Bellows extension and exposure

[squiress]

Just wanted to check on bellows extension effects on exposure. I haven't worried too much about this as much of what I have shot has been at near infinitiy settings, and when using an RB67, there was a wonderful little scale on the side of the bellows if I racked out far enough to need to worry about it. However as I get ready for my new Wehman 8x10, I am wondering about significant movements affecting exposure through contorted belows distances.

I understand that the focal length of the lens establishes a baseline for image brightness. And that when I reach half again that distance I need to increase by half a stop, twice that distance a full stop, etc. This camera has base tilt in front and rear and fairly significant rise. Besides bracketing, is there a way to handle this quantitatively or are the effects of movement not significant enought to worry about?

Thanks
Stew

[Geert]

Use the quickdisk: http://www.salzgeber.at/disc/index.html

It's a great tool to measure bellows extension compensation.

G

[Walter Calahan]

Bellows Extension Factor = (bellows extension measured from the film plane to the nodal point of the lens [usually around were the aperture is]) squared, divided by (lens focal length) squared.

Can be done with a small tap measure and a pocket calculator or in your head.

[Michael Kadillak]

Use the quickdisk: http://www.salzgeber.at/disc/index.html

It's a great tool to measure bellows extension compensation.

G

Calumet sells a device similar to the above mentioned Quick Disk that is laminated. Put the small square in your scene and measure the compensation on the ground glass. I had a tape measure and have done the math, but find this "visual" technique is much faster and more convenient for me.

Cheers!

[Geert]

Bellows Extension Factor = (bellows extension measured from the film plane to the nodal point of the lens [usually around were the aperture is]) squared, divided by (lens focal length) squared.

Thanks a lot for the formula, Walter, but I'm not that strong in math

G

[Ralph W. Lambrecht]

Stew

Check in the 'library' section of my website. You'll find a free pdf called 'BellowsTarget&Ruler'. Print it out and place the target into your scene. Then, use the ruler to measure the scale and the f/stop compensation on the ground glass. I laminated mine to protect it from frequent use.

[Robert Ley]

In his excellent book "Using the View Camera", Steve Simmons described an easy way to deal with bellows extension. For every 25% of bellows extension add 1/2 stop of exposure. So if you were using a 210mm lens (roughly 8 inches) and a bellows extension of 10 inches, add 1/2 stop, 12 inches= 1 stop. I have used this simple technique with good results and all it requires is a tape measure. Steve's book is an excellent source of good info on the basics of LF photography although a little dated on the information on film as so many have been discontinued or updated.

[Jack Flesher]

Another trick is to use the f-stop method -- if you can deal with odd numbered apertures and are happy being within 1/3 stop accuracy...

Say you are using a 150mm lens. Drop the zero and think of it as 'f15'. Now focus on your subject and measure the extension between the standards. Say your total extension is 200mm. Again, drop the zero for 'f20'. The difference in exposure between f15 and f20 is very near or just shy of one full stop. Make the change and go. I have used this method sucessfully for years even shooting transparency emulsions and it has worked well.

Cheers,

[squiress]

Robert,

Bought Simmon's book and read a little on this last night. I thought the extension was 50% for 1/2 stop. Have to reread that part. One of the best things about the view camera and movements is the whole composition thing under the cloth. Exposure though, especially with the much higher film costs at 8x10, is something I'd like to have nailed down before releasing the shutter.

Thanks

[tim atherton]

I'd second the quickdisc - quick and easy to use, print it out on card, laminate it and there you go (btw, I found the disk is a slight improvement over the calumet square for practical purposes and it's free...).

I just did a whole project with a couple of hundred images of varying magnifications in different sorts of lights and it worked out just fine