How can I calculate the visual magnitude of a star with only its flux data?

[Schmetterling]

How to calculate the visual magnitude of a star having only the flux of such star?
The equation involves 2 magnitudes and 2 fluxes, corresponding to 2 objects a and b:

m_a-m_b= -2.5log₁₀\left( \frac{F_a}{F_b}\right)

but I just have F_a and I have to obtain m_a.

Is it valid to take m_a=-2.5log₁₀ F_a?

 

[Simon Bridge]

the "b" indexed values would have to be some reference value that depends on the scale you are using.
http://en.wikipedia.org/wiki/Apparent_magnitude#Standard_Reference_Values

 

[Schmetterling]

Ok, but in that table the values for "m" don't appear, only the fluxes :-s

 

[Simon Bridge]

Ok, but in that table the values for "m" don't appear

... yes they do: right at the top of the 4th and 5th columns where it says "m=0". But you can use any reference values that make sense in terms of your coursework.
Specifically: you should not get your reference-values off wikipedia. You must have access to a library or a textbook or something like that?
In a pinch - go to wikipedias primary source.