Mechanics: Measuring Bellows Extension - with a ruler?

[JWMster]

So I've been reading up on this and want to make certain I understand that the key measurement is from:

1) From: Film plane (i.e. the back of the back standard)
2) To: Lens Plane. This will either be marked on the lens itself or assumed to be the back of the front standard.

Initially I thought we were actually measuring the relative distance from a lens's point of infinity focus to it's current location where the lens is focused for the specific shot, but this is inherent in the calculation.

My camera has a geared monorail, but no measuring marks. I assume we do this with a measuring tape or something. I did see somewhere that someone had come up with strings for different lenses and put marks on the string at different F-stops... which sounds clever but would likely end up lost at the bottom of my bag. I found the "Art of Foto" iPhone app which looks handy.

 

[BradS]

The two key distances are the image distance and the object distance.
The object distance is the distance from the lens to the plane of focus - the thing you are focused on
the image distance is, in simplified terms, the distance from the lens to the film.
So, to answer your specific questions...yes, close but you might be able to do better
I usually use the front surface of the lens board as a rough approximation of the lens plane (this is coincident with the flange which can be an interesting reference plane) and take the ground glass as the film plane. The film plane is actually, exactly where the film is but for practical purposes, the thickness of the ground glass hardly matters.

The important thing is to be consistent. So, if you focus at infinity and measure the distance from lens to film - using the lens board and ground glass surfaces as approximates of lens plane and film plane respectively then use those same surfaces when you re-focus on the close thing (I'm assuming you're trying to calculate bellow factor for example) then, the calculations will work out to be close enough.

oh, and yeah. I use a flexible plasticized cloth (?) measuring tape from the fabric store. It has both metric and old fashioned inches units of measure which is handy. The cloth means it light and will not damage anything in the camera bag.

 

[BrianShaw]

I do exactly as BradS, including the tape measure.

 

[Dan Fromm]

Extension is measured from the film plane to the lens' rear node. The rear node is invisible. However, for most LF lenses -- telephotos are the major exception -- the rear node is near the diaphragm. I use a tape measure to measure film plane-to-diaphragm distance when I have to adjust for magnification. Not perfect but good enough.

For me teles aren't a problem, I have only one LF tele and I don't use it for closeup work. If you have an LF tele and want to measure extension when using it, focus it at infinity. At infinity its rear node will be one focal length away from the film plane. If you're lucky it will be inside the lens' barrel and you can mark its location on the barrel with, e.g., a piece of tape. If it is in front of the barrel, record how far in front of the barrel it is. Or don't use it for closeup work and avoid the problem.

 

[Ian Grant]

As a rule of thumb most LF cameras have either Double or Triple extension, That's 2x or 3x the typical normal standard lens for the format, So A Wista 45DX, Speed, Pcemaker, Super, Graphics and similar have 12"/300mm maximum extension, probably DX means Double extension in the case of the Wista. It's assumed the common normal lens for a 5x4 is 150mm/6".

My MPP MicroTechnicals are Triple extension so half as much again 18"/450mm maximum and Linhofs are the same. It's similar with my 10x8 Agfa Anscos where 300mm/12" is the typical standard lens mu Commercial View has 36½" extension while the Universal View is 26½" that's just over Triple and Double. In fact the Double or Triple extension goes back to early field cameras and applies to all my wood and brass cameras Quarter plate to 12"x10"

Some cameras have scales for accurate bellows extension measurement, my Half Plate (7x5) Kodak Specialist 2 has an accurately engraved set of scales including the extension rail, surprisingly it's metric (late 1940's and only made & sold in the UK, The scale is great because there's also a chart for extension factor on the side of the camera.

On my Wista I marked part of the brass work with a fine indelible black pen (Sharpie in US) to indicate extension factors with my standard lens and they are still there over 30 years later D I

Ian

 

[jim10219]

I usually do the front of the front lens board to the back of the rear lens board. It's not 100% accurate, but for things like calculating the bellows extension factor, it works well enough. Fabric measuring tapes are the easiest to use, but I've also been known to section off parts of my dark cloth fabric to get a ratio out in the field. Or I'll even use my body to get a rough measurement if I need to (index and middle width are 1.5" wide at index's first knuckle. Index, middle and ring are 2" at ring's fingernail base, and most running shoes are exactly 12" on me, index finger is 15mm wide at base of nail. etc.).

 

[wiltw]

For what purpose do you want to measure the bellows extension distance? Is it to calculate magnification? to calculate exposure factor?

Sometimes the most pragmatic is to simply measure on the groundglass the image that you see, and compare that to the known actual measurement of the object being photographed. Depending on the 'why', that might not be adequate for your purpose.

 

[JWMster]

My purpose is to calculate the exposure adjustment for the bellows. Yes, I'm told you can measure the magnification of the object on the ground glass as opposed to the bellows distance. However, there are times when comparison of the size at infinity focus vs. current distance might not be practical. My understanding could be wrong, but it seems measuring the distance from lens plane to film plane is a lot more objective. THanks for your interest

 

[BrianShaw]

Calumet, when they were a viable business, sold a ruler and chip accessory for measuring magnification on ground glass. Very useful. Not sure where I saw it but I recall downloading similar from somewhere on the internet at one time.

 

[Bob S]

Just use the Rodenstock Pocket DOF/Scheimpflug calculator.
It computes image ration, exposure compensation, tilt angles, etc for all formats from 35mm to 810 directly off the gg and tells you where to position the film standard, and fits your shirt pocket. Eliminates any need for tape measures.

 

[BradS]

My purpose is to calculate the exposure adjustment for the bellows. Yes, I'm told you can measure the magnification of the object on the ground glass as opposed to the bellows distance. However, there are times when comparison of the size at infinity focus vs. current distance might not be practical. My understanding could be wrong, but it seems measuring the distance from lens plane to film plane is a lot more objective. THanks for your interest

I've always found it pretty easy to measure the distance from lens board to ground glass and do the arithmetic...

bellows factor = (image distance / focal length) ^2

 

[wiltw]

The convention is to merely measure between the front surface of the lens board and the focal plane.

 

[Doremus Scudder]

I made a table with all my lenses. I carry it in my field notebook along with a small tape measure. When needed, I measure the bellows draw (from film plane to center of lens or as close I can estimate) and consult the table. EZPZ

I've posted my table either here or over on the LF forum if anyone is interested.

Doremus