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I have a Nikkor 28mm f/2.8 prime lens. I understand that this focal length is only valid when the focus is held at infinity. Is there a way to easily measure how the focal length changes when I adjust the focus? Is the change so small that it can be disregarded?

  • Are you assuming your lens a single element thin lens? Or a real world lens with multiple elements that do not all move by the same amount and in the same direction when focus distance is altered? – Michael C Mar 25 at 21:15
  • I'm assuming my lens is a Nikkor 28mm f/2.8 prime lens. – B. Erickson Mar 28 at 22:18
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Focal length is that measurement taken when the lens is imaging a distant object like a star. An artificial star is a pin-hole placed about 2000 focal lengths distant. The unit of optical measure called the diopter is commonly used in the optical business. A 28mm lens = 1/28 x 1000 = 35.7 diopters. As you close focus to about 1 meter (3 feet), the back focus distance lengthens about 1 diopter. Now this lens functions at about 34.7 diopter. Converting --- 1/34.7 x 1000 = 28.8mm

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  • This assumes a single element lens or lens in which the entire optical formula moves in unison when the focus distance is adjusted. This is far from the case with most modern lenses that use internal focusing or have focusing compensators or use other designs that do not move the entire optical formula in the same direction by the same amounts. – Michael C Mar 25 at 21:14
  • @ Michael C -- No such assumption, as you close focus the back focus distaance elongates. This now elongated lens to image distnace is commonly called the "focal lenght". – Alan Marcus Mar 26 at 1:13
  • And lenses with many elements and internal focusing often decrease their focal length as the focus distance is moved closer. Take the Zoom Nikkor AF-S 70-200mm f/2.8G VR II, for example. When zoomed to maximum focal length (200mm when focused to infinity) and focused at MFD, it only has a focal length of 140mm. – Michael C Mar 26 at 2:22

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