T3D Re: Exposure at bbellows extension! (TECH-3D digest 361)

[Tom Deering <tmd@xxxxxxxxxxx>]

On 9/21/98, bobh@xxxxxxxxxxxxxxxxxx wrote:
>RER: Tom wonders if he puts a 45mm lens on a 100mm bellows
>extension..if he had a 145mm lens? Well yes and no.  It is a 45mm
>lens at a bit more that 4X extension..meaning it is going to focus
>close and magnify. BUT he is right that the diaphragm is now about
>145mm from the film so the f/stop opening say it is 4mm becomes
>4/145mm or f/36, whereas on the 45mm lens alone 4mm was
>4/45mm or f/11. BobH

I don't know why this didn't occur to me sooner.  The focal length depends
on the physical shape of the lens, which doesn't change when you use a
bellows.  It is sinking in, slowly.

So if I'm using the MAOFD formula, I still use 45mm as the focal length.

Now I understand why John put "operating focal length" in quotes, since the
actual focal length of the lens doesn't change.

Am I saying this right?  :^)  Now I have to go re-read all those messages.  :^)



------------------------------