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04-06-2009, 04:16 PM   #16
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QuoteOriginally posted by newarts Quote
The relationship between distances, focal length and magnification m is:

Sensor_subject_distance = focal_length(1+m)(1+m)/m

Lens_subject_distance = focal_length(1+1/m)

The first relationship holds for all lenses, while the second one is more difficult to apply because it is hard to know the exact location of the optical center of a complex lens (eg. it moves with internal focus lenses).

The number written on the side of the lens (like ,43m) is the minimum sensor_subject distance.

Iowa Dave

PS these are easy to derive from the thin lens equation and the definition of magnification:

1/Focal_length = 1/Sensor_lens_distance + 1/Lens_Subject_distance

m = Sensor_lens_distance/Lens_subject_distance = Image_size/Subject_size
These formulae are correct for a "simple" lens, but not necessarily accurate when it comes to distance between the front of the lens and the subject because of complex optical designs that move the focal node of the lens further to the front. Each lens designer can have created a different node point for the lens. All telephoto (not long focal length) lenses have the node in front of the physical lens.

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