Light Loss when using bellows/extension tubes?

[Jammy Dodger]

Quite simply, how do I calculate the light loss when using the above?

If I take a flash meter reading as normal how do I factor in the exposure settings I will need?

Thanks

Fraser

 

[Deleted member 95430]

View: https://youtu.be/aq6bwMdvhJ0?feature=shared


I use an app called Reciprocity timer does it all for me

 

[sk66]

The bellows factor works the same way as aperture does... i.e. ~ a 1.4x increment (it's actually slightly greater; sqrt(2) = 1.4142).

Let's say you are using a 50mm lens; 50 x 1.4 = 70mm. If you had a 20mm tube attached to the 50mm it would then be at 70mm, which would be 1 stop more light required. If the extension caused the resulting focal length to double then it requires 2 stops more light (i.e. a 50mm tube).

But an app is easier....

 

[Rodinal]

While the exact figure for any exposure increase when using tubes or bellows or indeed a macro 1-1 lens is the holy grail, in practise there is latitude with what you can comfortably get away with. With colour negative you can possibly get away with as much as +/- .75 of a stop. +-With B&W +1-.5 of a stop. If you use Ilford XP2 which is really a colour neg film but giving B&W negatives again +/- .75 of a stop. Even Ilford say you can expose it at a couple of stops out from the rated 400iso . I prefer 200iso.
I find making adjustments as accurate as .25 of a stop quite easy using simple calculations.