Bellows Extension Compensation

This large format camera business is strangely attractive to me! More film has been ordered and I’m out of Rodinal. It is a whole new photographic world out there to discover. It is fun!

Apart from the novelties of tilt, swing, shift and studying a large ground glass, exposures at short distances must be compensated due to the focal distance. For smaller formats the distance from lens to film is close to constant – or varies at least very little. In a large format camera the bellows extension can be considerable, not only due to the lens’s focal distance but also due to the distance between the lens and the object.

To project an image of twice its natural size the distance between the lens and the ground glass must be twice the focal distance of the lens. In my present setup with a 150 mm Xenar that translates to a 300 mm bellows! Only when focussing on infinity does the lens’s focal number coincide with the length of the bellows.

In the figure below this bellows extension compensation is demonstrated. Objects at infinity are projected onto the area a at focal distance f. Objects closer to the camera than infinity will be projected at a distance larger than f. In our special example of twice the size the focal distance will be 2f and hence the light that in the first case falls onto the area a^2 now is “diluted” onto an area four times as large, 4a^2. Of course the film situated at the focal distance will be underexposed  four times, i.e. 2 stops.

The upshot of it all is that that we need to compensate for this “dilution” of the image. For distances far away the compensation is effectively nil, but for closer objects it can amount to several f-stops.

The mathematical relationship for this quadratic relationship between extension and f-stop is

or simplified

For my 150 mm Xenar the following table helps. Given different bellows extensions it shows the correction to apply to get the actual f-stop. Or, if f-stops are not what I am after, the multiplier for measured exposure times (2^0=1, 2^0.36=1.28 etc).

A ruler with these numbers would be practical and easy to use. Just hold it along the bellows and note what f-stop number to add to the lightmeter value.

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