302.15Figure 1-30.–Depth of field on camera focusing ring.The terms object focal distance and image focaldistance are often used for these conjugate distances. Itis obvious from these two terms that the object distanceis outside the camera and the image distance is insidethe camera. Since the focal length denotes only thedistance from its center to the image when focused atinfinity, we need some way to account for the fact thatwhen we focus on closer objects the image focaldistance can be much more than the lens focal length,with a corresponding effect on image size, effectiveaperture, and other factors.50mm +^{(50mm)}1= 100mmThe various ratios between image and object focaldistances may be determined by a formula that containsthe focal length of the lens and the ratio (scale) betweenthe image size and the object size.That is:F = the focal length of the lensR = the ratio between the image and object sizeor the ratio between the conjugate foci ofthe image and objectWhen R is determined by the following formula:R =ImagesizeObject sizeObject focal distance = F + F R)Image focal distance = F + (F x R)For a 1: 1 reproduction using a 50mm lens, your objectfocal distance is as follows:and the image focal distance is as follows:50mm + (50mm x 1) = 100mmWhen the image formed by a lens is smaller than theobject, the larger conjugate is outside the camera. Whenthe image formed is larger than the object, the largerconjugate is inside the camera.These conjugate focal distances have someinteresting relationships that may be used in severalways. The following examples illustrate the practicalvalue of these distance relationships:EXAMPLE 1: A4x5-inch copy negative must be madeof a 16x20 print using a camera equipped with a 10-inchfocal length lens.Figure 1-31 .–Conjugate distances.1-27

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