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When you manually adjust your focus ring on the lens, what actually happens inside the lens? Is this changing simply the point at which the lens is focusing, so that anything at this distance away from the camera, within some tolerance, will be in focus? How does this work?

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Yes, the manual focus mode on lenses lets you manually change the point of focus. Actually what you're setting is the plane of focus, an entire plane, perpendicular to the optical axis, where the image is in focus.

The distance to the plane of focus is not constant, though - the larger the angle from the optical axis, the longer light has to travel to get to the camera from the plane of focus.

The "tolerance" you're referring to is called the depth of field which is the distance between the closest and furthest objects in an image that appear acceptably sharp.

What physically is going on in the lens is that by turning the ficus ring, is that lens elements within the lens move towards the front or back of the lens, changing the plane of focus. That the plane of focus does change when moving the lens elements is a result of optics.

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  • \$\begingroup\$ thank you hugo; my question spawned from another photo.stackexchange thread, where in an question about achieving a shallow depth of field and trying to maximize bokeh, someone made the comment about how the depth of field will quickly increase, the closer you are to the hyperfocal distance. so i read about hyperfocal distance focusing, and this technique is quite the opposite of the typical technique in focusing for a portrait with a shallow depth of field. and when i read about the technique for achieving this type of focusing they said it was necessary to use the dof marks and distance \$\endgroup\$
    – user74091
    Commented Sep 7, 2014 at 6:23
  • \$\begingroup\$ scales; neither of which my kit lenses come with. to express my understanding of the process, if you wished to have a shot focused to the horizon sharply you would wish to have lots of dof, and as such step up your lens to a f/16 (?) maybe. then the focus ring is calibrated to the aperature ring such that you set the infinity mark to line up with the selected aperature. this would then give you an indication by the distance scale of how far in front of the lens the focus point would be, and everything after that as well? \$\endgroup\$
    – user74091
    Commented Sep 7, 2014 at 6:26
  • \$\begingroup\$ to clarify what my confusion might have been, especially since my lens does not have a distance scale or dof marks; all of my focusing is done with af and by visualizing through the optical viewfinder, especially so when looking for achieving shots with shallow dof. however, in this case it seems that you almost calculate where the focus plane is manually. is there a similar technique for calculating the distance to a focus point for those shots with shallow dof? thanks again. \$\endgroup\$
    – user74091
    Commented Sep 7, 2014 at 6:30
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    \$\begingroup\$ @user74091 It's great that you have follow-up questions. I will try to answer them as best as I can, but in order to easily find them please start a new question for each of the different questions you want answers to (after you've searched the site for an already existing question + answer that is). Good luck \$\endgroup\$
    – Hugo
    Commented Sep 7, 2014 at 8:12
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    \$\begingroup\$ No lens has a flat plane of focus. Some come very close, but the field of focus for simple single element lenses is a curve. Actually, curves, as each wavelength of light has a slightly different degree of curvature. Compound lenses correct, for one degree or another, for field curvature with varying degrees of success. But the theoretical perfect plane is just that - theory. And bad theory at that since the nature of light itself and the nature of refractive optics means even a lens that perfectly matches its blueprint will not have a perfectly flat field of focus. \$\endgroup\$
    – Michael C
    Commented Dec 27, 2017 at 3:56

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