In normal black and white photography, almost all the visible spectrum of light is used. To get a sharp picture, the lens must be able to project an image on the film where all the color components are in the same plane, the light sensitive emulsion of the film. For long lenses this is seldom true. Only very expensive tele lenses with low dispersion optical glass have a reasonably low "longitudinal chromatic abberration".
For normal tele lenses, the different color components of the picture are actually focused on the film, behind the film or in front of the film. For instance, the red components are behind the film when light rays in the middle of the spectrum, the green parts, are focused onto the film. One could say the that the focal lenght of the lens is a little longer for red light than for green light. At the other end of the spectrum, the blue light rays are also focused behind the film.
Now, what about apo-chromatic lenses? They should have no chromatic abberration, the manufacturers say. Well, they are better than ordinary lenses but the same problems still exist even if they are smaller. In the figure below I have copied the curve for a very good lens, an Apo-Rodagon 1:4/80 mm enlarging lens from Rodenstock. These types of enlarging lenses have much lower color abberration than ordinary camera lenses. But even here one can see how the focal lengths varies over the wavelength.
The calibration point is around green light, 550 nm. At deep red, 644 nm, the focal length has increased by 10 um, that is 0.01 mm. At blue light, 405 nm, the difference is about doubled. Of course these deviations are small, mostly within the detectable depth of field. But when all parameters are optimised and the film is really low grained, such as Technical Pan, the perceived depth of field is much smaller than normal and the requirements are correspondingly higher.
As mentioned earlier, for cheaper tele lenses, the abberrations are much higher, maybe ten times higher than in the fine enlarging lens above. So something has to be done to get sharper pictures.
My simple solution is to use only one little segment of the spectrum, preferrably the red part. One advantage with the red part is that red light penetrates the haze (water vapor) much better than blue light. This creates higher contrast in distant views. When using red light, the sky in landscape photography is often more beautifully rendered with a darker tone. The foliage will often come out light and lucent. Without any filter, foliage is often rendered too dark because normal panchromatic film has lower sensitivity in the green spectrum. Surprisingly, the foliage has often high reflectance in the deep red or even infrared spectrum.
Normal black-and-white film has very low or no sensitivity outside 600 nm. So the interesting deep red part of the spectrum would give no picture at all if a deep red filter were used with normal film. The Kodak Technical Pan film however has the required extended red sensitivity.
To select the desired part of the spectrum, a dark red filer is used over the camera lens. The Kodak No. 25 red and No. 29 deep red are examples of such filters. There are also filters from B&W, Hoya and others available. Number 091 from B&W is a very deep red filter that I use myself.