Thread: DIY: tape measure for easy bellows extension calculations

1. Re: DIY: tape measure for easy bellows extension calculations

I also made up a chart to carry in my field notebook, all the calculations for each of my lenses out to 2-stops with corrections in both extension factors and EV. The chart makes visible all corrections finer than third-stops. I focused each of my lenses at infinity to find out where on the lens to measure to from the film plane, and I discovered that all my Super Angulons indicated the mark to be at the front surface of the lensboard, and all the Symmars showed the mark to be at the aperture ring - convenient. For the tape measure, I bought a small spring loaded Lufkin metal tape, in millimeters, and it's easily found in a pocket when needed.

2. Re: DIY: tape measure for easy bellows extension calculations

What I do for bellows extension is have a small card that lists the actual diameter of each of the lens f-stops in millimeters. This can be calculated by dividing the rated focal length by each focal number (i.e. for a 300mm lens at f/8, 300/8=37.5mm), and only needs to be calculated once, ahead of time.

Once out in the field, I can use a tape measure to get the actual focal length with the bellows extended, divide by the f/stop diameter referenced on the card to get the actual focal ratio already compensated for bellows extension.

~Joe

3. Re: DIY: tape measure for easy bellows extension calculations

Idea borrowed from Maris Rusis:

Using a black indelible marker, write a ruler scale (inches or cms) on the white part of your dark cloth.
That way it's always handy and it's one less thing to carry (or forget).

I just mark off whole inches, I don't bother with 1/2" or 1/4" increments.
If I'm shooting with a 12" lens and my bellows is extended 16 or 17 inches, I'll round up and add a half-stop.

4. Re: DIY: tape measure for easy bellows extension calculations

Have a look at the through the lens meter on this site, all your bellows factors will be resolved at a stroke

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5. Re: DIY: tape measure for easy bellows extension calculations

Originally Posted by Ari
Idea borrowed from Maris Rusis:

Using a black indelible marker, write a ruler scale (inches or cms) on the white part of your dark cloth.
That way it's always handy and it's one less thing to carry (or forget).

I just mark off whole inches, I don't bother with 1/2" or 1/4" increments.
If I'm shooting with a 12" lens and my bellows is extended 16 or 17 inches, I'll round up and add a half-stop.
Oh! That's handy! I might have to do that! And maybe write some other important info on there as well!

I've always just used my body for rough measurements. My right middle finger nail is a half inch wide. My first two fingers at the largest knuckle is 1.5". My first three fingers are 2". Both of my feet are literally a foot (without shoes). From finger tip to finger tip with one hand rolled down to the second set of knuckles is 6', etc.

You'd be surprised at how handy that stuff comes in. Just take some time to make a few measurements using your body, and you've got some fairly accurate rulers with you at all times.

6. Re: DIY: tape measure for easy bellows extension calculations

Originally Posted by AtlantaTerry
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.

Actually, there is. At the bottom of both 'rulers' there is a horizontal band which has the word "Stops" at the left. As you look to the right, you will see the 'hieroglyphs' faintly overlaid with the stop compensation which corresponds to it. For instance, the horizontal diamond areas are +.5 stop compensation. The vertical bar area are +1 stop compensation. The width of each area varies, of course, by focal length of the lens.

I've used this for a while (when I remember that I have to compensate) and have found it a fairly easy to use and practical tool. Although admittedly not as accurate as a 'to the mm measurement and associated detailed table' solution.
Regards, Jim

7. Re: DIY: tape measure for easy bellows extension calculations

No matter what you do, you have to make a scale for YOUR lenses. Many short and long focal length lenses have flange focal lengths completely different than their optical focal lengths. This throws standard calculations completely out of whack. If I'm using a Fujinon T 600mm with a flange focal length of 374mm or a Minolta 12.5mm with a flange focal length of -7mm, an unadjusted tape measure -- used by itself -- will be inaccurate for many lenses. Run the calculations yourself, for your lenses -- adjusting for the flange focal lengths -- and produce your own tape measures.

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