Lenses Exposure Compensation with Tubes and Bellows Using bellows or extension tubes means that the light has to travel further through the system before it reaches the image sensor. If you are not using TTL metering, perhaps using flash in the studio and assessing exposure with a separate flash meter, then some allowance will need to be made for the reduced amount of light reaching the sensor. A formula used to calculate the compensation factor required is: Compensation Factor = Metered Exposure ( M + 1 ) 2 For example, for a magnification of 1×: Compensation Factor = Metered Exposure × ( 1 + 1 ) 2 = 4 This factor means that four times more light is required to give the correct exposure (i.e., two stops). A metered exposure of f/16 would become f/8, for example.