Bellows extension for a 180mm macro lens

[felix5616]

I have a rodenstock 180mm apo macro lens for up to 2:1 reproduction range. How do i calculate how much bellows extension i need to get 2:1 magnification? i tried used an ebony non folding 4x5 but can seem to get enough bellows extension for 2:1 magnification so i thought i'd get a monorail 4x5 for studion use.
thanks

[Joanna Carter]

To get a 1:1 reproduction, you would need 360mm of extension. To the best of my knowledge, you would need around 4x the focal length of the lens to get 2:1. That would mean a bellows extension of 720mm with a 180mm lens. Apart from a monorail with extensions, I don't know of a field camera that would give that length.

[Dan Fromm]

The magic formula is: extension (from the infinity position) = f*(1 + m)

where f = the lens focal length, in your case 180 mm, and m = magnification, in your case 2.0

[Greg Lockrey]

The magic formula is: extension (from the infinity position) = f*(1 + m)

where f = the lens focal length, in your case 180 mm, and m = magnification, in your case 2.0

And your exposure factor is 8.

[Bob Salomon]

To get a 1:1 reproduction, you would need 360mm of extension. To the best of my knowledge, you would need around 4x the focal length of the lens to get 2:1. That would mean a bellows extension of 720mm with a 180mm lens. Apart from a monorail with extensions, I don't know of a field camera that would give that length.

Wista wood or technical cameras with the longest extension bed and bellows.

[felix5616]

thanks all, much appreciated.

[aduncanson]

Dan gave the applicable formula, but interpreted it incorrectly. f*(1+m) gives the image distance, not the extension from the infinity focus position. Consider the following familiar situations:

At infinity m = 0 and image distance equals f.
At life size m = 1 and image distance equals 2*f.

Unfortunately image distance is measured from the rear nodal point of the lens and since that location is not generally known, image distance is not as easy to use as the change from the infinity position is. The calculated change from the infinity position will be correct no matter the details of your lens design or camera configuration.

For the change from the infinity position use simply f*m, so for your 2:1 reproduction you would need 360mm more belows (or rail) than you use at infinity.

Good luck

[Greg Lockrey]

Dan gave the applicable formula, but interpreted it incorrectly. f*(1+m) gives the image distance, not the extension from the infinity focus position. Consider the following familiar situations:

At infinity m = 0 and image distance equals f.
At life size m = 1 and image distance equals 2*f.

Unfortunately image distance is measured from the rear nodal point of the lens and since that location is not generally known, image distance is not as easy to use as the change from the infinity position is. The calculated change from the infinity position will be correct no matter the details of your lens design or camera configuration.

For the change from the infinity position use simply f*m, so for your 2:1 reproduction you would need 360mm more belows (or rail) than you use at infinity.

Good luck

The formula is correct.... his description of "Extension (focus at infinity) " should have read "Extension (at the point of focus)". I have an old Kodak table from the handbook "Kodak Master Photoguide" and his formula works. The 180mm lens will need to be racked out to 540mm which is what both of you are saying in essence.

[Dan Fromm]

Alan, thanks for the correction. I gave the distance for the distance from rear node to film. I've got to drink more coffee before posting in the morning.

Re finding the rear node, for most of the lenses we use -- not telephotos, not retrofocus -- it is close to the diaphragm. Using the diaphragm's position or even the front of the lens board is usually a safe approximation.

[Leigh]

The magic formula is: extension (from the infinity position) = f*(1 + m)

That equation gives the distance from the rear lens node to the image, not the delta from the infinity-focus position.

Given that the rear lens node can be anywhere, inside or outside of the physical lens, the equation is only of academic interest. It doesn't tell you how much bellows draw you need.

The error becomes greater as the lens becomes significantly longer or shorter than "normal".

- Leigh