To guard against noise peaks, one imposes a lower limit to the number of contiguous pixels that appear above a set threshold. To guard against incompleteness, one sets the threshold as low as possible. Thus, galaxy counts tend not to be strictly magnitude-limited, but rather are limited simultaneously in area and in surface brightness. Small errors in the subtracted sky can easily dominate the net photometric errors. Software must be able to distinguish cases of confused images and to account properly for the flux in the individual components.
Some effects are peculiar to isophotes in noisy data.
Due to photon shot noise, even a centrally concentrated image may break up into small islands that become difficult to reassemble without bias in the derived fluxes. Moreover, objects will appear artificially bright because statistically low pixels will be excluded from the isophotal area. There will be a statistical redistribution of objects resulting in steeper counts because there are more faint galaxies than brighter ones and the photometric errors increase with increasing magnitude. Competing with this effect is increasing incompleteness with increasing magnitude. Accurate interpretations of faint galaxy counts should thus be based on detailed simulations of the selection function, random errors, and known systematic errors.