تقدير الرقم البؤرى الجديد بعد وضع العدسات الإضافية أمام العدسة الأصلية.. آلة التصوير

Estimating the new f-number after placing the additional lens in front of the original lens

As a result of placing the additional lenses in front of the original lenses, as we have previously shown, the focal length of the two lenses will change, and the focal number .f.no will change accordingly. In view of the importance of knowing this new focal number for the photographer when he evaluates the exposure factors, or when he searches for the appropriate depth of field for the new focal number, so we discuss in the following the bases for calculating this new focal number, whether the additional lens is negative or positive.

And since the additional positive lens, for example, may sometimes increase the relative area occupied by the image on the sensitive film, and at other times reduce it (according to the previously explained rules on pages 167, 168, 169), so we also see that the new focal number may increase or decrease at other times according to For shooting conditions and the nature of the design. the machine. This is what we explain as follows:

First: If the distance between the lens and the image remains exactly equal to the focal length of the original lens:

If the original lens in the camera was set at an infinity distance (i.e. the distance between the lens and the image is exactly equal to the focal length of the lens), then the original focal number of the lens will not change, as the additional A position did not result in changing the distance and the film, just as the aperture that was set did not change. Light passes through. Thus, we find that the two factors upon which the estimation of the focal number was built remained constant and did not change. Therefore, for all practical purposes, the f-number is considered to be constant despite the position of the additional lens
(Theoretically, the focal number may vary slightly.)

Second: If the distance between the lens and the image, after placing the additional lens, differs from the focal length of the original lens, and the difference between them is large, as is the case with bellows cameras:

If the extra lens is placed in front of the original lens in any camera, whether it is the body:

(a) Far away, which causes a significant decrease in distance. The lens and the image are the focal length of the original lens (which is the case of the additional lens performing its function with the original lens as a single obtuse angle lens).

(b) Or was the object close, which calls for a significant increase in the distance between the lens and the image from the focal length of the original lens (which is the case of the additional lens performing its function with the original lens as a lens for close-up photography).

It is possible to estimate the new focal number according to the following equation:

The new f-number = the old f-number x the new distance between the lens and the image over the focal length of the original lens

If we put the following symbols:

F ` = the new f-number after the additional lens is put on.

F = the first focal number before the additional lens is put on.

r = the distance between the lens and the image.

p = focal length of the original lens.

p` = focal length of the additional lens.

p`| = The focal length of the two lenses (the original and the additional).

It is possible to put the previous equation in the following abbreviated form:
q` = q x r on p