Rodenstock DOF calculator question from LF Newbie

[jmontague]

I picked up a Rodenstock DOF calculator and am having difficulty with step 3, the Scale of Reproduction. How, outside of the studio, do you know the ratio of the actual object to that seen on the GG? How is this done in the field? I get the rest of the steps, but this one has me stumped. Thanks in advance for the help.

[towolf]

You have this: http://www.weschoen.de/Bed.-Anl.%20Rechenscheibe.pdf

But there is also this: http://linhof.com/wp-content/uploads/2015/03/depth-of-field-chart.pdf

On the second page there they also explain how to get magnification factor

http://i.imgur.com/VioA5eT.png

[BetterSense]

That has never been intuitive to me. Why not just the ratio of the focal length to the distance? It seems like similar triangles would apply here. I don't get why you subtract the focal length from the distance before dividing.

[towolf]

If you look at the graphic it becomes clear.

http://www.elmar-baumann.de/fotografie/herleitungen/abbildungen/abbm_g.gif

B/G = b/g

due to geometry of the red triangles.


Now the ray crossing through F creates a similar situation on the image side

B/G = b-f / f

b-f is the right half (distance from the focal point to B) and f is the left half (from the cardinal point / dashed cross to F). Since this is a parallel ray it works from the image side.

The same works from the object side B/G = f / g-f


Here’s the three scenarios:

http://i.imgur.com/eupfHKk.gif

[jmontague]

How much bellows extension must be employed before adjustment of time/f-stop is required for proper exposure? Is it a given that you will be close enough to your subject (field or studio) to measure its size before the bellows extension required for focus is enough to require exposure compensation?

[Bob Salomon]

How much bellows extension must be employed before adjustment of time/f-stop is required for proper exposure? Is it a given that you will be close enough to your subject (field or studio) to measure its size before the bellows extension required for focus is enough to require exposure compensation?

You can read that off the calculator as you adjust its scales.