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

Question

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 1

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.

Answer 2

How it is possible?

If you didn't a mistake, most probably Nikon 18-200mm isn't 200mm at the long end. It isn't a surprise at all. Many times the companies 'adjust' the numbers in order to match to something round, to something standard, to something already known in the market.

Most of the time, this isn't a big problem - usually the photographers look for other things (sharpness, lack of distorsions etc.)

For example, from the lenses labelled "70-200mm f/2.8", Canon and Nikon have exactly 200 mm but Tamron is 186 mm and Sigma is 190 mm.

Also, the Nikkor 70-200 VR II is worse than the Nikkor 70-200 VR on this aspect, with an effective field of view of around 135mm at 200mm at the minimum focusing distance.

Manufacturers do this all the time, but usually stay within 5% of the claim. It’s considered a bit of cheating when they are further off than that (the Sigma 50-500 is an example that people often talk about, it’s really a 470mm, but that’s still way under 10%).

Answer 3

The Nikon 18-200 f/3.5-5.6 uses an internal focussing design. This means that in order to focus closer, it moves interior optical elements rather than moving the entire lens assembly away from the sensor. This design means the lens doesn't have to extend outward during close focus and can focus much closer than a conventional focus design, but the trade off is that the effective focal length of the lens becomes shorter as you focus closer, resulting in a wider field of view than expected for the nominal focal length.

You will probably find that the fields of view for both lenses will be very close when set to the same focal length and focused at infinity.

Answer 4

Both lenses, most lenses, even very expensive ones, exhibit "focus breathing". Its a terrible term that just means: the actual focal length the lens provides depends on how close you focus. There are youtube videos that will give you a demonstration of this effect.

Focus breathing means that lens manufacturers tell you "This is a 55mm -200mm fl lens", but when we take the lens home and focus at something far away, say half a football field, we see that the 200mmm end is about 200mm. But when we focus very close, 5 feet, say, we find that even if we set the lens to 200mm, its actual FL is a somewhat shorter.

While I was shopping for a lens recently, I looked up focus breathing for the nikon 70-200 F/2.8, it does have significant focus breathing when focusing close, even for a very expensive (to me) lens. Nikon doesn't state this in their specs, but one reviewer found that this lens is about 140mm max when focusing close. Some wedding photographers were furious at this since they could not move close and get a closeup of, say, a face, since focusing close the lens is only 140mm but they expected close to 200mm.

Back to your question: your two lenses exhibit different amounts of focus breathing, since they are very different design lenses. Focused at infinity, which is they way Nikon rates their lenses, they are probably both very close to stated range. But focusing close, neither lens provides actual 200mm fl. It looks like the lens used to take the lower image is exhibiting much larger breathing.

Try another test:

Go outdoors, measure off 150 feet from something of interest, e.g. a tool shed in your back yard, or a light post, and take a picture with each lens, both set to 200mm, then post the results. I will bet that the images will be a lot more similar.