[LargeFormat] Bellows Extension and light fall-off

[Clive Warren]

At 7:18 am -0700 27/5/02, Karl Wolz wrote:
snip
>2) This one's probably easier.  Light comes out of the back of the lens in a
>cone.  In macro work, you extend the bellows more, so you are dealing with a
>larger section of that come of light.  This, incidentally is why the lens
>needs to be opened more to allow for "bellows factor".  There is only so
>much light available for any given f:stop, so as you move further back on
>that expanding cone of light, the image gets dimmer.
snip

The scientific law associated with the light fall-off effect is the 
Inverse Square Law - "When a surface is is illuminated by a point 
source of light, the intensity of the illumination produced is 
inversely proportional to the square of the distance separating them."

So, if the distance between the light and the surface is doubled, the 
intensity of the illumination of the surface is reduced to one 
quarter. This law comes into serious effect for photographers when 
the distance (Extension) between the (to all intents and purposes) 
centre of the lens and the film plane exceeds the distance of the 
measured focal length of the lens. Also an issue for strobe or 
redhead lighting, ie distance from subject.

The formula for working out the Effective Aperture taking into 
account the bellows extension for extensions longer than the focal 
length of the lens being used is:

Effective Aperture = (Bellows Extension/Focal Length) X Lens Aperture

Another way of doing this is to work out the "extension factor" which 
is the amount by which you need to multiply the exposure to obtain 
the corrected exposure for bellows extension:

Extension factor = (Bellows Extension)squared/(Focal Length)squared

On the workshop Ole mentioned a good technique he uses in the field 
that doesn't need so much mental arithmetic which he can post!

Cheers,
        Clive