshots 70/220 or better 220vr on ebay.
one can get filmslitters to cut down 70mm-film. Ilford announced unperforated delta 100 and hp5 plus for this year.
see here a roundshot-panorama- 4 secs around-1/60 second) 65mm grandagon.
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Thanks for all the responses. I am gathering parts and materials and construction will start soon; I will start a thread in the camera building section once I have made some progress!