What does the graph of image distance and object distance show with lenses?
1 /image distance + 1/ object distance = 1/ focal length That is called the basic lens equation. When the object distance is twice the focal length of the lens the image distance is also twice the focal length of the lens and so a same-size image is formed. That is why for macro photography at 1:1 object/image ratio an extension tube is used to distance the lens from the film by an extra amount that is equal to the focal length. 50mm extension on a 50mm lens gives a 1:1 image with the lens 100mm from the film and the object 100 in front of the lens. 100mm extension on a 100mm lens gives a 1:1 image with the lens at 200mm from the film and thje object 200mm in front of the lens. and so on. A bellows unit can be used instead, giving more versalitily in changing the image size.
