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.