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Forum: Photographic Technique
10-01-2018, 04:00 PM
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AF fine adjustment should be done in PDAF mode because fine adjust only pertains to PDAF.
Evaluation of focus shift, OTOH, should be done with the most reliable focus method available, meaning manual focus under live view. As noted in Roger Cicala's article, significant shift should be seen for exposures at or around f/2.8 with lenses for which this is a problem. At narrower apertures the effect is masked by DOF.
To test your FA 77/1.8:- Focus (wide open, the default) using manual focus in magnified live view with a flat target at or near minimum focus distance. I used 1m.
- Make an exposure at f/1.8 aperture. Use a tripod with 2s delay (no SR)
- Make a second exposure at f/2.8 aperture
- Compare the two at full resolution using a computer monitor. If #2 is soft compared to #1, the lens may have focus shift.
The linked article as well as my prior comment indicate that the cause is uncorrected spherical aberration that results in the focus plane shifting as the lens stops down such that the effective focus is no longer at the face of the sensor. Focus shift is related to longitudinal (axial) color aberration (loCA) where different wavelengths focus forward/backward of the intended focus plane.
The focus ring and the whole physical mechanism stays where you left it. It is the light that is not behaving. Remember, this is an optical issue, not an autofocus problem. AF fine adjust is not a good solution in that the severity of the problem varies by focus distance. Here is a link with diagrams:
https://photographylife.com/what-is-focus-shift
and the diagram from that page...
Steve
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Forum: Photographic Technique
10-01-2018, 01:17 PM
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Yep! The example I often use is that of a studio shot of a face in profile with rim lighting. There is precious little for the camera to focus on that is non-ambiguous and few of the surfaces have detail addressable by the AF system.
Example
Visione by Daniel Zedda, CC-BY license
Found at Lighting setups for great portrait photos | Discover Digital Photography
Steve
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Forum: Photographic Technique
10-01-2018, 08:49 AM
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Optical evaluation for focus shift requires accurate and precise fine focus technique. That is why live view is used; it is the gold standard. Using PDAF fine adjust to evaluate focus shift just adds another set of variables. My opinion (based on reads of over a hundred threads on AF fine adjust) is that most people over-adjust or adjust based on spurious or inadequate data. That is why I added the qualifier that use of one of the available focus evaluation software packages might be in order when trying to detect subtle focus shift, even with manual focus under live view.* That being said and out of the way, a +8 adjust from wide open is anything but subtle. I did not see that kind of shift with my lens. My unsolicited advice is to redo using live view.
FWIW, focus shift results from uncorrected spherical aberration and is most evident at close distances. While once not unusual and part of the lore of measurebating film photographers when I was young, it was usually not an issue under field conditions. Ansel Adams covers the subject in the 1980 edition of "The Camera" with suggestions for refocus stopped down if there is any doubt. His solution was in regards to a particular class of large format lenses and involved refocus on specular reflections using a magnifying loupe on the view camera ground glass; think of it as magnified live view. Focus shift used to be part of photo magazine lens tests, but as optics improved, that aspect of testing was dropped. Now days, the term is more frequently applied to focus stacking with Nikon than to lens aberrations. Significant focus shift is more of a novelty of the past than a serious and ongoing concern.
Roger Cicala dedicates two paragraphs of his article on using fast lenses to the subject. His short discussion is concise and to the point. In fact, the whole article is excellent for putting the "must have fast primes" discussion in perspective and may shed light on why many of the experienced (older) users on this site take a fairly dim view of the current call for ultra-fast wide angles and such.
Lens Rentals | How to Shoot With Wide Aperture Lenses
Steve
* I have seen example scattergrams from PDAF evaluation and they are fairly alarming. PDAF, while quite adequate, remains a fairly blunt tool for fine focus.
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Forum: Photographic Technique
10-01-2018, 12:12 AM
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FWIW, I just tested my FA 77/1.8 in the range f/1.8 to f/8 with focus established at f/1.8 using split image (KatzEye) on my K-3 with no obvious shift. A more rigorous test would require something like the Reikan or LensAlign systems to allow statistical evaluation of focus accuracy to detect focus shift.
Edit: Repeated using magnified live view with same result.
Steve
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Forum: Photographic Technique
09-30-2018, 11:33 PM
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For the most part, focus shift with modern (post mid-70s) lens designs are an artifact of pixel peeping. Some shift is almost always there, but is mitigated by increased DOF; however, magnification (pixel peeping) defeats DOF.
I have not seen much focus shift in the lenses on my shelf with the exception of my MC Rokkor-PF (Minolta) 58/1.4. It has visible shift in the split-image finder from f/1.4 to f/2.8 amounting to about a centimeter at 1 meter distance! :eek: Other than that, it is a fine lens. ;)
Steve
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