[LargeFormat] Bellows Extension and light fall-off

[Ole Tjugen]

28.05.02 01:25:14, Clive Warren <cocam@blueyonder.co.uk> wrote:

>snip

>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
>


Right. As soon as I get round to it, I'l write this up properly for the forum. But here's a short version:

The f-stop numbers are very useful for a lot of things, among them estimating bellows factor. If you look at them, you'll notice that the 
difference between two adjacent f-numbers is a factor of 1.4; skipping one, the factor is 2. 

Now measure the bellows extension with the camera focused at infinity. What you measure in; inches, cm, mm, fingerwidhts or pennies,  
doesn't really matter, as long as you're consistent.

Then focus on your close-up subject and measure again using the same units.

If extension at infinity is 11cm and this is the same as the focal length of the lens (I'm making it easy on myself here, this would be a 110mm 
lens), and the extension when focused close is 16cm, the light-loss is one stop (like in f11,f16). If the extension is 22cm, the loss is 2 stops 
(and you're at life-size reproduction, but that's another matter). With an extension of 14cm, the loss is about half a stop.

With a tele lens, things get a bit more complicated since the rear node of the lens (where you really should be measuring to) is nowhere 
near the lens board. It helps if you use some sort of "known" measure, and also know where the node is supposed to be. On my 360mm 
Tele-Xenar, the lens board (at infinity) is at two indexfingers and a nail (21cm), while the nodal point is at three and a half indexfinger... If my 
new extension is three indexfingers, the nodal point will be at about four and a half indexfingers, giving a difference of 3.5 to 4.5 fingers, or 
about two thirds of a stop.

Yes, I measure bellows extension using indexfinger lengths, which just happen to be about 4 inches or ten cm. Fingers also tend to have 
useful marking along the length: A knuckle about halfway out (5cm/2in), another knuckle further out (2.5cm/1in), and a fingernail (1cm/half 
an inch). Except when using very short lenses, this is accurate enough for slide exposure!


Ole