In this article, I will walk you through two real-world scenarios on calculating the bellows factor when extending your bellows for macro work with large or ultra large format view cameras.
The bellows factor is a term used to describe the amount of light lost when using a bellows extension greater than the lens’s focal length. This is due to the increased distance between the lens and the film plane, which causes the light to spread out and become less intense.
The bellows factor is calculated using the following formula:
Bellows Factor = (Bellows Extension / Focal Length)^2
In the case of a 300mm lens being used with 600mm of bellows draw:
Bellows Factor = (600mm / 300mm)^2 = 4
This means that the amount of light reaching the film plane is reduced by a factor of 4 when using a bellows extension of 600mm with a 300mm lens. To compensate for this light loss, you would need to increase the exposure by two stops (i.e., double the exposure time or open up the aperture by two stops).
For example, if you metered the scene at 1 second, your new exposure based on your bellows draw would be 4 seconds plus any compensation that may be required for a filter you are using, plus reciprocity failure, if applicable for your negative medium.
How About a 210mm Lens with a 600mm Bellows Draw?
The bellows factor is calculated using the following formula:
Bellows Factor = (Bellows Extension / Focal Length)^2
In the case of 600mm bellows draw with a 210mm lens, the bellows factor would be:
Bellows Factor = (600mm / 210mm)^2 = 8.16
This means that the amount of light reaching the film plane is reduced by a factor of 8.16 when using a bellows extension of 600mm with a 210mm lens. To compensate for this light loss, you would need to increase the exposure by approximately three stops (i.e., double the exposure time three times or open up the aperture by three stops) OR multiply your metered exposure time by 8.16, plus any additional time required to compensate for a filter factor plus reciprocity failure of your chosen negative medium.