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# Thread: LF: Applying Bellows Extention To Exposure

1. Since I attended school decades before calculators became popular, I can still handle very elementary math. A small tape rule, preferably one with a metric scale, is the universal bellows extension compensation tool. When the lens is stopped down to the taking aperture, measure its entrance pupil and the film to lens distance. Divide the former into the latter to get the actual f/number.

2. Attached is a .bmp file containing the chart, focal lengths from 240mm thru 645mm. I printed out a nice small chart in Courier New font size 8 and it will fit perfectly in my notebook.

I am also attaching a line graph of the chart for those that prefer them.

If you want something made up for your series of lenses that you use on a particular camera, let me know. I need to know the total bellows extension for the camera and the FL of each lens.

So long as the requests don't get too big, I'll make up a chart or graph for your lenses\camera.

Thanks.

-R

3. Here is the line Graph.

4. I wrote a spreadsheet to help me with the bellows ext factor. I can't attach it because it's in Excel. I could send it to you and see what you think, or I can post it to my website. I'm curious if it works. pm me if you want.

-Dorothy

5. Originally Posted by djkloss
I wrote a spreadsheet to help me with the bellows ext factor. or I can post it to my website.
-Dorothy
oops...
http://eriephoto.com

6. Since I've been quoted here (I just now saw this thread), I would like to say that I have three good ways of determining bellows compensation (from simple to accurate):

1: Measure the extension at infinity focus using arbitrary units - finger lengths, finger widths or similar. Then measure extension when focused close, using the same units of measure. Convert the "measurements" to values similar to the f-stop sequence. A simple case could be a 180mm lens, which will have infinity focus at about 1 3/4 finger lengths or eight finger widths (my fingers). If close focus is at 11 finger widths, compensation is one stop (difference between 8 and 11, finger lengths are too difficult here but work great with a 210mm).

2: Replace arbitrary units with cm, and do the same.

3: Replace cm with mm using a proper tape measure or similar, and know exactly where the measure point (front focal node) of your lens is. Then use the quoted formula.

I have usually forgotten to bring a tape measure, so I apply the formula to finger widths or lengths by mental arithmetic (i will have neglected to bring a calculator, too).

7. Go here:
http://www.largeformatphotography.in...ws-factor.html
Look for the article by John Cook. It's simple and very understandable.

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