Re: OT: Lens distortion conundrum

[Robert Martinu <antediluvian(at)gmx.net>]

Adam Seeley wrote:

Hello,
from a still photography background, but shouldn't be that much difference

> My instant reaction is that this is wrong and will give us incorrect results when we get down to the undistortion, tracking and ultimately the line up of CG & live action. 
>  
Using the same lens with different film/sensor sizes leads to different 
distortion patterns.
The smaller format will record the information for the center of the 
image, but the data for the border regions goes to nirvana.


> So, I'm just curious to know whether this what's known in the industry as a technical faux pas (to put it mildly), or am I missing some basic technical savvy.
>
> My basic, possible naive, understanding of film sizes and focal lengths would lead me to believe that you would need double the focal length to compensate for the 35mm->16mm film size difference, or that to use the same lens you would have to shimmy the filmback backwards to observe the same amount of distortion across the 16mm film.
>  
At focus close to infinity:
The relation of focal lengths needed for the same field of view  is the 
same as the relation of the diagonals of the film.(crop factor)
At very close distances lens construction may may make things difficult

> I also assume that you would need some sort of lens conversion ring to mount a 35mm lens on a 16mm camera which would again slightly change the focal length.
>  
Depends on the camera system.
Most dSLRs are esigned to use  35mm lenses with sensors in the 24mm 
range, there is no inherent need for an adaptor.

If one is needed, then it depends on the flange focal distances, if the 
16mm has a sufficient smaller one then the 35mm an adaptor can be built 
without changing the lens behavior. Considerations regarding crop factor 
still apply.
In other cases a lens has to be built into the adaptor, with all kinds 
of influences on picture geometry.

Robert
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