Luminous Landscape Forum
Equipment & Techniques => Cameras, Lenses and Shooting gear => Topic started by: hassiman on March 11, 2016, 05:12:49 pm
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I was wondering if any of you would be kind enough to explain Focus shit to me. I am new to the Nikon 14-24 for my D810. Just wondering when it becomes a problem and how I can work around it? Does AF not work properly at certain focal lengths?
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I was wondering if any of you would be kind enough to explain Focus shit to me. I am new to the Nikon 14-24 for my D810. Just wondering when it becomes a problem and how I can work around it? Does AF not work properly at certain focal lengths?
Hi,
Focus shift is the shift of the (thin) plane of optimal focus when the aperture is changed (usually compared with wide open). Since most focus systems focus with the aperture fully opened, it can be an issue if the focus plane shifts enough and also changes the DOF zone too much (can result in front- or back-focus, despite correctly focusing on some features).
This is not to be confused with a perceived change in curvature of field which is usually most prominent towards the edges. It's usually not (much) affected by stopping down, but edges just gets more acceptably focused due to more DOF, and some residual aberrations get reduced at moderate apertures.
Cheers,
Bart
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What I did with the Nikon 28mm f1.8G, which is reputed to exhibit focus shift, was to focus at my most common shooting aperture (f8) on a target attached to a picket fence angling away from me down the sidewalk. Easy to see where the true point of focus fell along the fence, and then use AF fine tune to adjust so true focus would fall on the target. With a zoom you'd have to settle on a specific focal length or split the difference among two or more. BTW, I don't know if your lens is subject to noticeable focus shift.
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hassiman,
I was wondering if any of you would be kind enough to explain Focus shit to me. I am new to the Nikon 14-24 for my D810. Just wondering when it becomes a problem and how I can work around it?
To expand on what others have written, my understanding is that:
- The effect of focus shift is always to move the plane of focus farther from the camera.
- The rate at which the plane of focus shifts reduces as the aperture is stopped down. Hence, stopping down to, say, f/4 for the purpose of acquiring focus, will significantly reduce the residual focus shift which will occur when you stop down to, say, f/8 for image capture. Stopping down to f/8 from the outset is of course possible, but the increase in depth of field will, in principle, make it more difficult to determine if focus has been correctly acquired.
- The total amount of focus shift that will occur when the aperture is stopped down decreases as the distance from camera to subject increases.
Refer here (http://diglloyd.com/articles/Focus/FocusShift.html) for an article by Lloyd Chambers (for whom focus shift is a big deal).
Does AF not work properly at certain focal lengths?
Autofocus works as it should. The issue is that focus is acquired with the aperture wide open, but when the latter is stopped down the plane of focus shifts to an unintended location. How adverse the effect of focus shift is, is a matter of opinion.
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Hi, for critical work the only way is to focus in live view mode with the lens stopped down at the intended aperture (IMHO).
I have a Nikon 50mm 1.4 AIS which has considerable focus shift at large apertures and it is not compensated by DOF when stopping down.
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Never really noticed focus shift with the 14-24. But that's probably because, when stopped down at such focal lengths, there's so much effective depth of field that everything is in sharp focus anyway (even on an A7r). Doubly so with landscapes/cityscapes. Probably the only time you'd see it with a 14-24 is if you were pressing it into service as a macro lens...
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Hi,
It is mostly caused by spherical aberration being not fully corrected.
Look at this image, from this article:http://toothwalker.org/optics/spherical.html
(http://toothwalker.org/optics/spherical/undercorrected.png)
You see that the most periferal rays have a focal point at 'a' while the central rays ha a focal point at 'c'. With both central and peripheral rays present the best compromise is 'b'. Stopping down takes away the peripheral rays, so best focus is at 'c'.
Most lenses have a quite significant focus shift. Jim Kasson has done a lot of testing on this recently: http://blog.kasson.com/?p=13369
This image illustrates the focus shift on the Zeiss Otus (85/1.4)
(http://blog.kasson.com/wp-content/uploads/2016/02/Ptus-focus-shift.png)
Jim has focused manually and used a Stackshot rail to take 41 exposures with the camera moving 192 mm between first and last exposure. The blue curve (f/1.4) maxes out at around 80 mm, while the yellow (f/4) one maxes at around 40 mm. So focus shifts around 40 mm when stopping down from f/1.5 to f/4. Subject distance here is 3.3 m.
This is Jim's figure for the Zeiss Batis 85/1.8 , something like 60 mm focus shift from f/1.8 to f/4
(http://blog.kasson.com/wp-content/uploads/2016/02/Batis-focus-shift.png)
And the Leica Summicron (90/2) (around 50 mm shift in best focus going from f/2 to f/4
(http://blog.kasson.com/wp-content/uploads/2016/02/Leica-focus-shift.png)
Best regards
Erik
I was wondering if any of you would be kind enough to explain Focus shit to me. I am new to the Nikon 14-24 for my D810. Just wondering when it becomes a problem and how I can work around it? Does AF not work properly at certain focal lengths?