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the tilt being represented by the inclination of the baseboard from the horizontal, or of the plate from the vertical. This angle is measured in degrees, and can be distinguished as the angle A.
(2)
The focal distance from lens to plate when taking the original, which distance can be represented by ^{u} v.(3) " The focal distance from lens to negative when copying, which distance can be represented by V.
^{11} When working with near inclined surfaces the question of focus is an important one, and while a small stop will do a great deal, we cannot expect it to produce perfect focus if negative and copy are inclined in the same direction. This is, however, an essential condition of correction if the distance V is less than the distance v, hence V should always be greater than v ; the nega-tive and copy are then inclined in opposite directions, which is a condition conducive to good focus." If the negative and copy are inclined to angles proportional to their respective focal distances from the lens, perfect focus is secured without stopping down, provided we have a good flat field lens. But to do this, and at the same time secure perfect correction, we must use a lens the focal length of which is greater than
v, which fact prevents the use of the original lens. Also, we must enlarge to a certain precise scale. The use of the stop will free us from these inconvenient restrictions, but it is obviously desirable to depend upon it as little as possible, and for this reason it is always advisable to enlarge the image slightly in the copying process. By enlarging the image the angles are reduced, and more nearly approach to the correct proportions for perfect focus; but by reducing the image the angles are enlarged and diverge farther from the proper relative proportions. If we increase the focal length of the lens used for correction, the angles are increased for any given scale of copy-ing, but the increase is compensated for, as they also approach more nearly to the proper relative proportions for good focus. The most favourable conditions are secured by enlarging slightly with a lens of slightly increased focal length, or by enlarging on a rather bigger scale with the original lens."The following formulae are sufficiently true for all practical purposes, but the exact formulas require the use of trigono-metrical ratios instead of the simple measurement of the angles in degrees.
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