# LF: Applying Bellows Extention To Exposure

Discussion in 'Exposure Discussion' started by Bruce Osgood, Oct 31, 2004.

1. ### Bruce OsgoodMembership CouncilCouncil

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I'm reading Stroebel's "View Camera Technique" and have come to understandings that I would like to confirm as valid.

When shooting a subject greater than 10X the lens focal length (4x5 135mm = 5.1") a bellows extension factor must be applied to the camera exposure.

EXAMPLE: With a 4x5 camera and a 135mm lens, a subject greater than 50 inches from the camera will need a factor applied to either/or combined shutter speed and apature to achieve expected negative exposure.

BELLOWS FACTOR: From Ole Tjugen (oftjugen@online.no) on June 1, 2002.
Bellows Extension Squared Divided by Lens Length Squared = Bellows factor.

In the above example, a focused image of a subject longer than 51 inches from the film plane with a bellows 6 inches extended would require a bellows factor of 1.44 or one and one-half stop increase [(6X6) 36 / (5X5) 25] = 1.44 or 1-1/2.

In your estimation are these understandings of mine correct?

2. ### Helen BMember

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Bruce,
That looks correct, apart from the 'greater than 50 inches' bit. Shouldn't that be less than 50 inches? The example you give is correct in terms of the exposure factor - but a 5" lens that is 6" from the film plane will be focussed on an object about 36" (or thereabouts) from the film plane, not 51". However, if a 5.3" (135 mm) lens was 6" from the film plane, then the object would be about 52" from the film plane, but the bellows factor would then be 1.28 - not significantly different from 1.44.

As you can see, the 10x rule roughly corresponds to a maximum error of 1/3 stop - a 'standard' limit on error - before applying the bellows factor.

I hope this makes sense.

Best,
Helen

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4. ### markMember

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Brian, how does that thing work?

Bruce, how goes it with the Busch?

5. ### rbarkerMember

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I use the \$10 Calumet Exposure Calculator, #CC9201 , which consists of a small square target that is placed in the scene, and a little ruler used to measure it on the GG. Quick, easy, compact, and works with any format and focal length.

6. ### bmacMember

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It is basically a ruler that has the exposure compensation marked for various lenses. Sine I am now only going to use one lens, I will most likely just mark it on the bed of my camera.

7. ### roteagueMember

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I cheat. I have a meter (Minolta Flash III) that allows me to take readings directly off the ground glass.

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9. ### James BleifusSubscriber

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12. ### Bruce OsgoodMembership CouncilCouncil

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Busch

The Bush is okay. There is significant operator error that needs to be worked out but I will PM you on those operating questions. The S-K Xenar 135 seems to be a very soft contrast lens but I beleave the shutter is 'quite' accurate.

13. ### David A. GoldfarbModeratorStaff MemberModerator

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The method that I use, which is similar to the Calumet device and the QuikDisk, is to estimate or actually measure the magnification factor by comparing the width of the field of view at the plane of focus to the width of the format. If it's a still life or macro I'll put a ruler into the scene (and remove it before taking the photo), and if it's something like a portrait, I usually just estimate it. Then I have a table for converting magnification factor to exposure factor on the back of each camera.

So, for example, if I'm shooting 4x5" in horizontal mode, and the scene is 10" wide at the subject position, then the magnification is 1:2, calling for 1-1/3 stop additional exposure.

An attraction of this method is that it works the same way for any format, unlike the Calumet device and QuikDisk, which are only practical for large format.

15. ### reggieMember

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I was researching this topic on APUG and the internet so I would like to revive it and ask a question regarding the quoted formula. Shouldn't the formula be limited to cases where the bellows extension is greater than the focal length of the lens?

If not, the formula seems to imply that when the bellows extension is less than the focal length, then the exposure factor would result in a decrease in the exposure (that is the factor would be less than 1).

Deal or No Deal?

BTW, some years ago, I was out photographing with <DropName>Cole Weston</DropName> and he had a tiny tape measure and in a small notebook he would look up on a handwritten chart to find the factor for a particular lens. I have looked around for one of these charts, but I couldn't find it. I don't want a Calumet scale because they take up way too much space. I don't want to put ruler of some scale in the image.

I think I'll reproduce one of these charts and paste to a page in my small notebook and then I'll go on a hunt for a tiny tape measure.

