The apparent magnitude m of a star is related to its apparent brightness b by the relationship:-

On their variable star charts, the AAVSO supply a number of unvarying stars of known visual magnitudes with which to compare the brightness of the variable star. For example, you can have a look at the (a) chart for T Cephei. Comparison stars are marked with their magnitude written beside them (but with the decimal point omitted, so that 3.3 is written as 33). For each comparison star i in the region of T Cep, you have a measure of its apparent magnitude m_i and an observation of its apparent brightness b_i and so you can calculate an estimate of c_i:-

The constant c_i for each nearby comparison star should be similar so an average value <c>can be calculated:-

where the summation of c_i is over the n comparison stars available. Also, an estimate of the error in of computing the magnitude of a star is available from the standard deviation σ(c):-

An estimate of the apparent magnitude m_v of the variable star can then be obtained from a measure of its apparent brightness b_v:-

The total brightness b_s of a comparison or variable star is obtained from summing together the values of the brightness of the star b_j at pixel j for all the pixels where the star occurs in the digital picture. However, in practice b_j contains a contribution from the brightness of the sky b_sky so:-

where j runs over the m pixel values in a box which is large enough to contain the star. However, b_sky can be determined from placing a box of the same size near to the star but in an area where there is no stellar contribution to the light:-

where k runs over the pixel values in this new (but same size area) box. So:-