When focusing on some lenses, a very interesting effect can be noticed. Many lenses change (usually reduce) the angle of view when focusing from infinity to MDF (minimum focusing distance). In Western literature, this effect is called 'Focus Breathing'.
Dependence of the viewing angle of the lens on the focusing distance
This can be easily checked, it is enough to turn off the automatic focus on the lens and focus on anything from infinity to MDF, while you can notice that the frame borders expand and contract, and there is nothing to blur the borders of.
Lenses that give the same viewing angle when focusing on infinity can give different viewing angles when focusing on MDF. Here is an example with three class 85 / 1,8 lenses. Everything is taken from a tripod from the same focusing distance.
This effect is very highly visible when working with macro lenses that change the maximum aperture value depending on the focusing distance. For example, you can read about it in the review of macro lenses. Nikon 105mm f / 2.8D AF Micro-Nikkor и Tamron SP AF 90mm 1: 2.8 Di MACRO 1: 1 272E for Nikon. For a simple understanding, we can say that with focusing lens makes easy zoom. This also applies to fixed lenses and zoom lenses. Visually easy to understand the effect of the example of a fixed lens Tamron SP AF 90mm 1: 2.8 Di MACRO 1: 1, in which the trunk lengthens when focusing in macro mode, as if the lens were zooming, and at the same time the field of view changes.
Personal experience
Changes in the field of view when focusing is sometimes very unnerving, as it leads to the fact that parts of the desired composition fall out of the frame, and you have to move in or out of the subject. When I shoot macro, this effect is especially felt. Visually, it seems that the focal length of the lens is changing, while they say that the lens changes its focal length - this is not entirely true. Focal length is measured when focusing at infinityb, when the rays go parallel to the optical axis, but when focusing toward the MDF, it is no longer possible to speak of the focal length as such.
Conclusions
The effect described above is not always noticeable in practice and you should not worry too much about this. What is 'Focus Breathing' is well known to videographers, who are much more annoyed by this effect than photographers :)
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Material prepared Arkady Shapoval.
if the focal length 135mm is written on the lens, then it will be 135mm on the crop and on the plane !!! only the angle changes, the crop cuts the frame in view of reducing the size of the matrix and visually it seems that the lens is moving closer, the focal length never changes to which camera you put it on.
Just this effect made me almost turn gray!
I took test shots from 18-105 and from 70-300 (on focal lengths 70 and 105) I got 4 completely different shots !!! Indeed, as if the focal length is changing. I already thought that the DX 18-105 was immediately indicated by the equivalent focal length, but in time I realized that Focus breathing was to blame for everything.
Hello, why then dx lenses, if fx can be used on full frame and crop?
To reduce size and cost
Thanks for the answer and for such a helpful article!
There is so much to say here. Traditional primes breathe forward, many zooms breathe backward, and strongly. (I remember the comparison of the Tamron and Canon 70-200, where at 200 the difference in the actual focal length at a portrait distance can be really huge). Then zooms began to rule, at least on the long side. For example, if you look at how 24-105 class zooms work, many of them hardly breathe at 105, and as the focal length decreases, they begin to breathe more noticeably. They started doing this even before any video, because otherwise the effect would have been quite noticeable. It would simply steal the focal length, and not only at macro distances. In general, the longer the focal length, the more pronounced the effect. This can have consequences in that telephoto primes are actually longer than zooms if you are shooting small animals at relatively close distances. On the other hand, lenses that breathe forward also lose in aperture (the aperture does not become larger). It is also possible that this is one of the reasons why zooms do not feel like fixed lenses. For example, someone will buy a 24-70 and figure out “well, 70 mm is almost like an 85 mm fixed lens, I can live with such a difference, I will shoot portraits, crop a little”. But in reality, at a portrait distance, the 24-70 will have about 65 fr, and the 85 will have about 90-92, and this is already much more than the expected estimate of 15 mm.
You can roughly understand how the lens will behave if you look at the MDF and the maximum magnification.
On the left is the Sigma Art 70-200, which breathes forward, on the right is the Tamron second version, which is stable, which is known for its cruel breathing backward. With the Tamron, you can focus noticeably closer than a meter, but it still does not reach the magnification of the Sigma. For some reason, further than 7-8-10 meters it will not be noticeable. But if you decide to hunt for close-ups, squirrels, cats - it will feel like two different lenses.