Box camera focal length

[steveo_mcg]

I've got about 4 box cameras and they're all slightly different lengths, lens to film plane, does this make any difference to the focal length or is the box size simply related to the lens used?

 

[joxby]

The variable you are missing is the film frame size.

 

[joxby]

oh wait....I dunno why I read box camera as "pinhole camera"

 

[abdoujaparov]

The variable you are missing is the film frame size.

Frame size shouldn't matter. 50mm is 50mm whether it's on a micro4/3 digi sensor or a 10x8 sheet film. Obviously the coverage is different, but the focal length isn't.

Focal length is the distance from the optical centre of the lens to the film plane when it's focused to infinity. With the simple lenses on a box camera, the optical centre is pretty much in the centre of the lens (unlike complex lenses) so I image if the boxes are different sizes, it's because they have different focal lengths.

 

[StephenM]

In theory, a box camera will be focused just short of infinity - but not enough to make a great deal of difference to the lens to film distance if you're measuring with a tape measure. Given that the lens will probably be a simple meniscus, to a reasonable approximation the focal length will be the lens to film plane distance.

 

[steveo_mcg]

Cheers guys, I wasn't sure if the strength of the meniscus lens would be a factor.

 

[abdoujaparov]

Cheers guys, I wasn't sure if the strength of the meniscus lens would be a factor.

It is a factor - the focal length is determined by the geometry and the refraction strength of the glass/plastic - but whatever that results in, they'll still stick the film plane at the the focal length.

Obviously that only applies to simple lenses. I believe that in complex multi-element lenses, the optical centre doesn't even have to be within the lens.

 

[StephenM]

Obviously that only applies to simple lenses. I believe that in complex multi-element lenses, the optical centre doesn't even have to be within the lens.

Correct; but strictly the focal length isn't measured from the optical centre, but the rear principle plane. For what it's worth, that's what makes a retofocus lens a retrofocus rather than a simple wide angle - the rear nodal point (note the switch in terminology) is behind the last element of the lens. And a telephoto follows the same idea in reverse (making the difference between telephoto and long focus. Matters more to LF photographers.

 

[steveo_mcg]

It is a factor - the focal length is determined by the geometry and the refraction strength of the glass/plastic - but whatever that results in, they'll still stick the film plane at the the focal length.

Obviously that only applies to simple lenses. I believe that in complex multi-element lenses, the optical centre doesn't even have to be within the lens.

Ah cool, gotcha now.