It sounds like the lens itself. What aperture are you using? I usually focus wide open to minimize depth of focus, and print closed down to use the benefit of depth of focus. Try an aperture near the middle of the scale.Originally Posted by noblebeast
An additional consideration is the ratio of enlargement. Inexpensive lenses are designed to work best at relative small ratios with the idea that beginners will not need LARGE prints. The effects of field curvature (which your descripption sounds like) are more noticeable at higher ratios.
Will the use of a "high quality lens" make a substantial difference? At high magnification ratios, it is likely. Will it be immediately discernable? Nearly impossible to answer .. It depends on the subject, the print size, viewing distance, whether or not smoking is allowed in the gallery, level of alcohol consumption....
Be careful of one optical characteristic - color correction. It is as necessary for sharpness (resolution) in black and white images as it is with those printed in color. Different wavelengths of light (colors) should, ideally, focus to the same place; if they do not, there will be a "rainbow" effect at the image edges - usually most noticeable at the extremes of the field. The color *fidelity* - the accurate rendering of colors, is a different animal.