There's a pretty much linear relationship between the diameter of the star image and the magnitude, for digital imaging. If you blow up the image, you can estimate the diameters of numerous stars in the image, and then use the known magnitude of a few key stars (use a planetarium app or star atlas to get the magnitudes of the calibration stars) to calibrate the diameter vs magnitude relationship for that particular image. You should be able to use the same calibration for other images taken with the same exposure settings, but if you change your set up (exposure time, gain / sensitivity, focal length, etc), you'll get a different relationship, but it will still be linear.
You can then estimate the magnitudes of all the other stars in the image. Rather than using a ruler to measure the star image diameters, it's better to have a template of a range of circle sizes of known diameters, which you can move around until you get a good match between the star image and a circle of the same diameter.
Here's a brief write-up on the technique which I applied to an image of the Butterfly Cluster (M6):
http://julianh72.blogspot.com.au/201...-with-zwo.html
