Dependence of the viewing angle of the lens on the focusing distance
When focusing on some lenses you can notice a very interesting effect. Many lenses change (usually reduce) the field of view when focusing from infinity to MDF (minimum focusing distance). In Western literature, this effect is called ‘Focus Breathing’.
This can be easily checked, it is enough to turn off the auto focus on the lens and focus on anything from infinity to MDF, and you can see that the frame borders are expanding and tapering, and there is nothing to blur the boundaries.
Lenses that give the same viewing angle when focusing on infinity can give different viewing angles when focusing on MDF. Here is an example from three 85 / 1.8 class lenses. All shot from a tripod with the same focusing distance.
85 85 85
This effect is very noticeable 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 Nikon 105mm f / 2.8D AF Micro-Nikkor and Tamron SP AF 90mm 1: 2.8 Di MACRO 1: 1 272E for Nikon macro lenses. For a simple understanding, we can say that when focusing the lens makes an easy zoom. This also applies to the fix lens and zoom lenses. Visually, it is easy to understand the effect using the example of the Tamron SP AF 90mm 1: 2.8 Di MACRO 1: 1 lens fix, in which the trunk is extended when focusing in macro mode, as if the lens performs a zoom, and the field of view also changes.
Changes in the field of view when focusing sometimes unnerve at times, since this leads to the fact that parts of the desired composition fall outside the frame, and you have to approach or move away from the subject. When I shoot a macro, this effect is especially noticeable. Visually, it seems that the focal length of the lens changes, while they say that the lens changes its focal length – this is not quite so. The focal length is measured when focusing at infinity, when the rays go parallel to the optical axis, but when focusing in the direction of MDF, it is no longer possible to talk about focal distance.
The effect described above is not always noticeable in practice and you should not worry much about it. What is ‘Focus Breathing’ well known to videographers, who are more annoyed by this effect than photographers 🙂