[Brightcolours]

24mm is 24mm.... Uncorrected one would call it "fisheye", and will have a wider FOV than corrected. If you correct barrel distortion, you crop "the corners" and end up with 24mm corrected FOV.

No, it is not a fisheye. Neither is the barrel distortion all that bad.
Have you actually shot with any of these lenses - I actually have.

I was explaining to you how it works.
Totally uncorrected gives a wider FOV (see fisheye vs rectilinear).
Not totally corrected sits in between. 
When you correct such an image, you end up with 24mm rectilinear FOV.

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I have always hated the discussion about focus breathing. Although this is a fine trait for video, it actually mean sthat teh FL is decreasign when focusing more closely.
I have never considered it a great treat for stills, or, IOW, photography.

It does not quite mean that. Focus breathing means you see the FOV widening and narrowing when focussing back and forth, like a chest widens and narrows when breathing. It can also mean that the FOV narrows when focussing more closely, or, indeed, that the FOV widens when focussing more closely.

Somebody started throwing aroudn this term at soem stage, and somehow a lot of people forgot that there really is a big difference between stills photography and video or filming.
I really hate it when you start at 200 mm far away, and end up with 135 mm in close-up.

You hate that it is called focus breathing and you hate focus breathing itself?

Focus breathing is a load of crap, or nonsense, IMO.
A lens created to prevent focus breathing, which happens to be a totally nonsensical term, effectively means that it shortens it FL when focusing closer.

Not quite, see above. (FOV can widen (RF 70-200mm f2.8 IS USM), but it can also get more narrow (EF 70-200mm f2.8 IS USM III)) when focussing closer. Both FOV change is focus breathing, the view breathes while going through (part of) the focus range.

It is something I have hated for stills photography whenever it was applied, and it with macro lenses it gets even worse, because it means that you are left with less space between your subject and your setup - the FL is shorter after all.

It should have been called focal shortening upon focusing, or something like that, because that is what it really is.

No, what it really is is a change in FOV (breathing) when going through the focus range (hence focus breathing).

In zoom lenses it is a lot worse, i.e., focal shortening, because of the inherent design compromises that are made. Funnily enough, a few years ago a lot of fuzz was made of zoom lenses that shortened FL when focusing closer. I remember the discussions about the Nikon 70-200, with the 200 mm end becoming 135 mm at shortest focusing distance, vs the Canon 70-200, which stayed around the 190-195 mm mark or thereabouts. Nowadays it is acceptable, because it means it suffers less from "focal breathing".

That Nikon is the Nikkor AF-S 70-200mm f2.8 VR II. The Canon RF 70-200mm f2.8 IS USM now is as bad or worse.
That Nikkor had its FOV widen towards MFD (focus breathing). That EF 70-200mm f2.8 IS USM had its FOV narrow towards MFD (also focus breathing).
Also notorious for the negative focus breathing issues are/were all the superzoom lenses, like the Nikkor AF-S 18-200mm VR (and Sigmas and Canons and Tamrons of the same ilk).

Why is that an issue at all, with stills photography?  Or even with video for that matter, as in the olden days nobody was bothered by it. To me, "focal breathing" looks good, because it shows that you are getting in close-up. The movement that "focus breathing" causes, makes it intrinsically clear that you are gettign closer. But maybe that is just old-fashioned me.

In video the issue is clear. Imagine a scene with a conversation of two people, who are not on the same focal plane. The director wants to go back and forth in focus depending who is speaking. All during this scene you'd see the FOV breath in and out.

In stills photography, the issue is a bit different, and is mainly centered around the widening FOV issue. You can get a more narrow FOV with above mentioned EF 70-200mm, than possible with above mentioned AF-S 70-200mm (or RF 70-200mm). Less possible is less desirable.

Lenses designed primarily for still photography almost universally suffer from focus breathing. Convention would dictate that the nominal focal length would be at infinity focus.
It follows that, since most lenses breathe to some degree, exact focal length is rather academic - more so in zoom lenses.

True Cine lenses are a different kettle of fish.

IMO still photography lenses which are designed properly do not suffer at all from anything, They have the great ability to not have any FL shortening when focusing closer, unlike most modern zoom lenses, or CINE lenses ....

Kind regards, Wim

A lens which does not change focal length when going through its focus range shows focus breathing. The FOV will narrow towards MFD (and widen towards infinity). Reason: you move then lens to focus, but the image plane remains at its spot. The change in FOV is not the result of a change in FL, in that case.

If a lens shows a widening towards MFD, there is a big fall in FL. If a lens has no focus breathing, there is a moderate fall in FL. If a lens is like a Canon EF 70-200mm, it has a slight fall in FL towards MFD, and shows a clear narrowing of FOV. If a lens has no fall in FL, it shows an even stronger narrowing of FOV.