This is one cause of ill-formed foci. The light emitted by any natural object is always mixed light that can be analysed into light of very various colours, and as an uncorrected lens always performs such an analysis it represents one object point by a number of differently coloured image points at various distances from the lens, some being visible and many invisible. When visually focussing an object in the camera we place the focussing screen in the plane of the visible image points. The plate, which is afterwards substituted for the screen, is, however, very little affected by the visible rays, the developed image being formed almost entirely by invisible " actinic " or chemically active rays; hence, if chromatic aberration is present, the plate is in the wrong place and the resulting negative is blurred. The actinic rays come to a focus nearer the lens than the visible rays (provided the aberration is not over-corrected), and we can roughly adjust the plate to the former by pushing it nearer the lens; but in a corrected lens both visible and actinic rays have the same focus, and no such adjustment is necessary. A corrected lens is styled ' achromatic," or " apochromatic" if corrected for more than one set of actinic rays. With an uncorrected single lens the actinic focus is about 2 per cent, nearer the lens than the visible focus, but the latter focus is the nearer in an over-corrected combination. The test for achromatism is to focus carefully, then expose and develop. If the developed image is as sharp as the visible one the lens is achromatic.
The most convenient object for this test is a printed surface slightly inclined so that each line of print is at a different distance from the camera. If we focus on one line but find another is the sharpest in the developed image chromatic aberration obviously exists.