Focus breathing.

Started 3 weeks ago | Polls thread
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snellius Senior Member • Posts: 1,066
Focus breathing.

In the thread, "70-200 2.8 S focus breathing?" I found out that there are apparently two definitions for Focus breathing. I always thought that Focus breathing as the name implies is the term used to describe the increase or decrease in image size (breathing) when focusing. But to my surprise, focus breathing is also defined as changing the angle of view or focal length. The angle of view is completely clear, but that or focal length I find very confusing. On zeiss' Lenspire website they also flip it around and even mention the change in focal length first.

To me, the term focus breathing refers only to changing the angle of view when focusing. Whether or not the focal length is changed is of course closely related to this, but without information about the optical construction and distance between the principal planes, any change in focal length says nothing about what happens to the angle of view. Calling changing the focal length also focus breathing creates a lot of confusion. Most people will think that this works the same as when choosing a lens (the longer the focal length the narrower the angle of view and the shorter the focal length the wider the angle of view). But if focusing close makes the focal length shorter, it does not at all mean that this will increase the angle of view. Depending on the optical construction, this may be different. Moreover, if the focal length remains absolutely the same the angle of view will actually change. See the examples below.

It is clear from the above examples that in focusing, there is no uniform relationship between changing the focal length and changing the angle of view. This is probably why the extended definition also speaks of "angle of view or focal length". Without explanation, the word "or" actually creates immediate confusion, because is it now a choice where one excludes the other, or is it something that is still in doubt? One thing is certain, the definition for focus breathing which only talks about the angle of view is always clear.

So there are other parameters that "breathe" when focusing and thus affect the angle of view, such as the already mentioned changing of the distance between the principal planes, but also the ratio between the object and the image distance. Fortunately, these are not also called focus breathing. Not to mention the aperture value, the blur, the focus number, and the bokeh which also change with focusing. Any noticeable change in such parameters, especially the focal length, might as well be mentioned separately, without also calling it focus breathing.

I'm curious to hear what you guys think about this.

POLL
Focus breathing is only changing the angle of view when focusing.
14.3% 1  vote
Now that I read this I also find that focus breathing is only changing the angle of view when focusing.
14.3% 1  vote
Changing the angle of view or focal length when focusing is definitely a logical description for Focus breathing.
42.9% 3  votes
The focal length just now has a popular status within the term focus breathing, it is now too late to change it. That it is confusing does not matter.
14.3% 1  vote
I have no idea, but I'm curious to know what others think about it.
14.3% 1  vote
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