Calculating bellows length

[mickster]

Hi

I'm shooting some close up head shots on 5x4. I'm currently using a 150mm and wanted to move to something longer ~250mm to get a flatter perspective.
I'm trying to work out whether I have enough bellows to do this. The 250mm is a non tele lens.

The framing (ie subject magnification) will remain the same...a magnification of approx 0.33. Accordingly, my subject to lens distance will increase with a longer lens, by a distance of 250/150mm, or 1.66. Given that I know the magnification, subject distance and bellows extension with my 150mm, how do I calculate how much bellows will be needed for the same framing on a 250mm. Is it just the 150mm bellows length x 1.66?

I feel that this is simple but i've managed to over think the whole thing and left myself completely confused All help is greatly appreciated!

rgds

[MumbleyJoe]

I found this explanation, though I don't know exactly whether this is true or not (I have no reason to doubt it). I had just been wondering the same myself (well, different question, but same core issue)

Core formula is:
1 / f = 1 / u + 1 / v
Where f is the focal length of the lens, u is lens to subject distance, and v is lens to film distance.
More properly, v and u are measured from the focal point of the lens. For a symmetric lens, that's usually smack-dab in the middle of the lens. For a telephoto lens, it's way in front of the lens -- that's the definition of telephoto. For a retrofocus lens, it's way behind the lens.
Magnification is v / u.

Source:
http://photo.net/large-format-photography-forum/00Sswe

Hope that helps!

[aduncanson]

That will work. Ignoring fine points like the difference between focal length and flange focal distance, (almost always within 10% of focal length and often offset to provide shorter than calculated distance), image distance is proportional to focal length. Specifically:

Image Distance = (1+Magnification) * Focal Length

[Jack Dahlgren]

m = (di - f)/f
m = magnification
di = distance to image from lens nodal point
f = focal length

Just do the math :-)

-Jack

[mickster]

That will work. Ignoring fine points like the difference between focal length and flange focal distance, (almost always within 10% of focal length and often offset to provide shorter than calculated distance), image distance is proportional to focal length. Specifically:

Image Distance = (1+Magnification) * Focal Length

Thanks! So, assuming that 'Image Distance' = bellows length I get a figure of 1.33 * 250 = 332.5mm of extension for the 250mm lens. Hmmm, time to look into a top hat / Thalmann-style extender board...