LF: Applying Bellows Extention To Exposure

[seadrive]

I believe that bellows extension factors work just like f/stops, so an 8" lens that is 11" inches from the film plane requires one additional stop of exposure, just as stopping down from f8 to f11 would require you to double the length of the exposure, to compensate for the smaller aperture.

At the moment, I only have a 210mm lens (just a little over 8"), so I've marked the bed of my camera at 8", 11" and 16". By looking at the position of the front standard between the marks, I can guesstimate the additional exposure required to about a third of a stop.

 

[reggie]

...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.
-R

I did the math and created a chart that I will use with my 8x10. I carry a 240mm lens and a Cooke XVa with focal lengths of 311mm, 476mm, 645mm.
My Canham 8x10 max bellows extension is 36". The chart starts off with a minimum extension of infinity focus of my smallest lens plus 1" which equals 10". The chart has 5 columns, one for the total bellows extension (not the extension amount) and one for each lens focal length. The numbers in the chart are increases in F-stop that must be made to compensate for the extension. They are based on the formula:

factor=(E / F)**2

where:
factor = calculated exposure factor
E = Total bellows extension
F = bellows extension with lens focused at infinity

To make it easier, the exposure factor is then converted to an F-stop value using the formula:

(log factor)/(log 2)

and the result is rounded to 1 decimal place.

Example:

Using a 240mm lens, I focus on an object such that the total bellows extension is 18-inches.

factor=(18/9)**2 = (2)**2 = 4

(log 4)/(log 2) = (.6)/(.3)=2

Thus the exposure compensation is 2 F-stops.

I am pasting the chart in below. It would be easy to create a graph for each lens so that values could be interpolated from the graph. I will paste this in the notebook I keep in my camera bag along with a cloth tape measure.

One more note. One member mentioned that he used a rule of thumb of adding 1/3rd of a stop for each 1-inch of bellows extension. The chart shows that this is accurate in a very limited scenario. It can introduce large errors in exposure beyond that.

Here is the chart. I would appreciate it if someone would do a sanity-check on it for obvious errors, but it looks correct to me.

**Note: at press time I added in a column for a 210mm lens


EXT 210mm 240mm 311mm 476mm 645mm
---- ----- ----- ----- ----- -----
8 0.00 0.00 0.00 0.00 0.00
9 0.30 0.00 0.00 0.00 0.00
10 0.60 0.30 0.00 0.00 0.00
11 0.90 0.60 0.00 0.00 0.00
12 1.20 0.80 0.00 0.00 0.00
13 1.40 1.10 0.20 0.00 0.00
14 1.60 1.30 0.40 0.00 0.00
15 1.80 1.50 0.60 0.00 0.00
16 2.00 1.70 0.80 0.00 0.00
17 2.20 1.80 1.00 0.00 0.00
18 2.30 2.00 1.20 0.00 0.00
19 2.50 2.20 1.30 0.00 0.00
20 2.60 2.30 1.50 0.20 0.00
21 2.80 2.40 1.60 0.30 0.00
22 2.90 2.60 1.70 0.40 0.00
23 3.00 2.70 1.90 0.60 0.00
24 3.20 2.80 2.00 0.70 0.00
25 3.30 2.90 2.10 0.80 0.00
26 3.40 3.10 2.20 0.90 0.10
27 3.50 3.20 2.30 1.00 0.20
28 3.60 3.30 2.40 1.10 0.30
29 3.70 3.40 2.50 1.20 0.40
30 3.80 3.50 2.60 1.30 0.50
31 3.90 3.60 2.70 1.40 0.60
32 4.00 3.70 2.80 1.50 0.70
33 4.10 3.70 2.90 1.60 0.80
34 4.20 3.80 3.00 1.70 0.90
35 4.30 3.90 3.10 1.80 1.00
36 4.30 4.00 3.20 1.80 1.00

 

[removed account4]

wow, what a whole bunch of complicated "stuff" ...

i am sure if john cook were able to read this thread he would chime in. he wrote of a very interesting (and seemingly foolproof) way of computing exposure compensation for bellows extension.

it is about 3 articles down from the top of the page
... http://www.largeformatphotography.info/bellows-factor.html

i've done this, and it has worked every time, and it's much easier than all that complicted math too ...

john

 

[reggie]

wow, what a whole bunch of complicated "stuff" ...

i am sure if john cook were able to read this thread he would chime in. he wrote of a very interesting (and seemingly foolproof) way of computing exposure compensation for bellows extension.

it is about 3 articles down from the top of the page
... http://www.largeformatphotography.info/bellows-factor.html

i've done this, and it has worked every time, and it's much easier than all that complicted math too ...

john

Before I made my chart, I read that thread and enjoyed John's post. I used the formulas posted by Richard Koser and validated by Roy Harrington.

Note that once all the math is done, I am left with a chart and a tape measure. This is a pretty simple way to make exposure adjustments out in the field and the studio. I like it. It is simple, quick, takes up little space and doesn't require doing any math in the field. Just measure, look up the chart and make the f-stop adjustment.

Also, I wrote a program that will calcualte this chart for any focal length lens(I also need to know the max bellows draw for the camera) so it is easy to produce the chart - I don't have to go thru all the math again.

If anyone wants a chart of their own, just send me the focal lengths that you use and the maximum bellows draw of you camera and I'll calculate one for you.

Thanks.

-Mike

 

[rbarker]

Nice chart, Mike. Now all you need to do is similar charts for actual versus marked shutter speeds at various ambient temperatures for each of your shutters, and you'll be all set.

 

[Jim Jones]

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.

 

[reggie]

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

 

[reggie]

Here is the line Graph.

 

[djkloss]

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

 

[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

 

[Ole]

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).

 

[gbogatko]

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