Calculating lightloss on a cambo 4x5

Measure from the ground glass to the rear element of the lens. Square it (multiply it by itself). Then square the focal length. They both have to be in the same unit of measurement. Both mm or inches. (1 inch = about 25mm). Divide the bellows extension squared by the focal length squared. That gives you an exposure factor. If the focal length is 12 inches, and the extension 17.5 inches, your factor would be about 2. So double the exposure. It does not have to be exact. Do not wrestle with very small differences. For example 12 squared is 144. 17.5 squared is 306. That is close enough to double to use a factor of 2.

Exposure compensation = Total bellows extension - infinity bellows extension divided by bellows extension at infinity. Measure from lens board to ground glass. Example: a 150mm lens focuses infinity at 150mm; you focus the lens on an object and the total bellows extension is 275mm. 275 - 150 = 125, 125 divided by 150 = .833 or 5/6 stop.

The other way that I find easier to understand is: take the size of the object, and the size of its image on the ground-glass. Call that ratio M. Then multiply the exposure-time by (1+M). Eg if you have an object 2cm tall and it appears 4cm tall on the glass, multiply the exposure by 1+4/2 , ie 3x as long.

You'll find variations on this theme at http://www.largeformatphotography.info/bellows-factor.html .

Hi Calumet has a little device just for that problem, and it is under ten bucks! It consist of a little target piece, and a ruler to measure with. You put the target in your picture, and measure it's size on the ground glass. It give you the exact exposure compensation for that shot! Very simple and easy.

I have all of my students get one and they never have exposure problems.

Don Cameron

www.doncameron.com

Think of the bellows length compared to the focal length of the lens as one f-stop compared to another.

In the example given above, using the same units of measurements, with a focal length of 12 inches and a bellows length of 17.5 inches. Compare those two measurements as if you were comparing f-12 with f-17.5. ie: f11-f16, a bit more than a one stop difference in exposure would be indicated

You can use inches, millimeters or centimeters as long as the same units of measurement are used for the bellows length and focal length measurements.

No need to do any division, multiplication or squaring of numbers. And no need to try placing a small target in your scene when there's no room for one or no place for it to be placed.

Just get a ruler and measure.

i second brook's post. thats exatly what i learned.

The very first thing to understand: exposure compensation for bellows extension is only needed for closeups. If the subject that you are focused on is more distant than ten times the focal length of the lens, than no compensation is needed.

Here's a way to use google, which provides advanced calculator functions, to figure the bellows compensation. Obviously you can't do this in the field, but it is a simple matter to prepare a table in advance, for example, listing the correction for every extension in 1 cm increments for each of your lenses.

Simply enter into the search field of google "6.64 * log (B/F)", replacing B with the numeric value of the bellows extension and F with the value of the focal length. Both have to be in the same units. Using Bruce's example of B=17.5 inches and F=12 inches, enter into google

6.64 * log (17.5 / 12)

You will get back: 6.64 * log(17.5 / 12) = 1.08800917.

Rounding, this means that the exposure correction is 1.1 stops, or rounding more, 1 stop, which is the factor of 2 that Bruce calculated.

May I suggest that you obtain the following book for your reference's Using the view camera!by Steve Simmons Great book and has a lot of info and even how much to correct for different films: I did Great book\\\\\

Tim H gave the correct way to determine what is called the effective f-number, not the time. To get the adjusted time, you have to multiply the original time by the square of the factor described, one plus the magnification. By the way, an example where the image is actually larger than the object would be a rather unusual situation.

There is one additional problem with these methods. If the times are shorter than one second, unless you have an electronic shutter, you have to increase the shutter speed in increments of one stop, which means increasing the time in powers of two. Thus if the ratio of extension to focal length just happens to work out to 1.414, or thereabouts, squaring it will yield 2 and you can just double the time. So if the indicated exposure were 1/16, you would choose 1/8 instead. (Of course, if the time is over one second, you can just count it out or use a timer.)

But if you need to make some other adjustment, you will need to change the f-stop by some amount measured in f-stops or fractions of a stop. To determine how much requires the use of logarithms. Fortunately, Michael Briggs tells you how to do that without knowing anything about the subject.

Even better than Don C.'s suggestion, is if you can get the same thing for free. The QuickDisc provides a target and calibrated measure to give exposure compensation. All you need is a printer.

Link to the QuickDisc

When you can just multiply the f-stop number by the increase in bellows extension, then why would logs, squares or any other mathematical jiggery-pokery be necessary?

