Compensating for 35mm still lenses' exposure when mounted on a 16mm camera

BBDFIB · Jun 27, 2023

I've been looking into using 35mm lenses on a 16mm camera I've got. The crop factor from still 35mm to 16mm is roughly 2.8x, but does this directly apply to the aperture as well? Or is that a slightly different calculation to make? Thanks in advance.

RAZKY · Jun 27, 2023

BBDFIB said:
I've been looking into using 35mm lenses on a 16mm camera I've got. The crop factor from still 35mm to 16mm is roughly 2.8x, but does this directly apply to the aperture as well? Or is that a slightly different calculation to make? Thanks in advance.

No compensation for aperture is needed.

BBDFIB · Jun 27, 2023

RAZKY said:
No compensation for aperture is needed.

Is it not like, for example, an old variable focal length lens where the aperture is pulled far enough forward or backwards to, in effect, reduce the size of itself as the film/sensor would "see" it?

Because a C mount to PK mount doesn't seem long enough to create that effect by itself.

Soocom1 · Jun 27, 2023

There are a lot of physics taking place here.

But the core of it comes to what is called the registration or flange distance.
Thats the distance that the lens sits from where it physically touches the mounting surface to the image plane.

It doesn't matter the size of the image area itself (format size) , but in order for the lens to focus it has to be at the designed registration distance in order for it to work correctly.

So the effective apature doesnt change, only the image format size.

RAZKY · Jun 28, 2023

BBDFIB said:
Is it not like, for example, an old variable focal length lens where the aperture is pulled far enough forward or backwards to, in effect, reduce the size of itself as the film/sensor would "see" it?

Because a C mount to PK mount doesn't seem long enough to create that effect by itself.

I cannot comment on "old variable focal length" lenses, but all the C to PK adapter does is place a PK mount lens to the proper flange-focal-distance for infinity focus. That FFD would be the same for any format, and does not change the aperture value.