focal length - Why do two zoom lenses set to 200mm on the same camera produce different fields of view?

I've been testing out a Nikon 18-200mm F3.5-5.6 against a Nikon 55-300mm F4.5-5.6

I took two pictures of a door knob from an equal distance of about 3 meters away with the same Nikon d7100 DX camera but each with the different lenses both set to the same focal length of 200mm.

To my surprise, the image taken with the 55-300mm lens gave a considerably tighter crop than the one taken with the 18-200.

Is this normal, and if so how is this possible? I was under the impression that the field of view should be the same if both lenses are set to the same focal length. I'm asking this question because I bought the 18-200mm lens second hand and I'm worried there is something wrong with it even though it does appear to be fine.

Here are the example photos. Note the meta data on the images do confirm both were taken at 200mm. I was not using a tripod, so there was probably a slight discrepancy, but I did make sure to sit in the exact same position and leaning back against a wall in the same spot to take the photos. The difference in crop was immediately apparent

Answer

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.

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  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.

  
  



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  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.

  
  


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  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.