View Full Version : Lens focal length

Marie Dohoney
10-Oct-2004, 06:12
I want to use a Sinar Norma. The book I was given with the camera says that exposure needs to be increased because of the "bellows factor" and gives the formula Bellows Length squared/Focal Length squared = Exposure Increase. I can measure the bellows length, but I don't know what Focal Length means. Is that the number on the lens, as in 1:4.5/150? Would the Focal Length be 4.5? Thank you for your patience. I am very much a beginner, and this camera will keep me humble for a long, long time.

Ernest Purdum
10-Oct-2004, 06:25
In the example you give, 150 is the focal length expressed in millimeters. 1:4.5 is the aperture, usually expressed as f4.5. Focal length is the distance between a particular point in your lens and your film when focused at infinity. When you focus a lot closer than infinity, that's when you need to use "bellows factor". It only becomes needed when your subject is within maybe eight focal lengths away, in your case 8X150=1200 or about four feet, so for most subjects you can forget all about it.

The Sinar Norma is a great camera to learn and have fun with.

Tom Westbrook
10-Oct-2004, 06:25
If you haven't, take a look at the article on the Home page: http://www.largeformatphotography.info/bellows-factor.html

Also to save yourself all the math, Calumet (among other's) make a cheap little tool to do the calculation for you: http://calumetphoto.com/ctl?PAGE=Controller&ac.ui.pn=cat.CatItemDetail&ac.item.itemNo=CC9201&ac.cat.CatTreeSearch.detail=y&type=SPDSEARCH

neil poulsen
10-Oct-2004, 13:04
For my purposes, I calculated a table for each of my lenses. I picked two points on my camera to represent the distance between the front and back standards. (Distance from back of front standard to back of rear standard.) I placed a lens on the camera, focused on something at "infinity", and measured the above distance between the standards. The closer the object being photographed, the further apart will be the two standards, and the greater the aperture adjustment needed to correct for bellows expansion. Knowing the focal length of the lens, I was able to calculate a table that would give me the aperture correction for successively greater distances that I might measure between the two standards. My table for each lens is accurate to the nearest 1/10th stop. In use, I bring whatever object into sharp focus, measure the distance between the standards, and use my lookup table for that lens to determine the correct aperture adjustment. Very easy.

I'm often accused of overcomplicating things! But I like this system, because it allows me to determine and make all bellows expansion corrections at the camera. I don't need to measure distances to the object, etc.

If you're interested in this approach, I could probably put together a spread sheet that would automatically make the calculations. Let me know.

Marie Dohoney
10-Oct-2004, 16:35
What a great place to ask a question, however simple! I think I'm going to have a great deal of fun with this, and really appreciate this kind of support at the outset.
Neil, I think I would like a spreadsheet a little further down the line. This sounds like a system I could work with. If I may, I'll get back to you in a bit. My task just now is to take 'Norma' someplace beautiful and practice getting things in focus. Then I'll be able to take the measurements you've suggested.
Thank you all very much!
Marie Dohoney

Řyvind Dahle
11-Oct-2004, 07:05
Try quick disc! I will.



Marie Dohoney
12-Oct-2004, 18:45
Way cool!
Thank you!