...or just measure the bellows extension and the diameter of the aperture through the front element, then divide the former by the latter. This gives you the actual f/stop at the externsion and aperture your camera and lens are set for.
There are quite a few simple methods, but the books all tell you to divide the square of this by the square of that, which seems unnecessarily complex.
The real problem with digital photography is that now I'm the only one who stinks of fixer...
Since this is a sticky thread and since I do not see my particular solution (which I thought was standard procedure...), here goes.
1. Figure out bellows extension factors for all your lenses for a set of exposure factors (I use 1.5, 2, 3, 4, 6, 8, 12, 16, and 20, dropping the higher ones as the bellows length is exceeded).
2. Make a chart and carry it with you in the field, along with a small tape measure.
3. When you need to apply a bellows extension factor, measure your bellows extension, look up the factor in the chart for the lens you are using and apply it as you would a filter factor.
Voilá! Done and no fussy calculations needed in the field.
FWIW, I'm attaching the bellows extension factor tables for my lenses for up max bellows extension 16 in/400mm in both inches and mm. Maybe they will be of use to someone.
i have a different question about bellows extension. how do you calculate the extension needed to focus at a certain minimum distance? if i had a 240mm lens, how much extension would i need to focus .7m away, for instance?
If all this hasn’t made you want to go back to 35mm lets now start on the Scheimpflug Rule.
That's just so fun to say isn't it? Scheimpflug, Scheimpflug.
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Bellows Draw for a Given Diaphragm-to-Object Distance
If you could restate your question then it then it can be answered after a fashion.
In a second semester university physics course the material takes a brief look at optics. It doesn’t go into great depth and doesn’t consider more than 2-element systems. Most of the section on optics involves a theoretical “thin” lens that is thin relative to its diameter and is usually a single-element double convex lens like you’d see in a magnifying glass with each surface the same spherical radius.
I seem to recall that for the purpose of some calculations a symmetric compound-element lens is roughly equivalent to a thin double convex lens whose single element is centered at the plane of the diaphragm. With this simplifying idea it’s possible to calculate something similar to what you asked.
The Thin Lens Formula is:
1/f = 1/p + 1/i
where f = focal length of the lens, p = distance from the object to the center of the lens, and i = the distance from the image to the center of the lens.
1/i = 1/f -1/p = (p-f)/fp
i = fp/(p-f)
If by “0.7 meters away” you mean 0.7 m from the object to the plane of the diaphragm, then p = 700mm. You said that f = 240mm, so now we can calculate i, the distance from the film plane to the plane of the diaphragm.
i = 700mm*240mm/(700mm-240mm) = 365.22 mm.
To test the calculated prediction I used a 240mm f/5.6 Rodagon enlarging lens to project the image of the lit filament of a 60-watt light bulb onto a piece of white mat board. The lens was placed with the plane of the diaphragm approximately 700mm from the center of the bulb. I measured the distance from the plane of the diaphragm to the mat board where the position of the board gave the sharpest image of the filament. It’s somewhere between 355mm and 375mm so that suggests that the calculated value is close to the actual diaphragm-to-image distance. With a proper optical bench I could have refined the measurement.
Very interesting thread this.
What if you are using a 35mm camera with a zoom lens for metering. Is it not so that the bellows factor is "factored" in when using an SLR reading? A zoom lense has variable max f-stop from min to max. So if I use a zoom lens to get about the same size as on the ground glass, will the metered value be usable on the folding camera?
Or am I as per usual completely off?
That the field of view is the same, doesn't mean that the bellows factor is taken into account. The lens designs are very different, at least if we are comparing typical 35mm lenses to LF lenses, so lens-to-film distance is very different, and falloff can be very different too. There are Scheimpflug issues, differences in lens coatings and contrast etc. etc.
Originally Posted by shimoda
In the limit of infinity focus, yes, you can get a good idea from 35mm metering, but for anything involving substantial bellows factor or movements etc.... usually not wise.
If you are using a 35mm camera to meter, it's best to take that meter reading and then apply the bellows factor to that reading. I do this quite often for landscapes... again, for situations with large bellows factor, beware! The 35mm cameras have the advantage of spot as well as area and average and matrix metering, which all have their plusses and minuses. Plus, if you use a digicam to meter, you can also get colour temp and scene brightness range (a histogram) and a "proof" image too.
Page 36 of my 1981 Kodak Master Photo Guide has the same friggin cool bellows compensation factor gizmo that is exactly like Jason Brunners. All you need is a two inch target to place on the subject, then measure the GG with the factor scale in the guide, and presto, instant extension factor.
This is a great little 5 page PDF, has an explanation of movements but has a nice chart and explanation of bellows reciprocity failure and how to compute it. Not mine, just thought I would share.
"A picture is a poem without words"