There are several considerations that could be affecting the results of your test:

• Focal lengths for most lenses are measured with the lens focused to infinity. Depending on the design of the lens the field of view may change significantly when focused at much closer distances.

An example: Nikon's 70-200mm f/2.8 VR set at 200mm and focused at MFD gives an FoV of only about 145m, but Canon's 70-200mm f/2.8 L IS II set at 200mm and focused at MFD gives a 196mm FoV. The difference is that the Nikon lens has internal focus elements located in front of the back focal plane (the crossover point inside the lens) while the Canon design uses rear internal focusing in which the elements are located behind the back focal plane.

• Focal lengths for many zoom lenses are rounded to the next "standard" focal length. Have you ever seen an 84-183mm zoom lens? If you have you didn't realize it, because the manufacturer would have likely marketed such a lens as an 80-200mm. With such a lens, the EXIF data will likely report the focal length as 200mm when the actual focal length was only 183mm, because the lens is programmed by the manufacturer to report "200mm" when at the maximum focal length of 183mm.

• The markings on the barrel of many zoom lenses are less than precise in the middle of the lens' zoom range. If you line up the 55-300mm lens barrel with the 200mm mark and take a photo what does the EXIF for a shot taken say the focal length was? Don't be surprised if the markings on the barrel are a little off.

• If you handheld the shots of the doorknob taken from about 3 meters you may have had slight variances in distance. After all, you changed lenses in between each shot. Unless you use a tripod or other stable mount with the camera securely attached while changing lenses you can't be sure the shooting distance was exactly the same.