Bellows Extension

[bill_rosser]

I have a Mamiya C220-F Professional. There is a scale indicating 1x. 1.5x, 2x on the left side of the camera. It is obviously a table by which to increase exposure given a certain distance between the lens and film planes. My question is, "Does 1.5X mean to multipy the time by that numer, multipy the ASA by that number, increase the exposure by the square root of 2 (rounded off) or what? How best to use that calculation will be much appreciated. Thank you.

[mark_hubbard]

"Compensate the exposure after reading the figure indicated by the pointer while focusing. For instance, assuming that the correct exposure value measured by an exposure meter is 1/125 sec. at f/11, compensate the exposure as follows: If the pointer indicates 2, 1/125 sec., f/8. If the pointer indicates 3, 1/125 sec., between f/8 and f/5.6." (So says the C330f manual.) Anyone want to translate that for Bill and me?

[paul___]

The C330 manual is telling you how to compensate using the aperture. Perhaps its easier to understand if you compensate using the shutter speed. If the scale indicates 2 and the meter reading is 1/125 at f/11, you can divide the denominator of the shutter speed (125) by the scale indicator (2) to get roughly 1/60 at f/11. That is the same light exposure as the 1/125 at f/8 they calculate. If the scale indicates 3, then divide 125 by 3 to get roughly 1/45 at f/11. Which is also the same light exposure as the 1/125 between f/8 and f/5.6 they calculate.

 

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Looking at it from a "light" point of view; take the number on the scale and divide it by 2. That is the amount of light, in stops, that is being lost due to the bellows extension:

 

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1 = 0.5 stop loss

2 = 1.0 stop loss

3 = 1.5 stop loss

 

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I hope this helps.

[mel_brown1]

Bill, my older C-33 has no such convenient scale, but I can give you a couple of simple formulas that will work with ANY camera/lens configuration.

 

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The first formula establishes the magnification factor: m=ed/fl (where m=magnification, ed=lens extension distance from infinity focus, and fl=focal length).

 

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Magnification (m) may or may not be useful to you, but it ties right in to the next formula: eif=(m + 1)^2, where eif=exposure increase factor.

 

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Let's conjure up an example, using an 80mm lens. To start, focus the lens at infinity. You will probably notice that the lens standard is up against the camera body. That's the "zero" reference point.

 

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Now, extend the lens to focus on an object some 30cm (about 1ft.) away. Measure the lens extension distance. It should be about 40mm. That establishes the magnification (m=ed/fl) as 40/80, or 0.5.

 

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Crank that 0.5 number through the second formula [eif=(m+1)^2], and you get: eif=(0.5+1)^2, or 1.5^2, or 2.25. That tells you to use 2.25 times more exposure than if you were focusing at infinity. Just a bit more than one step of added exposure.

 

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That should get you right on target, but if you really want to know the number of steps of exposure increase needed, compute it with your scientific calculator as: log(eif)/log(2).

[graham_patterson]

I recently updated my Mamiya pages with an exposure compensation table, along with some observations about the scale versus the auto compensation system. This was the result of some correspondence with Joachim Hein, who did most of the math (thanks!). The scale compensation on the C220s and the pre-C33 errs up to half a stop over.

http://www.btinternet.com/~g.a.patterson/m_faq.htm

[jim_chow]

I read something on bellows extension that is far simpler. Take the

example above where one uses an 80mm lens. Focus on the subject. Say

there is 40mm extension. Then, in cm, the original focal length is 8cm

while with the extension, it's 12cm. The difference between f8 and f12

is a little over 1 stop, so it requires one stop compensation.

[gordon_vickrey1]

I wrote the following response but decided against posting it because Bill Rosser's original question had already been well answered, and because this particular item might be a bit far removed from that original question. But since the previous poster already "brung it up" (in the immortal words of John Travolta), it might be helpful to flesh out this topic just a bit.

 

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Calculators are not needed for determining compensation for bellows extension. A faster, simpler method is simply to read your aperture scale, which has the powers of the square root of 2 built in. Using an example from an earlier post: You have an 80mm lens. Think: "f8." You have added 40mm extension and so are shooting at 120mm. Think: "f12." the difference between f8 and f12 is the amount of compensation you need for this shot, so just read that difference right off your aperture ring: slightly less than 1 1/3 stops. And if your lens's focal length more easily lends itself to using inches as the unit of description, well, that works just fine, too. Compensation for a 6 inch (150mm) lens focused at 12 inches (300mm) can be read as the difference between f6 and f12 (using inches), or as the difference between f15 and f30 using mm). It's two stops in either case, and getting it that way sure beats standing there poking your calculator.

[joachim]

For the conversion of exposure factors to f-stop you have to use logarithms.

 

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A = log(V)/log(2)

 

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where I denote with V the exposure factor and by A the number of F stops. Just by accident Pauls formular works for V=2 and V=4 and is approximately correct in between. For V=1 it is however off by 0.5 stops.

 

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All the formulars discussed on this side, as well as those I used for the tables in Grahams web page and the exposure compensation scales of the C330 neglect the pupil magnifcation of the lenses. This should be a reasonable approximation for standard lenses, but can be severly off for wide and tele lenses.

The pupil magnification is discussed e.g. in http://www.photo.net/photo/optics/lensTutorial.html.

 

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When measuring the bellows extension at a Mamiya TLR, please notice that you have to extend the bellows allready to reach the infinity.