Jasons' Bellows Factor Compensation Thingy
I've been using JB's bellows factor thingy for a few years. I've misplaced the little photo, though I still have the ruler. I had some spares in an envelope, but I can't find that either. I've just been looking up other ways of figuring bellows factor, but none are as easy as that little ruler. Can anyone tell me the size of the image that gets put on the subject? I tried looking for it on his website, but the link doesn't work.
Must admit, I have a dress maker's tape sewn to the edge of my dark cloth and use John Cook's method about a third of the way down this page.
I'm not familiar with that tool, but you may find another tool being just as useful.
You'll find it on this page:
by scrolling down to 'Bellows Ruler & Target'.
I carry a set of your target and ruler in both my medium format and large format kits Ralph, they are easy to use, and I've not made any mistakes because of them.
Here is the new version of the "classic" Calumet one for only eleven bucks: http://www.calumetphoto.com/item/CC9201/. They shouldn't replace knowledge of how to calculate extension factor by crunching the numbers, but they certainly are a welcome way to speed things up when shooting!
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Just to complete the list of thingys - here's what I use: http://www.salzgeber.at/disc/
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Real easy way to compute bellows extension is to think of lens and bellows extension in terms of F stops.
If using an 8" lens and bellows is extended to 11" then it would be 1 F stop of compensation, i.e. F8 -- F 11 If using an 10" lens and bellows is extended 20" then you are @ life size and a 1:1 ratio requiring 2 full stops of increased exposure.
The shorter the lens the less the bellows is drawn to cause a full F stop increase, just as the lower F stop numbers are closer together, it is a proportionate ratio.
In theory the shorter the lens the greater margin of error, however, this method has served me well in the field and years ago in the studio with exacting Ektachrome and table top work ranging from half to full life size set ups.
The great part, it's all in your head, nothing to carry or fumble for in your bag.
Thanks Jerevan - same kind of thing, so I printed and used it this AM. It gave me 1 1/2 stops more, agreeing with the dial in an old (but still very useful) Kodak guide.
Steve - the lens is 9.5 inches, and the bellows was 16.5 inches. Your method gives me about the same, but as neither number corresponded with nice familiar f-stop numbers, my math-phobic brain feels squirmy just looking at it. I can do it, but I have to write it down and think about it 2 or 3 times to be sure I'm not making a mistake, and then I'm still not sure.
Ralph - looked at yours too - same basic idea. I couldn't figure how to import and print it. It was 3 in the morning when I tried, so it's probably my fault. I'm no more intuitive with computers than I am with numbers.
Depending on your browser, it either downloads as a pdf, or it opens in your browser and you save it as a pdf.
Originally Posted by sly
I prefer to think in terms of factors rather than f-stops, because if I'm using filters in conjunction with bellows extension, the filter factor and bellows factor would be multiplied to get the final correction. f-stops cannot be multiplied. The "disc & ruler" is certainly convenient and speedy, but the target would have to be in focus in order to measure its image on the GG, so what to do if the plane of focus is inaccessible (e.g., "in the swamp").
I use a small pocket-size extendible scale to measure the bellows extension, then check my pocket-size notebook for the needed factor; the notebook is really a mini ring binder that contains a chart I made up for each of my lenses, showing the bellows factor for incremental extensions. Another page has the filter factor for each of my filters. A pagefinder with built-in calculator takes care of the multiplication chore (numbers hurt my head, too).
Last edited by silveror0; 11-13-2010 at 02:45 PM. Click to view previous post history.
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