DIY: tape measure for easy bellows extension calculations

[AtlantaTerry]

I was just sitting here at 5am on a Sunday morning thinking about an easy way to calculate bellows extension factors and thought of something.

Maybe it has been done before or maybe it hasn't.

I have seen some inexpensive small spring-loaded tape measures for sale in fabric stores, craft stores, etc. Unlike hardware store tape measures these can be only 4 or 5 feet long.

OK, so what if I bought a tape measure for each of my lenses where I would need to know bellows extension? Let's say for example, my 300mm f/5.6 Rodenstock Sironar-N that I use for portraits. I would certainly be extending that lens out in order to photograph someone 8 or so feet away therefore I would need to know the bellows factor.

It would seem to me to be very handy if I could simply place the hook of the tape measure on the film plane then pull it toward the lens. Then if I had already marked the bellows factor on the tape using a black Sharpie pen, I could adjust the shutter speed to make up for light loss.

A second idea which is far lighter weight and cheaper would be to use some cloth gift wrap ribbon then note the shutter speed changes needed on it. I could glue a loop on one end that would attach to the back of my Cambo then when I had roughed in the portrait, I could lift the ribbon up to the front standard and quickly know the light loss.

How fine should I make the correction marks? One second increments, half second, etc.?

Does this make any sense to those assembled here today?

Thanks,
Terry

Golly, I've got to get some sleep, I need to be at church at 7:45 to warm up for choir.

[rbultman]

This sounds like the exposure compensation scale on the RB67. Great idea.

[cowanw]

It would be hard to do it in seconds, as the lighting and aperture would have to be consistant.
I would think 1/2 or 1/3 stop intervals would be fine. You can likely estimate smaller intervals between your marks.

[AtlantaTerry]

It would be hard to do it in seconds, as the lighting and aperture would have to be consistant.
I would think 1/2 or 1/3 stop intervals would be fine. You can likely estimate smaller intervals between your marks.

Yes, you are right. But I was not thinking of actual seconds but more about how much to increase the time.

Let's say for example that my light meter said the exposure should be 1/30th at f/8. But the lens was extended so much that I needed to double the amount of light hitting the film. So the tape measure or ribbon might read "2X" which would tell me to use a shutter speed of 1/8th.

Or let's say the tape measure or ribbon indicated "2.5X". I could over compensate with the shutter speed by going to 1/4th which would be 3X but then stop down 1/2 of an f/stop to f/8½ which would bring me back into line.

Besides, for most B&W or color negative film, I don't think being a half stop off either way isn't going to be the end of the world. If I were using transparency film, that would be a different story.

›

Choir went fine, we sounded good. But now I'm sitting here yawning. I may need to take a nap.

[Jac@stafford.net]

I like the idea of using a metric stick along the bellows. Several approaches are outlined on the LF main page article here.

A clever, simple device was worked out by Philipp Salzgeber. See: http://www.salzgeber.at/disc/index.html

And this is cool, too.

[AtlantaTerry]

I like the idea of using a metric stick along the bellows. Several approaches are outlined on the LF main page article here.

A clever, simple device was worked out by Philipp Salzgeber. See: http://www.salzgeber.at/disc/index.html

And this is cool, too.

I have known about Phillip's bellows factor tool for many years. But it's designed for close-up photography, so would not work well for portraits when I use my 300mm Rodenstock Sironar-N. That's what got me thinking about a simple alternate.

Thank you for the link to the Cooksey-Talbott tool. It is exactly what I had in mind. What bothers me about his website is there is no explanation of the Egyptian hieroglyphs used in his ruler design.

[Daniel Stone]

You can use those bellows-extension factor rulers for general shooting as well. I used one a few months ago when doing a series of 4x5 shots of a friend and his wife, and it allowed me to dial in bellows extension perfectly. Approximate distance of camera to subject was 6 feet(using 350mm lens on 5x7).

Just sayin

[Joe Smigiel]

I have seen some inexpensive small spring-loaded tape measures for sale in fabric stores, craft stores, etc. Unlike hardware store tape measures these can be only 4 or 5 feet long.

I use one of the loose rubber-coated fabric rulers found in such fabric stores. They are large enough to not get lost easily but easy to stuff in a pocket and carry about. (A small Velcro square would allow hooking the tape end to the camera if desired though I think such an arrangement might generate more problems than it solves.)

As far as determining what to do with the measurements it makes when figuring bellows extension, I just mentally do the distance as f-stop trick to figure out the exposure compensation for any lens. That has been discussed in detail on the forum but basically you think of the measured extension as an f-stop to determine compensation. For example, an 8" lens extended out 11" gives a one-stop loss in light (analogous to f/8 vs. f/11). At 16" the compensation would need to be 2-stops and so on. I just estimate the in-between extensions visually.

If you actually want to calculate the value, the key multiplier to remember is 1.4 (or √2) for any lens. For example, 8" x 1.4 gives 11.2" as the one-stop loss point of extension. Likewise, 150mm lens x 1.4 gives 210mm as the one-stop loss point. For that matter, 4 finger-widths at infinity x 1.4 gives 5.6 finger-widths as the one-stop extension loss point, etc.

Here's an extension table I made for reference that is also easy to carry:

[AtlantaTerry]

Here's an extension table I made for reference that is also easy to carry:

Thank you.

[Paul Metcalf]

I pretty much use Joe S's approach, but only when I remember to consider the bellows extension factor!