The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Lens
be focussed sharply at the centre of the screen, and the image examined with an eye-piece. If a halo is seen round the flame, it may be assumed that some spherical aberration is present. The objection to this method is that the fringes may not be due to spherical aberration, but to a kind of halation or ghosts. Another test, is to fix to a window two small, dark-coloured wafers, with their edges just in contact; the camera is set up at least ten times the focus of the lens distant, and then the images sharply focussed as stated above, with full aperture; on the insertion of a small diaphragm there should be no increase of sharpness. Another method is to proceed as above, and affix to the centre of the front lens of a doublet or the centre of a single lens a piece of black paper three-fourths of the diameter of the lens; the image which is now formed by the margins of the lenses is sharply focussed. The black paper is removed, and a small diaphragm inserted, and the image examined; if it requires refocussing by racking in or reducing the distance be-tween lens and screen, the lens suffers from negative spherical aberration ; if the camera requires racking out, positive spherical aberration exists. Chromatic aberration, or practically non-coincidence of the visual and actinic foci, is rarely found in modern lenses. No lens is actually achromatic - i.e., without colour - because the finest lenses corrected as much as is possible show fringes of colour, as may be at once practically proved by any one in the following manner: - Arrange a thermometer with a naked bulb, so that a spot of light is reflected from the mer-cury, and focus this spot of light (the optician's artificial star) on the focussing screen of plain glass by the aid of a magnifier; on racking the camera in slightly, a faint fringe of green is seen surrounding the spot, and by racking out beyond the focus a fringe of reddish purple comes into view. This is best seen with an eye-piece. To test, however, whether the visual and actinic foci actually coincide, place the newspaper previously described squarely opposite the lens, but leaning away from it; focus sharply, with full aperture, on one particular line of type - focussing must be effected by placing the screen in the dark slide, or the fact of non-coincidence of the plane of the focussing screen and sensitive plate may cause error ; then on exposing a plate, if the same line of type as • focussed does not come out the sharpest, it is proved that the lens possesses a distinct
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