Explanation please.

Discussion in 'Beginner Questions' started by za33photo, Jul 21, 2022.

  1. What is meant when it is said that a lens "Breathes".
    This old ignoramus would like to know :confused:.
    Thanks.
     
  2. Imagine your view camera; you rack focus back and forth and work that bellows, add focal length to close up. Videographers don't like the zoom efect going along with that. Portrait shooters complain when their zoom's 200mm end provides a conventional 135mm's FOV for head shots.
     
    luis triguez and za33photo like this.
  3. Watch this video (4-5 mins) for a demo on 4 lenses.

     
    za33photo likes this.
  4. Thanks very much for the replies.
    I now know and understand what "focus breathing" is all about.
    I never even noticed this before :D.
     
  5. Well, yes, but,

    the term is also used to refer to the pumping in and out of air in the extension and compression of the tube of the lens.

    Many lenses sucked in dirt quite freely in the process.
     
    za33photo likes this.
  6. “Lens Breathing – Focus Breathing” --- As you know, the job of the camera lens is to project an image of the outside world onto the surface of film or digital image sensor. To achieve, light rays from the subject (vista) traverse the lens. The figure (shape) of the lens and the density of the glass forces these rays to change their direction of travel. This action is called “refraction”, Latin to bend backwards.

    We can draw a diagram showing the revised path of these rays. Such a drawing is called a ray trace. Such a trace reveals the path of the light as it traverses the lens. Such a ray trace from a single image point resembles two ice cream cones place pointy end to pointy end.

    When we focus, we are adjusting the distance lens to sensitized surface. When the sensitized surface just kisses the apex of the cones, focus is achieved. It a fact that the distance lens to apex is a variable based on subject distance. The apex of this cone of light is at its shortest if the object is at infinity (about 3000X the focal length of the lens. Objects that are closer than infinity will generate a longer cone of light (increased back focus distance).

    Now the size of the image of objects is a function of their distance from the lens. Objects that are close generate longer cones of light. Objects at infinity generate the shortest projection distance. Thus, the size of the projected image of an object is proportional to this projection distance.

    As we focus our camera, we are adjusting the distance lens to projected image. Since the size of the image of an object change with projection distance, we will likely notice that image size (magnification) changes somewhat as we focus. This phenomenon is called “focus breathing”.

    Focus breathing is often considered disturbing in the motion picture trade as it is viewed as disrupting the smoothness of a zoom. In Cine photography some zoom lenses have a correction applied that minimize focus breathing. In still photography, focus breathing is likely moot.
     
    Last edited: Jul 21, 2022
    PapaTango, NHSN, kmac and 2 others like this.
  7. What you said is the reverse of what people nowaday called "focus breathing". Yes what you describe would make the lens appear to have slightly longer focal length when focus close. That is normal and it does bother a bit on movie/video but never on a still camera system. What people are talking about focus breathing nowaday is that the lens actually shorten the focal length when focused at close range. The reason for this is many new lenses have internal focusing and they actually shorten the focal length to avoid having to move a lot when close focusing. People are talking about lenses of 200mm focal length that are actually only 135mm when focused at closet distance.
     
    za33photo likes this.
  8. @ BeBu Lamar -- I stand on my answer - To further illuminate - Focus Breathing is an observed change in angle-of-view induced when the lens is focusing on different distances. In other words, a change in magnification when focusing different subject distance. As you focus on different distances, the back focus changes. Not to be confused with focal length, a measurement made when the object being imaged is at infinity. At infinity, light rays arrive parallel. At closer distances light rays arrive diverging. I will concede, that being 84 I could be wrong and outdated.
     
  9. I've never heard about it or noticed it either. FWIW, I think that it's wise to take photos - where possible - with a 'margin of error' that can be corrected. So digital photos and film photos that are (or can be) digitized prior to or after development,
     
    Ludmilla likes this.
  10. Isn't that "eye blow" or "air pump"?
     
  11. I don't say your answer is not correct but it's not what people are complaining about focus breathing today. What you describe is ver slight and only noticable when you do a video not still. People are complaining that they lose (not gain as in your case) when they focus close. This problem only happens with newer lenses with internal focusing.
     
  12. Since we have to breathe both in and out, it seems that it should work both ways.

    The change in angle of view with focus is well known to those doing close-ups,
    especially the very-close ups.
     

Share This Page

1111