Maybe I will post it here if anyone is interested. I will probably cover focal lengths from 240mm to 1000mm at 1/3rd stop increments. Or, could I make a graph with x axis=bellow ext, y axis=factor and each plotted line representing a different lens?

I'm not great at math, but this seems pretty simple.

-R

16. ### agGNOMEMember

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Alternately, you can do all of the math calculations once , and make a scale for each lens out of paper and tape (fancier if you wish) that you attatch to the rail or bed of the camera.

17. ### Bruce OsgoodMembership CouncilCouncil

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Well, what I am doing is carrying a card that I made up for my two lenses based upon the formula that Ole suggested. It seems to work, I have not had negatives using this chart that I would say were a problem.

LENS inch
135 < 4' 5" 6" 7" 8"
factor 0.9 1.3 1.7 2.3
< 4' 11" 12' 13'
factor 4.3 5.1 6
LENS <7' 5" 6" 7" 8"
240 factor .3 .4 .5 .7
<7' 11" 12' 13'
factor 1.4 1.6 1.8

I don't know how this will line up when sending.

18. ### Shawn DoughertyMember

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Keeping bellows extension factor simple

I use the following formula with great success:

"Add 1/3 of a stop for every inch of bellows draw past the focal lenth of the lens."

For example, my 12 inch lens focused at 18 inches would get 2 stops extra exposure. This has worked extremely well for me. Honestly, if you're comfortable trying to be creative and doing mathmatical equations in the field, great. I'm not and this works. Best of luck. Shawn

19. ### Allen FridayMember

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Quick and dirty bellows exstension calculation. Use a tape measure (I keep a cloth tape in my LF case.) Convert the length of your lens to inches, a 150mm lens is 6 ". Measure the distance from the lens board to the film plane in inches, for example 11 inches. Calculate the adjustment by thinking of the two distances in terms of F-stops. In the example, what increase of exposure would you make to go from f6 to f11. (A fuzzy less than 2 stops.)

20. ### Charles WebbMember

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This device has been around since before dirt was invented, is totally fool proof, needs no batteries and you don't need a calculator for doing math. I have seen it in several different forms, put out by Kodak, Ansco and who ever. You cant beat it unless you are metering off the SatinSnow. As for taking up too much space, ahhh a give us a break!!!!!!! The money you save on the card and ruler device will buy several sheets of film.

Charlie.........................

21. ### rbarkerMember

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If the bellows extension is less than the focal length, you're probably focused on a point beyond infinity, where exposure doesn't matter. There's nothing out there. Trust me. We went ther in the '60s.

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The bellows factor is always given by the equation that you quoted. It is just that the factor isn't too significant until your bellows starts getting "long". That is, the factor doesn't amount to much until the plane of focus is closer than about five or ten focal lengths from the lens.

To apply the equation, measure the distance from the lens board to the film plane. This is the bellows extention. Divide that distance by the focal length (make sure use use the same units on top and bottom) and square the result.

So, for example, if you have a 6 inch lens and you're focussed on something such that you have 10 inches of bellows, then the bellows factor is:

(10*10) / (6*6) == (10/6)^2 = 100 / 36 = 2.77777

to convert any factor (bellows or, filter) to stops, take the log of the factor and divide by the log of 2 so, continuing with the above example,

log(2.777777) / log (2) = 1.47 stops.

make sense?

23. ### reggieMember

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So you advocate using the magnification method with the tool sold by Calumet? I think Shawn's simple method that would use a cloth measuring tape and the rule of thumb of his much simpler and takes fewer steps. I will do some math to validate the rule of thumb and simply use that when I need to.

"Add 1/3 of a stop for every inch of bellows draw past the focal lenth of the lens."

Seems pretty simple to me.

-R

24. ### Donald MillerMember

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Simple is not always accurate and in the grossly over generalized method that you describe, the equation is grossly inaccurate in addition to be overly generalized.

25. ### reggieMember

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Like I said: "I will do some math to validate the rule of thumb ..."

If the math shows it not accurate enough, then I'll use another method. But thanks for the useful sermon...

-R

26. ### Jon ShiuSubscriber

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Isn't the rule of thumb: add one stop for every 50% greater than focal length? Measure the bellows extension. For example a 150mm/6in lens and if you measure the bellows at 9 inches = 1 stop more. Bellows at 12 inches = add 2 stops.

Jon