1:1 on 8x10 bellows length question?

[David Brunell]

I have a relatively simple question. I know on a 4x5 it is recommended that the bellows length = twice the lens focal length for a 1:1 reproduction with a standard lens (not telephoto design) May I follow this rule for 8x10 as well? i.e. for 1:1 on 8x10 with a 12" lens I need at least 24" of extension. Furthermore, is there a formula I can follow that will provide me with a rough estimate of the closest focus distance (in feet) with a known lens length and a known bellows extension length combination?

[Leigh]

The rule has nothing to do with the lens or the format.

For any lens, a 1:1 ratio requires that the distance from the lens to the film = 2x the lens focal length.

Please note that this is an approximation.

More accurately, a lens must be advanced from its infinity focus position by a distance equal to its optical focal length.
The infinity focus position is referred to as the "flange focal length" of the lens, and may differ from its optical FL.

The error is not large for most lenses, the exception being telephoto designs.

- Leigh

[David Brunell]

The rule has nothing to do with the lens or the format.

For any lens, a 1:1 ratio requires that the distance from the lens to the film = 2x the lens focal length.

Please note that this is an approximation.

More accurately, a lens must be advanced from its infinity focus position by a distance equal to its optical focal length.
The infinity focus position is referred to as the "flange focal length" of the lens, and may differ from its optical FL.

The error is not large for most lenses, the exception being telephoto designs.

- Leigh

Thanks Leigh, I confused the infinity focal position into the equation. So, it is dependent on the lens infinity focal distance which varies from lens to lens.

[Leigh]

Hi David,

If you're just selecting a camera to work with a lens, or the longest lens that will work on a particular camera, you can
use 2x the Optical Focal Length (OFL). That will give you a slight safety margin, since the FFL will work in your favor.

The Flange Focal Length (FFL) is the distance from the front of the film to the front of the lensboard when the camera
is focused at infinity. With short lenses, FFL is usually > OFL, while with long lenses FFL < OFL.

Obviously when actually taking a shot, you go by the image on the GG rather than the calculations.

- Leigh

[ic-racer]

If you are doing 1:1, a 4x5-coverage lens will suffice and only require 300mm of bellows. In fact, if you like the angle of view of a 300mm lens on 8x10, a 150mm lens at 1:1 will be identical.