How to calculate required bellows extension?

[Pawlowski6132]

So, I'm sure there is a formula or online calculator or spreadsheet somewhere to help calculate the answer to this but, here's what I'd like know:


On a 8x10 camera, using a 450mm lens, with a person standing 15 feet away from the lens, what is the required bellows extension to get a head-and-shoulders shot?


Can someone point me to some resources I can study?

thanx much

jrp

[stawastawa]

try playing with this simulator,
though I think you will have better luck playing and measuring the bellows extension yourself.

my quick test shows that a 450mm lens will be close to torso / full body than a head and shoulder image at 15ft subject distance.
http://apenasimagens.com/en/focal-le.../?preview=true

[Emmanuel BIGLER]

Hi!

The key factor to properly answer your question is the magnification ratio.
For a head & shoulder portrait on a 8"x10" format, you'll need something 2x or 3x bigger as a frame on the subject.
This implies a magnification ratio M = (image size) / (object size) between 1:2 = 0.5 and 1:3 = 0.33

Hence the different working distances are given as follows:

Distance from the subject to the lens = f(1+1/M) = between 450 mm x (1+2) and 450 mm x (1+3) = between 1350 mm and 1800 mm

Total bellows draw (for a non-telephoto, quasi-symmetrical lens) = f(1+M) = between 450 mm x (1+0.5) and 450 mm x (1+0.33) = between 675 mm and 600 mm

Additional bellows draw beyond the focal point (infinity-focus setting) = f M = between 450 x 0.5 and 450 x 0.33 = between 225 mm and 150 mm.
The formula f M giving the additional bellows draw beyond the focal point is valid for all kinds of lenses including telephoto lenses (and including retrofocus lenses, but no such lens exists, to the best of my knowledge, for the 8"x10" format).

[Pawlowski6132]

Both responses spot on what I was looking for.

thanx for taking the time.

I'm a little smarter...

jrp

[Pawlowski6132]

Oh, one more thing (and I should know this by now)...

With a 450M, lets say, when I am focused at infinity, the bellows extension is approximately 450mm correct? if so:

1. What does this mean, in practice, with regards to what will or will NOT be in focus??
2. The closed focus plane to the lens is the "minimum focus distance"?

thanx

[Ken Lee]

You might find this article helpful: http://www.kennethleegallery.com/html/tech/bellows.php

[toyotadesigner]

All the math in the field is pretty useless, because you are focused on the composition.

Use Quickdisk from a photographer in Austria and enjoy your life as a large format photographer. It is free of charge. You just need a decent printer, a cutter, scissors and 15 minutes.

Quickdisk

[Vaughn]

Nicely done, Ken.

Personally, if I am worried about bellows extension w/ B&W film, I usually double the exposure -- same with reciprocity failure compensation. Close enough to get what I want.

[Drew Bedo]

I regularly work in 4x5. My most commonly used focal lengths are 90mm, 150mm and 210mm.

What I do::

I have pre-calculated the extra exposure for the two longer lenses. The 150 is a six inch lens. For each one inch of extension beyond infinity (6") I open up 1/3 stop.

The 210mm is close to (but not exactly) an eight inch lens. With this lens, I add 1/4 stop more exposure for each inch beyond infinity, which I approximate as 8".

I have used the 210mm on my 8x10 with small front5 movement and this method works fine.

Your 450mm is nearl y(but not exactly) a twelve inch lens (at 11.7"). Fot this lens, every two inches beyond infinity will need 1/3 stop extra exposure.


After focusing, I measure from the front of the lens board to the film plane and work it out. No formula or head math, and I can do it with a cold wet wind blowing in my face when I am tired.


You can work through the formula with an app on another piece of expensive gear, but this method has worked well for me hands-free.

[Bill Burk]

The Quickdisc look like a very easy and sound solution.