Terminology - Focus Breathing

The [link=https://www.pentaxuser.com/forum/topic/dfa-f2-8-70-200mm-63792 ]thread on the DFA 70-200[/link] refers to "Focus Breathing".

- What is this please, and
- what effects, if any, might it have on usability and photograph quality?

I'm going to have a go at this.
I think if you for example have the lens set at 200mm on a distant subject the lens does indeed give you the 200mm magnification that is claimed for the lens however if you then focus on a close subject with the lens still set at 200mm it gives you a lower level of magnification, not 200mm
As far as I'm aware this has zero impact on image quality as such, just your subject matter might be a little smaller in the frame than you might expect.
Now I wait to see if I've got it right.
Just noticed, you're another Suffolk Boy

Yes Nigel, and difference can be surprising. Several years ago I observed this using a 135 prime and the 18-135 zoom set at 135. For a distant scene the angle of view for both was similar, but for a subject at about 5 feet, the zoom at 135 gave a FoV more like a 90 mm lens.

Philip

The Pentax 60-250 is known for this. At closest focus distance the FL is more like 190mm.

This effect happens to most zooms and is quite normal.

I've never looked for this phenomenon but those variations are much greater than I would have imagined.

From what I understand, it occurs more strongly on internal focus lenses as a bit of a necessary compromise. I'm sure that Physics would allow for internal focusing lenses that also don't have focus breathing but this would probably require additional complexity in the lens design.

Gets a bit more fun in video too, where you change the focal point in the scene and suddenly the field of view is changing as well.
Similar fun when you want to zoom in or out, but that changes the focus point in non parfocal lenses.

Video tends to highlight flaws you can usually gloss over or adjust/work around in stills.

I suppose if the angle of view widen as the lens breathes it is less of a problem if you are critically framing something rather than having to recompose and focus constantly if your composition is significantly cropped.

Probably explains why this phenomenon hasn't really bothered me much. As a former owner of DA*60-250 and current owner of Nikkor 70-200VR2 and D-FA*70-200.