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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?

mattdm
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karthikeyan
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    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... – twalberg Feb 26 '19 at 19:55
  • Other factors that come to mind are distortions and principal point positions, but the latter is probably negligible outside macro photography. – relatively_random Feb 26 '19 at 20:04
  • And one more factor is mark on the lens barrel is more or less wrong – Romeo Ninov Feb 26 '19 at 20:15
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    "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? – scottbb Feb 27 '19 at 12:46
  • @scottbb the image plane marking on the camera! Thanks for pointing it out. I will update my question. – karthikeyan Feb 27 '19 at 12:52
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    Yet another good reason photographic cameras are not measuring devices.... – mattdm Feb 27 '19 at 15:01
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    @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. – relatively_random Feb 27 '19 at 18:49
  • @relatively_random thanks! I felt using a distance of 3m would compensate for differing barrel lengths and lens thicknesses as those sizes are negligible compared to 3m. – karthikeyan Feb 27 '19 at 19:07
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    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. – relatively_random Feb 27 '19 at 23:08

3 Answers3

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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.

Michael C
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  • 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? – karthikeyan Feb 27 '19 at 07:26
  • @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? – Michael C Feb 28 '19 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).

xenoid
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  • 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)..... – rackandboneman Feb 26 '19 at 22:22
  • @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. – relatively_random Feb 27 '19 at 18:17
  • It literally does - the focal plane moves further away from the focal point, and this has a bearing on field of view too.... – rackandboneman Feb 27 '19 at 21:09
  • @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. – relatively_random Feb 27 '19 at 22:57
  • 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"? – rackandboneman Feb 28 '19 at 08:36
  • @rackandboneman Focal length or focal distance is a property of the lens, not the imaging system. It is a measure of how strongly the beams of light get bent by the lens. A unit lens doesn't have a mechanism for changing this focal length so an object at a fixed distance from the lens creates a sharp image at a fixed distance on the image side. If you want to focus that kind of an imaging system, you need to put the sensor at that exact distance from the lens where the sharp image is. This is usually achieved by a built-in mechanism that moves the entire lens relative to the sensor. – relatively_random Feb 28 '19 at 22:04
  • @rackandboneman An internal focusing lens, on the other hand, doesn't rely on this movement to focus the imaging system. It provides a mechanism for changing the optical characteristics of the lens itself so that the image moves towards the sensor. Since the lens itself gets modified and not just moved around, focal length (in other words: light-bending power) can change. – relatively_random Feb 28 '19 at 22:15
  • @relatively_random does field of view change when moving a unit-focusing lens into focus or does it not? – rackandboneman Feb 28 '19 at 22:23
  • @rackandboneman Yes, both methods of focusing an imaging system can produce field of view changes or breathing. But that doesn't necessarily mean that the focal length is changing. In fact, a unit lens can't change its focal length by definition. Breathing there can only possibly be caused by changes in relative distances between the object, the lens and the sensor. – relatively_random Feb 28 '19 at 22:26
  • @rackandboneman And in both systems, breathing can be compensated for. In order to do it with a unit lens, though, the focusing mechanism would need to move both the sensor and the lens at the same time. I don't know if such systems are used in practice. – relatively_random Feb 28 '19 at 22:31
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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.

RoseEatsRice
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