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Old May 19, 2015, 1:27 PM   #21
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Originally Posted by VTphotog View Post
What TCav and the online 'experts' who are obviously reading each others' blogs, are telling us is that, the lens focus changes at the same distance, depending on the aperture setting.
I'm aware of circular aberration, and willing to accept that a lens that costs 3 grand or so might not be well corrected for it. (!? I probably wouldn't accept it if I had spent the money for it, though) It makes the image soft at the periphery, while being sharply in focus at the center. Pretty basic optics - available with just about any cheap telescope. Stopping down the lens will most always make the image sharper (but dimmer) by eliminating some of the peripheral light, but it doesn't change the focal plane of the lens.
I googled "circular aberration" and came up with nothing. It seems that you're mistaking what FaithfulPastor is experiencing and the experts are describing, as Petzval Field Curvature, and it's not. The well documented phenomenon at issue is that the outer portions of the lens focus in one place in the center, while inner portions of the lens focus somewhere else in the center. Since cameras autofocus while the aperture is wide open, the spherical aberration causes the focus to be influenced by the outer portions of the lens, so the camera focuses in one place, but when the aperture is stopped down, the influence of the outer portions of the lens are blocked and the focus shifts to only what the inner portions of the lens sees.

Quote:
Originally Posted by VTphotog View Post
However, if the camera tries to compensate by adjusting the focus when the lens is wide open vs stopped down, one would get this phenomenon.
Once again, the camera doesn't try to compensate for the focus shift. It doesn't know about it. It only happens during the exposure which is when the mirrors are out of the light path and the PDAF sensors are not seeing anything.

Do you get it now?
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Old May 20, 2015, 11:39 AM   #22
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BTW, the term "spherical aberration" may mislead you into thinking that this is an aberration that occurs in only some spherical lenses. That's not so. All spherical lenses focus imprecisely; it only becomes a problem when the lenses have a very large aperture. A way to correct for the imprecise focus is to use lenses that are not spherical, or aspherical.

Canon's 85/1.2 lens uses an aspherical element, as do its 35/1.4, 24/1.4, and 28/1.8 lenses, but the 50/1.2 does not.
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Old May 20, 2015, 3:18 PM   #23
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It's an interesting phenomenon, though to be frank it does seem logical really, given that light has less distance to travel when the aperture is opened fully (off center light ray paths) during focus- as opposed to the light travelling further through a smaller aperture- in effect a curved path to the sensor- except the light coming through the center of the lens... and given the very shallow dof wide open on say an f/1.2 lens I can see how this could indeed result in a slight focus shift...
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