Originally Posted by Bruce (Camclicker)
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.
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.
Originally Posted by Charles Webb
"Add 1/3 of a stop for every inch of bellows draw past the focal lenth of the lens."
Seems pretty simple to me.
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.
Originally Posted by reggie
Like I said: "I will do some math to validate the rule of thumb ..."
Originally Posted by Donald Miller
If the math shows it not accurate enough, then I'll use another method. But thanks for the useful sermon...
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.
Sponsored Ad. (Subscribers to APUG have the option to remove this ad.)
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.
"What drives man to create is the compulsion to, just once in his life, comprehend and record the pure, unadorned, unvarnished truth. Not some of it; all of it."
- Fred Picker
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.
Originally Posted by reggie
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
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.
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
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
i've done this, and it has worked every time, and it's much easier than all that complicted math too ...
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.
Originally Posted by jnanian
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.
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.
[COLOR=SlateGray]"You can't depend on your eyes if your imagination is out of focus." -Mark Twain[/COLOR]
Rio Rancho, NM