Re: OT: Lens distortion conundrum

[Francois Lord <francoislord(at)gmail.com>]

Who the hell decided to shoot the grids on 16mm?
Yes, it's a faux-pas. If you put a normal DSLR lens on a digital DSLR 
(under 2000$), it creates a "crop factor" usually around 1.5 or 1.6. 
This means the image circle the lens produces on the sensor is "too big" 
and the sensor only sees the center part of it, making the field of view 
smaller. Since the lens distortion is usually most visible on the edges 
of the image, this crop factor will give you incorrect results. The same 
phenomenon happens when putting a 35mm lens on a 16mm camera.

If you can't get the distortion from the shots, you could find the crop 
factor (I don't know how, the lens adapter also has an effect on it) and 
create a new image with the grid image smaller in it. It will be less 
precise but it's better than nothing.

hope it helps.

--
Flord

Adam Seeley wrote:
> Hello all,
>
> Here's a head scratcher for the day, (well mine anyway).
>
> We have a whole bunch of plates that have been shot 35mm Full Aperture on various Prime lenses. e.g . 10mm, 24mm, 50mm, 180mm
>
> Lens Distortion Grids have been shot for these plates using exactly the same lenses (matching serial numbers), but have been shot onto 16mm.
>
> 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. 
>
> 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.
>
> 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.
>
> Any answers/speculation/inherent knowledge & experience gladly received.
>
> Cheers,
>
> Adam.
>
>
>
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