Lenses introduce their own quirks into the mix. Some lenses are simply sharper than others. (The new Smart Sharpen filter in Photoshop CS2 contains a routine that specifically addresses lens softness.) A second lens problem, which we encounter a great deal more with digital capture than we did with film, is chromatic aberration, where the lens fails to deliver the red, green, and blue wavelengths to the same plane of focus, producing color fringing. It's a particular problem towards the wide end of wide-angle zooms, which is where we see it most often.
We suspect that we see chromatic aberration in digital capture more than in film simply because digital is much less forgiving to lenses. Film grain and interlayer scattering of the light tend to mask chromatic aberration where digital capture reveals it quite brutallyshooting film and digital with the same lens tend to bear this out. Camera Raw's Lens tab has controls for addressing chromatic aberration in digital raw captures, while a second new filter in CS2, Lens Correction, addresses it in Photoshop.
We'll cover noise reduction and lens corrections in the course of this chapter, but while only some images need noise reduction or lens fixes, every image needs sharpening, so that's where we'll start.