This is a spot diagram and graph of longitudinal abberation (LA) for an apochromatic lens quadruplet. I know that the LA graph should ideally come to a point for all wavelengths and it appears to in the graph but what is the 'y' axis? Is it the direction of focus? What does the 'x' axis mean? The spot diagram seems to indicate the degree of coma as you move the lens off axis from the incoming ray bundle. I guess small spots are better than larger ones but how small is good? Any help? Thanks.