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?
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