Bellow compensation.

[Drew Bedo]

You can get into extension compensation pretty deeply if you need to.

I try to make it as simple as my simple mind can deal with.

If the lens is extended to twice the focal length, two stops more light is needed.
If the total extension is one and a half focal lengths, only one more stop is needed.

For my 150mm lense, each additional inch of extension requires one third of a stop more light.

For my 20mm, the correstion is (about) one quarter stop for each inch.

This keeps it easy enough to do in my head while working on location. For critical close-up shots, I may get out the tape and calculator.

[Louie Devincentis]

Thanks everybody for the advice but lets see if I understand this not so easy subject. I only have to worry about light compensation when I am at very close distances and if I have to, compensate up to 2 f/stops.
Thanks again for your replies
Cheers.
Louie

[goamules]

I learned from Mark Sawyer to simply measure the focal length wherever the bellows is extended, then divide by the apparent aperture looking in the front of the lens. If my camera with an 8" F4 Petzval (with a 2" diameter circle of light seen looking into it's front) is stretched out to 16", it just became an F8 exposure. Then I meter the scene and calculate based on the f-stop I just determined.

[Mark Sawyer]

What Garrett said! There are all sorts of formulas for figuring bellows extension factor, but in the end, the very definition of an f/stop is just the focal length divided by the aperture. And figuring it from scratch helps keep you in touch with what it is.

[Rudgey]

If you have a iPhone, iPod Touch or iPad there is a great app called Reciprocity Timer that helps calculate bellows compensation and several other things effecting exposure.

Thanks for the link to that great App, I was surprised to see reciprocity kick in as fast as 2 sec on FP4 of 1 stop!

[Ian Gordon Bilson]

I'm with Drew Bedo on this subject - and probably less enabled than he.

This,from a Kodak Photo Guide. "Bellows Factor for View Cameras" : " Place a white card, 2 inches long,at the subject position.

Measure the width of the card on the ground glass. Apply the following ( +F stops) correction. Note the 1/3 f stops.

7 millimeters : + 1/3
14 : + 2/3
22 +1
30 +1 1/3
41 +1 2/3
52 +2F
64 fill in the gaps..
76.5
93 +3
110

Sorry about the Imperial/Metric combinations, but metrics are more precise for this application.
I use a cheap pair of dividers at the glass, and read off the mm to a scale on my Horseman LF.

[Brian Ellis]

It's easy to get way too involved and worried about bellows compensation, especially when using negative film which generally has a good bit of exposure latitude. I used a rule of thumb under which no compensation was made as long as the thing on which I was focusing was farther than 8 times the focal length of the lens from infinity (some people use 10 times which probably works fine too) and if it was just a little more than 8 times I added a half stop of exposure, if it was somewhat more I added a stop, and if the bellows was fully extended or thereabouts I added a stop and a half or two. I just eyeballed the distance, I didn't carry a ruler or other measuring device around with me. I had four lenses and I converted the focal lengths into feet (rounded inches up or down) and typed a little table so that I didn't have to do the "8 times focal length" math in my head but after a while you remember them and don't need a table. Always worked fine for me, this isn't nuclear physics.

[Drew Bedo]

Does anyone lower the blast visor and access THE FORCE?

[cowanw]

the thing on which I was focusing was farther than 8 times the focal length of the lens from infinity

This is where English screws me up. Lets assume a 12 inch lens; 8X12 inches is 8 feet.
Does this mean the thing you focus on is 8 feet closer than infinity? or 8 feet father than infinity away?
Or it is farther than 8 feet away from the camera?
Or is the bellows 8 feet from its infinity position?

[buggz]

I used this the other day, was easy, and worked great for me.

If you are doing close-up still life, macro etc then yes, you do need compensation. Have a look at Quick disc http://www.salzgeber.at/disc/ I've found that quite helpful.