An easy way to figure bellows factor

[ImSoNegative]

I know there are several ways to do this but this one I read about works great and its easy for me and that is what counts someone else posted this in a thread on here someplace and im just reposting it for future searches by others, so cheers to the OP.

step 1: convert the mm of your lens to inches ex. 90mm lens is roughly 3 1/2 inches
step 2: convert that to an f stop 3 1/2 inches is close to f4 the number 3 1/2 is close to the number 4 get it?
step 3: when doing a close up measure from the lensboard (roughly) to the film plane ex. say its ten inches to the film plane convert that to an fstop which would be
f11 (closest f stop to the number 10)
step 4: figure the difference between the two fstops ex. between f4 and f11 3 stops
step 5 add 3 stops of exposure

this of course will only work accurately if you don't use tilts

[C. D. Keth]

It'll be close enough if you use tilts. When was the last time you tilted so much it was more than a few mm difference between the top and bottom of the lensboard?

I just calculate the bellows extensions that correspond to full 1/3-stop corrections. I make a table of those out to the maximum my camera can do and I stick that on the lensboard. Simply measure and choose the closest correction. I don't like doing math when I'm making a photograph.

Here's what is stuck to the lensboard of my 150mm caltar (with the formatting all messed up by the forum software):

150mm

+1/3 - 168mm
+2/3 - 190mm
+1 - 212mm
+1 1/3 - 237mm
+1 2/3 - 268mm
+2 - 300mm
+2 1/3 - 335mm
+2 2/3 - 379mm
+3 - 424mm
+3 1/3 - 477mm
+3 2/3 - 535mm

Say you measure your bellows for the current shot at 229mm, you'll see that 1-1/3 stops is the closest correction.

[redshift]

There's an app for that for $2.00. https://itunes.apple.com/au/app/reci...459691262?mt=8 I like the way it that it works specifically for each brand of film and includes filter factors. It doesn't replace knowing how to do it though.

[Leigh]

I like the way it that it works specifically for each brand of film...

Since when does bellows factor care about what film you're shooting?

- Leigh

[Robert Oliver]

I use that same method... Learned it from Bruce Barlow's manual. I just carry a big plastic ruler with the mm conversion of my lenses written on it.

[redshift]

Since when does bellows factor care about what film you're shooting?

- Leigh

I thought reciprocity failure varied with the film and that extended exposure due to bellows extension brought reciprocity failure into play. I'll leave that discussion to the know it alls.

[ImSoNegative]

yes it can definitely bring it into play depending on far you stop your lens down and how long your exposure is, I use ilford film which there failure starts at 1 sec. I normally use the delta and rate it at box speed, so usually I have to add time, found a really cool formula for figuring reciprocity for the ilford films,
(1.7 x metered time) + (.12 x metered time squared) = corrected time. this has worked perfectly for me every time. for example if my metered time is 8 sec. 1.7 x 8=13.6 sec or just round it off to 14 sec. then .12 x 64=7.68 round it off to 8. add 14+8=22. 8 sec metered time is 22 sec corrected time.

[ImSoNegative]

It'll be close enough if you use tilts. When was the last time you tilted so much it was more than a few mm difference between the top and bottom of the lensboard?

I just calculate the bellows extensions that correspond to full 1/3-stop corrections. I make a table of those out to the maximum my camera can do and I stick that on the lensboard. Simply measure and choose the closest correction. I don't like doing math when I'm making a photograph.

Here's what is stuck to the lensboard of my 150mm caltar (with the formatting all messed up by the forum software):

150mm

+1/3 - 168mm
+2/3 - 190mm
+1 - 212mm
+1 1/3 - 237mm
+1 2/3 - 268mm
+2 - 300mm
+2 1/3 - 335mm
+2 2/3 - 379mm
+3 - 424mm
+3 1/3 - 477mm
+3 2/3 - 535mm

Say you measure your bellows for the current shot at 229mm, you'll see that 1-1/3 stops is the closest correction.

150 is a 6 inch lens so convert that to fstop 5.6 229mm is 9 inches plus a bit. so yes if you convert that to f9 the correct difference between 5.6 and f9 would be 1 1/3
this type of math can be pretty much done in your head as long as you know that 25mm is an inch and one knows the f stop numbers. you can get the exposure really close using any lens.

[StoneNYC]

yes it can definitely bring it into play depending on far you stop your lens down and how long your exposure is, I use ilford film which there failure starts at 1 sec. I normally use the delta and rate it at box speed, so usually I have to add time, found a really cool formula for figuring reciprocity for the ilford films,
(1.7 x metered time) + (.12 x metered time squared) = corrected time. this has worked perfectly for me every time. for example if my metered time is 8 sec. 1.7 x 8=13.6 sec or just round it off to 14 sec. then .12 x 64=7.68 round it off to 8. add 14+8=22. 8 sec metered time is 22 sec corrected time.

Delta films reciprocity starts at 1/2 not 1 according to ilfords documents FYI

[ImSoNegative]

Thanks I always thought it started at 1sec.