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Old May 15, 2015, 11:04 PM   #11
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Originally Posted by SIMON40 View Post
The mirrorless brigade chuckle....
... because they don't have any f/1.2 lenses?
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Old May 16, 2015, 11:12 AM   #12
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This is a problem with focus shift. The camera focuses with the lens wide open, but during the exposure, the aperture stops down and as a result, the focus shifts.

This could happen with mirrorless systems too (if they had any very large aperture lenses.) It would, however, be a lot harder to fix in a mirrorless system.
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Old May 18, 2015, 4:45 PM   #13
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Yes, I guess it would be hard to remedy on a mirrorless system.....

I guess using the DOF preview (assuming the cam' has one such button) and manually focusing at the same time (ideally on a tripod) could work... if you've a clear viewfinder and a good eye...
I wonder if you could use live view, DOF preview and manually focus at the same time..!!
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Old May 18, 2015, 6:28 PM   #14
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It shouldn't be necessary on a mirrorless camera, which has the focus built in to the sensor, not a separate system.
Until someone explains the physics of how aperture change affects focus distance, I'm going to believe it exists only in some people's heads.
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Old May 18, 2015, 7:53 PM   #15
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It shouldn't be necessary on a mirrorless camera, which has the focus built in to the sensor, not a separate system.
This has nothing to do with the fact that the PDAF sensors are or are not on the actual focal plane. It's about the lens focusing on one point when the aperture is wide open, and a different point outside the depth of field when the aperture is smaller.

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Until someone explains the physics of how aperture change affects focus distance, I'm going to believe it exists only in some people's heads.
Have you done any research on the subject?

https://photographylife.com/what-is-focus-shift

http://www.lensrentals.com/blog/2010...perture-lenses

http://www.aravind.ca/Reviews?Canon50_f12/focus_shift

http://diglloyd.com/articles/Focus/FocusShift.html

http://www.reikan.co.uk/focalweb/ind...-reikan-focal/

http://www.dpreview.com/forums/post/50043319

https://www.youtube.com/watch?v=2aTKSjMIFyk

That's a lot of heads.
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Old May 18, 2015, 9:46 PM   #16
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Yeah, it seems to be catching.
I have read some of those articles, and to me, it still looks like the problem is with the camera, rather than the lens.
Unfortunately (or not, perhaps), I don't have one of the expensive lens/camera combinations which seem to have the problem, so I won't be losing any sleep over it.
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Old May 19, 2015, 8:30 AM   #17
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Yeah, it seems to be catching.
I have read some of those articles, and to me, it still looks like the problem is with the camera, rather than the lens.
It's called Spherical Aberration, an optical effect, as in, it happens in the lens.
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Unfortunately (or not, perhaps), I don't have one of the expensive lens/camera combinations which seem to have the problem, so I won't be losing any sleep over it.
... and yet you're willing to publicly dismiss the phenomenon in the face of multiple experts demonstrating the effect, and a friend experiencing it?
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Old May 19, 2015, 9:50 AM   #18
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Can also vary by distance.
I just adjusted a 600mm using 50x as the test distance (about 100feet) and it is now bang on at long distances.
But completely misses focus when trying to focus close around 15feet.
I've heard of zoom lenses doing that, but this one might need a visit to the lens doctor.

With a 1.2 lens there might be a minute shift with focus distance as well, but it just might be enough to throw it out of whack for the cameras accuracy.
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Old May 19, 2015, 11:51 AM   #19
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Peter;
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.
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.
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Old May 19, 2015, 12:58 PM   #20
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Ouch! Kind of makes me happy I've stayed to the slow f/2.8 and f/4 lens. Would not need yet another focusing headache added to the mix!
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