How much bellow-extension do I need?

[ThePenguin]

Is there some sort of formula to calculate how much bellow-extension you need?

Take this example (on 8x10):

How much do I need the bellows to extend if I'm going to focus on something that is 2m away with a 480mm lens?

Is there a formula to calculate this?

[Greg Lockrey]

FD= (SDxFL)/(SD-FL)

Focal Distance= (Subject distance x Focal Length)/(Subject Distance - Focal Length)

FD= (2mx480mm)/(2m-480mm)=960000mm/1520mm=631.58mm

[IanG]

Camera's were sold as Double or Triple extension, that's bellows extension that extends 2 or 3 times the normal focal lenght for the format, in the case of a 10x8 camera that's a 300mm/12" lens.

Most 10x8 cameras have at least double extension, in the case of an Agfa Ansco 10x8 it's 27.5" 70cm on the more basic models.

Ian

[ThePenguin]

Great, thank you!

[Lynn Jones]

Magnification plus one times focal length = bellows draw.

Lynn

[Emmanuel BIGLER]

... and (inverse magnification plus one) times focal length = distance from camera to subject.

[banjo]

to make it easy if you have 480 mm that infinty no movemunt 480 x 2 is double extension 480 x 3 triple I can't remamber but I thank you need double extension
for a 1 to 1

[ThePenguin]

Allright, but if we do it the other way, how much bellow-extension do I need if I wanna shoot with a 210 on infinity?

[unixrevolution]

Allright, but if we do it the other way, how much bellow-extension do I need if I wanna shoot with a 210 on infinity?

Infinity focus extension = focal length, so 210mm.

[banjo]

yes Infinity focus extension = focal length, so 210mm
but for any thing closer then Infinity is when bellows extension
is needed. so for a 1 to 1 you will need 420mm that would be as
close as about 2'. for a head & shoulder or for focus at 8' you will
need about 260mm