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Hi Robert, A 67 camera uses roll film and produces a frame size that measures 60mm x 70mm. To select a lens focal length you need to know:
A lens about equal to the diagonal measure simulates the human experience. That means the angle of view obtained will about equals to that view as seen by our unaided eye which is said to be about 53ยบ. A camera fitted with a lens focal length about equal to the diagonal measure of the frame accomplishes this goal. Thus the three important frame measurements:
60mm or 2.36 inches wide
70mm or 2.76 inches wide
92.2mm or 3.6 inches diagonal measure (calculated using Pythagorean theorem) Thus a normal fixed lens of this format camera is about 100mm which is about 4 inches. Longer falls in the telephoto reign we might choose something over say 135mm
For wide angle we would choose something shorter than 75mm. For a zoom we would choose one that centers up on 100mm. It is generally accepted that for portraiture we choose a lens 2.5 x the diagonal (Hollywood chooses 3 X). This focal length duplicates the human perspective experience of self-image i.e. the mental self image one sees via the make-up or shaving mirror. Portraits made this way sell best. Thus 2.5 x 92mm = 230mm about 9 inches The above portrait stuff is true and is derived:
We shoot a frame; to preserve the true prospective the image is best viewed from a distance about equal to the focal length of the taking lens. Thus if we were to use a 9 inch lens we need to view a contact print placing it about 9 inches from our eye. We might need to use a magnifying glass to allow this to happen. The good news is, follow this rule and all distortions due to taking lens focal length are negated. The bad new is this rule is sometimes impartial to follow. However we can come close if we chose a lens 2.5 times the diagonal. As an example we use the 6x7 format camera to make a portrait using a 9 inch lens. We choose to make a print 8 inches x 10 inches. We must enlarge the 6x7 by magnifying it 3.7 times.
2.36 inch width x 3.7 = 8.7 inches
2.76 inch length x 3.7 = 10.2 inches
We crop to 8x10 So the magnification factor 3.7x
The viewing distance by the above rule is 3.7 magnification times the 9 inch lens = ideal viewing distance is 33 inches
Consider your customer viewing his/her 8x10 on the wall or desk. Same apples to a 35mm camera:
Frame is 24mm x 36mm or 0.94 inches x 1.5 inches
To make 8x10 magnification required is 8.5x
Image size becomes:
0.94 x 8.5 = 8 inches
1.5 x 8.5 = 12.75 inches (we crop to 8x10) The 35mm diagonal measure is 43.3mm
2.5 x 43.3 = 108mm (the recommend portraiture focal length is rounded to 105mm or 4.13 inches) If we use a 105mm lens and the magnification to make an 8x10 is 8.5 than: 4.13 inches times 8.5 inches = an ideal viewing distance of 35 inches. More marginal technical advice or gobbledygook from
Alan Marcus
ammarcus@earthlink.net
November 20, 2007
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