To reiterate the method that Brooks outlined. Measure your bellows length with the lens focussed on infinity (say 8"). Measure your bellows at the working distance (say 12"). Divide one by the other (12/8 = 1.5). Multiply your working f-number (say f/22) by the multiplication factor you've just worked out (1.5 x 22 = 33). So, in this case your actual aperture would be f/33. Finally, use your exposure meter dial to work out the correct shutter speed for the new aperture number, and the job is done.

This method is dead accurate (well, as accurate as simply measuring the bellows can be), and there's no squaring, logs, calculator tables or swearing needed.

Good point, Pete. I think for some people it's more about the process than the result. Why use a simple intuitive method when you can do all that fun math and make it seem so much harder than it really is ?

Actually the method I posted is even easier than doing simple division. Just compare the two measurements of focal length and bellows length as f-stops. It's elegantly simple.

Pete,

Let's modify your example a bit. Suppose instead the 'multiplication factor' is 1.2. Multiplying 22 by 1.2 yields 26.4. How do you set the aperture to f/26.4? If your exposure meter dial is like mine, it is marked in thirds of a stop. And you can estimate half a stop pretty well. Is f/26.4 closer to one third of a stop or one half a stop beyond f/22? Applying the formula to the factor 1.2, you come up with about 0.53 stops, which means you should set your f-stop on your meter to halfway between f/22 and f/32.

Also, if you've made the calculation, you can use it as follows. Determine your exposure as you would without accounting for bellows extension. Say in the example, you've decided on f/22 at 1/15 (really 1/16) of a second. Having discovered that the f-stop correction is about 1/2 stop, open up the lens by that amount to compensate for the dimming of the image by bellows extension. Of course, you might then be concerned about the slight decrease in depth of field, but you needn't worry. If you open up by the indicated amount, the effective f-stop for calculating DOF will be your original value, and if you decided that would have been sufficient without accounting for bellows extension, it will be after bellows extension.

In most circumstances being precise in terms of fractions of an f-stop is not important, but occasionally it may be, particularly when using transparency film in a difficult lighting situation. Also, after you have done this a few times, you will quickly learn that certain ratios mean certain numbers of f-stops or fractions thereof. If, as Michael suggests you compile a table and carry it with you, it doesn't take very long to figure the correction. But let me give a table now so everyone who wants it can print it out.

Ratio---------Number of f-stops to open up by

1.0-------------0

1.1-------------0.28

1.2-------------0.53

1.3-------------0.76

1.4-------------1.0

1.5-------------1.17

1.6-------------1.36

1.7-------------1.53

1.8-------------1.7

1.9-------------1.85

2---------------2

Let me add one other advantage of finding the amount to open the lens in terms of f-stops or fractions therof. You don't have to do any multiplication or refer to your exposure meter. In addition, the amount to open up is indepdendent of where you set the aperture, so if you change your mind about the time-aperture combination, you don't have to redo the calculation or look at your meter again. If you carry the table with you, you don't have to remember any rule, no matter how simple and intuitive it may seem to someone.

You do still have to find the ratio, which you can do either by dividing the bellows extension by the focal length or by adding one to the magnification. If you use the Quickdisc, then the calcuations are built into the scale, which simplifies even more, but you have to put something in the scene at the proper location and then remember to remove it again.

One further quibble which I don't think was mentioned. finding the bellows extension when the rear principal point is not near the lensboard can be tricky. One easy way around this is to first focus at infinity and then focus where you want and measure the displacement from infinity along the rail. Divide that by the focal length and add one. That gives you the appropriate ratio.

Myself, I carry a small paper scale marked, for each of my lenses, in the number of f-stops correction needed. It includes a correction for each lens's rear flange focal length, so I just place it between the standards and read off the correction in stops. I don't think anything could be simpler than that, though I admit, it did require some knowledge to construct the scale.

Leonard, the most accurate formulae for DOF automatically take the focussing factor into account, and work out the EFFECTIVE aperture, which has the correction factor built-in. Opening up the aperture again manually will negate the calculated DOF.

And is anyone really that bothered about whether to allow 1/3 or 1/2 of a stop? The difference is 1/6th of a stop - unnoticeable - and most shutters ain't that accurate anyway.

If you end up with a figure of f/27.6, or f/36.4 then it's pretty easy to guess at the interpolated time. If you don't think this is accurate enough, then you can work out your reciprocal powers of 2 and make a little table of 1/3rd stop increments. Whatever floats your boat. But remember, while you're messing about fishing out your table and looking things up, working things out, jiggling the aperture, etc. the light might easily have changed by 1/6th of a stop, or the subject just got up and walked off!

Wow, so many ways to do it.