View Full Version : Bellows extension for a 180mm macro lens

felix5616

24-Sep-2011, 12:37

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

24-Sep-2011, 13:40

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

24-Sep-2011, 14:43

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

24-Sep-2011, 16:01

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

24-Sep-2011, 16:05

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

24-Sep-2011, 16:58

thanks all, much appreciated.

aduncanson

25-Sep-2011, 08:50

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

25-Sep-2011, 09:34

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

25-Sep-2011, 10:15

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.

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

Dan Fromm

25-Sep-2011, 12:55

Leigh, I'm sorry that I can't thank you for the correction. Mr. Duncanson beat you to it. So did I.

I'm sorry that you don't agree with me and the lenses' makers that most of the lenses used for LF have nodes very near the diaphragm. In LF, the exceptions are telephoto lenses. For smaller formats, retrofocus lenses are also exceptions. I'm not aware of retrofocus lenses for formats larger than 6x7; if there are any, please tell me about them.

I don't at all agree that knowing where the nodes are is only of academic interest. If one doesn't know internodal distance one can't calculate total extension. This may seem like a trivial quibble, but it makes a difference when designing macro flash rigs.

In addition, as you didn't point out, f*m gives extension from the infinity position.

I'm sorry that you don't agree with me and the lenses' makers that most of the lenses used for LF have nodes very near the diaphragm. In LF, the exceptions are telephoto lenses. For smaller formats, retrofocus lenses are also exceptions. I'm not aware of retrofocus lenses for formats larger than 6x7; if there are any, please tell me about them.

All of the Rodenstock Grandagon-N short FL lenses have Flange Focal Lengths (FFLs) greater than their optical focal lengths. The same is true for comparable offerings from Nikon and Schneider. This information is from the published data sheets, so apparently the manufacturers and I agree. :p

I don't at all agree that knowing where the nodes are is only of academic interest. If one doesn't know internodal distance one can't calculate total extension. This may seem like a trivial quibble, but it makes a difference when designing macro flash rigs.

The nodal points are of academic interest because it's difficult to locate them on a real lens, for the purpose of determining proper lens position for a given magnification. That is after all the subject of this thread.

It's easy to use the FFL because it's a published value based on the lens mounting surface, i.e. the lensboard.

Certainly you can use the more detailed optical design points if you have access to them, and if you're doing a design that requires such information.

In addition, as you didn't point out, f*m gives extension from the infinity position.

There's another thread currently running on this same subject, and I'm getting them confused. http://www.largeformatphotography.info/forum/showthread.php?t=81060

I did make the point about extension from infinity focus position in post #2 of that thread.

- Leigh

Greg Lockrey

25-Sep-2011, 14:28

There's another thread currently running on this same subject, and I'm getting them confused. http://www.largeformatphotography.info/forum/showthread.php?t=81060

I did make the point about extension from infinity focus position in post #2 of that thread.

- Leigh

Learned a lot with this one today.... Thx Leigh

Not to confuse further the Macro180 at 2:1 would be racked out 540 mm from film plane to lens board. To put it another way 360mm from infinity position. As I learned today not all lenses have an infinity positions (FFL or flange focal length) as stated for the focal length. Cool to know....:cool:

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