in the market for modernish lens but....

[Dan Fromm]

I would rather say that it can reduce the effect of chromatic aberration. The aberration itself is unaffected.

I am very puzzled, please explain further.

[Ole]

Chromatic aberration means that the optical system ("lens") fails to focus all colours at the same place. All lenses have this to some extent, but some more than others. Most modern lenses are apochromatic (or better), meaning that light of three colours (wavelengths) will focus in the same plane. The correction in the region between these colours is also usually quite good, but it might get worse very rapidly outside this range. That is part of the reason for the OR focus marks on many lenses; the long-wavelength IR light focuses in a different plane than visible light.

With coatings that block UV (which all films are sensitive to) and IR (which only special films are sensitive to), the effect of the chromatic aberration is much reduced. But since chromatic aberration is a result of the lens construction - glass types, spacings and curvature - the aberration is still present.

Using a narrow-band filter also removes the effect of chromatic aberration, whih is why it is often recommended to use a yellow filter when shooting with one healf of a convertible lens. Any colour filter will do, and narrow-bandpass filters best of all.

[Dan Fromm]

Ah! So you were thinking of the invisible but photographically important (sometimes) parts of the spectrum.

The dread yellow filter, though, works on the visible.

Thanks for the clarification, I'd wondered whether you'd gone all mystical on us.

[Ole]

Thanks for the clarification, I'd wondered whether you'd gone all mystical on us.

Me mystical?

It's more like having a stereo with a bad speaker which rattles with deep bass notes: It may sound perfectly good as long as you don't play music with deep bass notes, but the problem hasn't gone away just because you play only string quartets...