I think your main problem is going to be that the 16mm is going to have a
smaller fov than the 35mm and you're not going to get as much information
for your grid as you need. Plus, of course, distortion generally tends to be
greater toward the edge so you've got the bit that matters least.
But generally, though, yes, I think it's going to be pretty much useless to
you.
> 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.
The focal length of a lens never changes, regardless of what you do with it.
> 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,
Not really double. The relationship - without actually thinking about it as
hard as I should - is trigonometric.
> 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.
Again, no. If you move the film plane back and forth it just goes in and out
of focus. You'd need an entirely different lens to get the same fov and this
is why your grids are wrong. Bearing in mind that you can't shift either the
lens or the film plane (same thing, effectively) then you start to see that
all you're doing is changing the fov when you change film formats. The 16mm
is just smaller.
I really don't see a way around this.
> -----Original Message-----
> From: owner-xsi(at)Softimage.COM [mailto:owner-xsi(at)Softimage.COM] On
> Behalf Of Adam Seeley
> Sent: 09 January 2007 14:36
> To: XSI(at)Softimage.COM
> Subject: OT: Lens distortion conundrum
>
> 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.
>
>
>
> ---
> Unsubscribe? Mail Majordomo(at)Softimage.COM with the following text in
> body:
> unsubscribe xsi
---
Unsubscribe? Mail Majordomo(at)Softimage.COM with the following text in body:
unsubscribe xsi
