I have a Nikon DSLR D5500 and a 18-55mm lens and a 50 mm lens. I was calibrating the camera to get the pixels per inch using each of the lenses and all other factors like the distance from the object is same in both cases. However, I get different pixels per inch values when I set the focal length to 55mm and 50mm lens. It is acceptable and that's what shall happen. But is there a way how I can correlate and come up with a factor that can be used in future to get the ppi for 55mm lens using 50mm lens image?
PS: I am calibrating the camera to know the size of object from the image.
Let me give an example to clarify my question: I image a ruler with same camera settings, same distance from the object, same angle but just two different lens - a 50 mm lens and 18-55mm lens set to 55mm focus. As I have markings on the ruler, I try to find how many pixels are in 1 inch of the scale so if I image some other object with all same settings I can know the physical size of the object from the image. However, by doing this the number of pixels occupied per inch of physical scale change in both images. And taking a ratio I can probably know the correlation between use of each lens and how many pixels are occupied per inch of physical scale. But I want to know if there is any correlation when I change the lens? Say now instead of 55mm I want to use a 200mm lens and want to know the size of the object. But instead of using a 200mm lens physically, can I use the same 50mm lens calibration and use some standard factor which I can scale to whatever focus I want (200mm, 35mm, etc.) I hope this clarifies my question.