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I am using two lenses - Tamron 75-250mm F/3.8-4.5 Model 104A and AF-P DX NIKKOR 70-300mm.

When shot with lenses set to 250mm of focal length, the field of view for the Tamron lens is greater than the Nikkor lens. The camera used was Nikon D5200 with a sensor size of 23.5mm X 15.6mm.

It is commonly understood that the focal length decides the field of view (similar discussion in this forum), but here is a case where the field of view is different in different models of lenses with identical focal length.

Here is an image of a scale/ruler with markings shot with both the lenses from a distance of 3m(distance between the plane of interest and image plane marking on the camera body) enter image description here

How can this be explained?

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    \$\begingroup\$ There are probably multiple factors to the full explanation, but one of the primary ones is that the stated focal length on almost all lenses is approximate - a "50mm" lens may actually be a 47mm lens or a 53.4mm lens... \$\endgroup\$
    – twalberg
    Commented Feb 26, 2019 at 19:55
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    \$\begingroup\$ "From a distance of 3m" — measured from where, precisely? The image plane mark on the camera body? The tripod mounting screw? The front of the lens? \$\endgroup\$
    – scottbb ♦
    Commented Feb 27, 2019 at 12:46
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    \$\begingroup\$ Yet another good reason photographic cameras are not measuring devices.... \$\endgroup\$
    – mattdm
    Commented Feb 27, 2019 at 15:01
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    \$\begingroup\$ @karthikeyan Image plane alone is not enough for setting accurate object-to-lens distance because lenses have different thicknesses (as in, the distance between front and rear principal points). Without compensating for that, your two images were effectively taken at two different object distances. Though like I said, probably negligible outside macro. Others have already answered what are probably the most significant causes of the differing images. \$\endgroup\$
    – relatively_random
    Commented Feb 27, 2019 at 18:49
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    \$\begingroup\$ If you wish to measure the actual focal length, object distance, distortions etc., look into camera resectioning (also called camera calibration, but this term is also used for pixel intensity calibration). Basically, you take a bunch of images of a known object with easy to find points and let an algorithm calculate all the geometric camera parameters for you. Fun subject. \$\endgroup\$
    – relatively_random
    Commented Feb 27, 2019 at 23:08

3 Answers 3

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There are at least two issues involved here:

  • Focal lengths, particularly with zoom lenses, are approximated to the nearest "standard" number. You'll almost never see a lens marked as a 242mm or 256mm lens, even if that is their actual focal length. They'll both be sold as 250mm lenses. In the case of a 70-300mm lens, the mark on the barrel for 250mm may not be in the precise spot it should be, either.
  • Focal lengths are measured when a lens is focused at infinity. That is, the focal length is measured based on how far collimated light striking the front of the lens will be focused behind the lens. As lenses are focused on distances closer than infinity, their angles of view (AoV) often change.

Two lenses that have the same FoV at 200mm focused on far objects may have very different FoVs when focused at closer distances. The AF-S Nikkor 70-200mm f/2.8G VR II, for example, was notorious for giving a roughly 145mm AoV when set to 200mm and focused at the lens' minimum focus distance (MFD). In contrast, the competing Canon EF 70-200mm f/2.8 L IS II gave an approximately 195mm AoV at a similar MFD. The newer AF-S Nikkor 70-200mm f/2.8E FL VR also gives an AoV much closer to what one would expect from a 200mm lens, even when focused at shorter distances.

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  • \$\begingroup\$ I see the comments as well as your answer mentioning how the barrel markings are wrong. But by how much per cent are they off typically? And what is the way out? I intend to use an alternative to the Tamron lens mentioned, as it is affected by Fungi. Anticipating a better perfomance from the 75-300mm lens left me disappointed as the magnification is subpar. Any suggestions? \$\endgroup\$
    – karthikeyan
    Commented Feb 27, 2019 at 7:26
  • \$\begingroup\$ @karthikeyan Every lens is different. The only way to know is to measure it, or find a report by someone else who has measured the same lens. Why is this level of precision so vital? \$\endgroup\$
    – Michael C
    Commented Feb 28, 2019 at 17:25
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From Wikipedia

One issue internal focusing lens can have is that the true focal length of the lens is reduced when not focused at infinity.

And the Nikon seems to be a lens with internal focus, and in your picture it has a slightly wider FOV (and therefore a shorter actual length).

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  • \$\begingroup\$ Also, the opposite is true for unit focusing lenses - their focal length increases when you focus closer. Calibrated focal length regardless of focus is what makes cine grade lenses so godperished expensive (probably a balancing combination of internal and unit focusing in these)..... \$\endgroup\$
    – rackandboneman
    Commented Feb 26, 2019 at 22:22
  • \$\begingroup\$ @rackandboneman By definition, unit focusing lenses don't change when focusing. All optical elements move as one fixed unit. So focal length can't change. What does change, if you keep the camera body fixed, is the distance between the object and the lens. So the focal length is not changing, you're just effectively moving closer towards the object when you focus closer. \$\endgroup\$
    – relatively_random
    Commented Feb 27, 2019 at 18:17
  • \$\begingroup\$ It literally does - the focal plane moves further away from the focal point, and this has a bearing on field of view too.... \$\endgroup\$
    – rackandboneman
    Commented Feb 27, 2019 at 21:09
  • \$\begingroup\$ @rackandboneman "the focal plane moves further away from the focal point, and this has a bearing on field of view" Yes, but this has nothing to do with focal distance, which is a property of the lens itself and doesn't change if you move the lens forwards, backwards, sideways or put it on a shelf. \$\endgroup\$
    – relatively_random
    Commented Feb 27, 2019 at 22:57
  • \$\begingroup\$ An internal focusing lens on a shelf would also only be focused on anything at all if there was a sensor, film, or screen? Or are you using "focused" to mean "a focusing distance has been dialed in that would result in focus if there was anything at the focal plane"? \$\endgroup\$
    – rackandboneman
    Commented Feb 28, 2019 at 8:36
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The answer is NO But the reason you are seeing this is normal. In almost an focal length each manufacturer is govimg a different TRUE focal length close to that number labelled. 10% variation exists in almost every focal length if you look at 3-4 makers. 35 mm 50 mm 200 mm

Not to mention zoom are even more varying

Then there is near focus focal length may not be infinity focal length. Much like max AND min change with zooming

Lastly very close up internal focus versus ext focus can vary as distance to film may be same but elements could be much closer (more in macro situations)

Basically, you have 10-15% difference and this is normal and within the variation from one maker to another. It would be EXTREMELY rare to happen with two copies of the same lens.

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