Surface photometry

I used to think that galaxy photometry is difficult only in that sense that it necessarily includes errors. As galaxy edges are diffuse, it's impossible to decide where the limit is, so any estimate is subjective and uncertain. I used to think that those are only second-order errors, important in statistical sense (every approach brings its own systematics), but not for individual galaxies. Last week I was struck by a simple plot I made -- a graph showing cumulative flux, flux per pixel and flux at a ring of given distance vs. distance from the galaxy center. The cumulative flux curve does not flatten or grow linear as one would be inclined to think -- as the number of pixels at each distance increases quadratically, their contributed flux increases as well. Mean flux per pixel, however, does flatten (and gives us a rough estimate of the sky value). It might just be a pretty good measure, but I'm worried about gradient in SDSS sky.