bclaff Loc: Sherborn, MA (18mi SW of Boston)

You could be right but I'd like to see a citation on that.

bclaff Loc: Sherborn, MA (18mi SW of Boston)

They also often care if a lens is parfocal, does not change focus when focal length is changed (zoomed). Most of our zoom lenses are not parfocal.
I have one such lens, it can be very helpful.

rmalarz Loc: Tempe, Arizona

Actually, it does. Changing the focal length will change the amount of volume withing the lens housing. Manufacturers try to make lenses dust resistant. In order to do that, the cylindrical parts need to have a tight fit, or some sort of seal. If the lens is at it's shortest FL and pulled to its longest FL, there will be a change in volume withing the lens' mechanical envelope. That will cause a reduction in pressure within.

As one changes focus, this again moves various elements of the lens and can cause an FL shift. The reverse happens when going from longer to shorter FLs, except pressure is built up inside the lens' mechanical envelope. If the lens is left to its own devices, changing that volume, however slight, by focusing will cause the lens to react to the change in pressure by either shortening or lengthening slightly to equalize the pressure inside the housing.

If the lens was completely airtight so no dust whatsoever could get inside, it would be near impossible to change focal lengths. Thus, the air moving in and out of the internal portion of the housing is referred to as breathing.
--Bob

Fotoartist Loc: Detroit, Michigan

Focal length "breathing" a dirty little secret in lens manufacturing and marketing. Been going on for years with all lenses.

marki3rd Loc: Columbus, Indiana

Absolutely incorrect. I repeat, the term breathing has nothing to do with air moving into or out of the lens body. You guys just make that explanation up because it sounds reasonable and right to you. In short you made that explanation up!!!!!

rmalarz Loc: Tempe, Arizona

Absolutely incorrect on your part. You need to get a grip on how air pressure and sliding cylindrical surfaces work. Think about it. Where is all of that air contained inside the lens assembly supposed to go? Or conversely, how is that reduction in air pressure within the lens supposed to behave?
--Bob

Fotoartist Loc: Detroit, Michigan

I assumed the OP term of focal length "breathing" was a term used for the shortening effect that occurs on focal lengths of some lenses when focusing. A better term than "shortening effect" or "fudging" on the specs which I was using.

JD750 Loc: SoCal

It means holding your breath when focusing so as to get a sharp image.

Seriously, Matt Granger's video, posted by catchlight on page 1, illustrates the phenomenon quite well.

PaulBa Loc: Cardiff, Wales

marki3rd Loc: Columbus, Indiana

I did not and do not argue that air does not move in and out of the barrel of zoom lenses when focal length is changed. What I said is that the term "focus breathing" did not come about because of the in or out movement of air during zooming. Think about it.... the term describes the CHANGE OF FOCUS coincident with the CHANGE OF FOCAL LENGTH. Where in that description do you see anything about movement of air. DUHH!!

Technical people have described some effects as "breathing" since long before zoom lenses. Want an example? In electronics amplifiers are adjusted for "offset" and "slope" (slope is another word for gain). Often when you adjust "slope" it affects "offset" and this is often refered to as breathing. Where is the air movement in that operation?

marki3rd Loc: Columbus, Indiana

Sorry, you are just showing your ignorance, all the more so when you attempt to belittle me. Yes air moves in and out, but just because that sounds like a description of breathing to you, it does not mean that is the basis for the term focus breathing. You get a grip!!

I spent 60 years as an EE working on USAF Space Track and NASA installations as well as working in the field of power measurement. I do have a "GRIP" on the technical issue here.

rmalarz Loc: Tempe, Arizona

Apparently, you are not completely familiar with the movement of element, barrels, and support hardware when zooming and focusing. Zooming changes the focal length. Focusing also changes the lens to film plane distance. When done with electronics the motion is very quick. That changes the pressure inside the lens and can cause elements to shift slightly, at least within the tolerance of manufacture.

I'm very familiar with speakers and speaker design from both theoretical and practical application. Just think about this for a second. Without air movement, where would speakers be?
--Bob

marki3rd Loc: Columbus, Indiana

What in gods name does this gobbly gook have to do with it. Trust me - pressure inside the lens is not the cause of focus breathing.

Bultaco Loc: Aiken, SC

It does nothing but change the focal lenght of a lens. The question is how do I determine what the focal lenght actually is?

marki3rd Loc: Columbus, Indiana

That is a separate issue.