Bellows factor

One of the problems that arise when using large format cameras, is the difficulty in determining exposure when the bellows are extended beyond normal ‘landscape’ use. By that I mean, when you are shooting anything from say, 10ft away to infinity, your exposure will be whatever your meter indicates, but if you decide to shoot something close-up, say a flower, the lens will be much further from the film plane and light will be lost.
There is a calculation which can be employed to work out the extra exposure needed for a given lens and extension, (Measure the film to lens nodal point distance, then use the formula; bellows extension over focal length equals compensation factor) but for me, doing maths out in the field takes away a lot of the pleasure of photography. If only there was a simple way of working out the exposure difference……….
Well there is! There is a simple device called ‘Quickdisc’ available online. It is a handy (and free) download from http://www.salzgeber.at/disc/
Once the page has been downloaded and printed, the shapes are cut out and stuck to a piece of thin card.
The disc is placed in the scene at the distance to be photographed and the card with the scale is held on the camera screen. If the longest dimension of the disc image on the screen is compared to the scale on the measuring strip then the exposure correction scale will indicate the amount by which the exposure needs to be altered. The disc is unlikely to be absolutely square on to the lens, so using the longest dimension overcomes the problem of having a distorted shape on screen.
I have used one for years and it has been really useful.