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. Thanks

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: http://www.darkroomagic.com/DarkroomMagic/Camera.html 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!

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. Cheers!

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.

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).

To find the distance from the infinity position: distance = (focal length in mm)* square root(f/stop change wanted) in mm distance = (focal length in mm)* square root(f/stop change wanted)/25.4 mm/inch in inches Example: distance = 135mm * square root (4 f stops) = 135mm * 2 = 270mm I used this to mark off changes in f/stops from 0.0 [infinity] to 4.0 in 0.5 increments. Steve

the version I made from JBrunner's idea has a 1.5" square target. I don't recall if I did any scaling though.

But if you want a 1 stop change, you get 135mm, and that should be a 0 stop change. Or am I missing something? For a 135mm lens I calculate a 1 stop difference at 191mm, 2 stops at 234mm, and 3 stops at 270mm.

Yes the equation is off. I ended up downloading the spreadsheet. The spread sheet was locked. I opened my own spreadsheet. The format of the equations had changed. Finally got it working. I cannot get it to line up here, but here it is ... Enter Lens Focal Length 135 Bellows Scale f/stop Extension Distance Change 60 0 0 65 0 0 70 0 0 75 0 0 80 0 0 85 0 0 90 0 0 95 0 0 100 0 0 105 0 0 110 0 0 115 0 0 120 0 0 125 0 0 130 0 0 135 0 0 140 5 0 145 10 0.25 150 15 0.25 155 20 0.5 160 25 0.5 165 30 0.5 170 35 0.75 175 40 0.75 180 45 0.75 185 50 1 190 55 1 195 60 1 200 65 1.25 205 70 1.25 210 75 1.25 215 80 1.25 220 85 1.5 225 90 1.5 230 95 1.5 235 100 1.5 240 105 1.75 245 110 1.75 250 115 1.75 255 120 1.75 260 125 2 265 130 2 270 135 2 275 140 2 Steve

Math makes my head hurt. I learned the algebra in the '60's at a photo school. I always had a headache. Then I found out about the f/stop/inches method that Steve Sherman describes. I've been pain free ever since. I keep half a dressmakers tape in my bags and studio roller cabinet, that lets me hit my exposures within a third of an f/stop. But even using the guesstimate in your head method it puts you in the half an f/stop range. Then you just go over for negs and under for trans.

The only time Salzberger's "Quick Disk" has failed me is the time I left it in the middle of a frame... nicely focused and perfectly exposed.

I crunched the numbers backward to find length and marked a small retractable metal measuring tape accordingly for each lens (3) in a different color permanent ink. It is attached with a key ring to the strap of a spotmeter. Its a no-brainer -- just measure from the lensboard to the film plane and see which exposure factor is the closest . http://www.jeffreyglasser.com/

I agree. What can be easier than using a focusing target and measuring the exposure modification off the screen with a ruler. It may not work in all and every situations, but it works in most cases and does so without math.