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PostPosted: Sun Jul 28, 2013 12:46 pm 
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Day later and still testing... now moved to an indoor studio with controlled lighting and carefully setup focus-targets again measuring at 1, 1.5, 2.4, 7 feet, then outdoors for Infinity distance.
Use of the Dock is a bit cumbersome as it can't be applied while shooting; lens has to be removed from camera, attached to the Dock, the Dock USB wired to the computer to open the Sigma focusing program and set whatever desired adjustments (4 distance zones of choice). Then remount the lens onto the camera body (with the new focus-tweaks) and shoot again. Basically pure "trial and error." Can NOT be adjusted -while- shooting; an attribute I consider its Achilles heel.
Using Canon's long-standing advice on setting up any lens-test I apply their calibration formula: millimeters of the lens times 50 (35 x 50 = 1750mm). Converted to inches: 1750 / 25.4 = 69 inches (or 7 feet) - the camera to test-target distance for lens focus-distance calibration that Canon deems best.
Then using the camera body's lens micro-adjustment feature, dial in the necessary +/- correction for a test target (can be done while shooting, results viewed at 10X on the camera's back LCD, ideally also using a hand-held 2X magnifier for better judgement.)
Above setup with 35mm lens and my specific 5D Mk III body calls for a +10 adjustment (much greater than any other Canon red-lens I have). Plus-10 means the camera's AF is set to focus farther back than is normal to achieve sharp AF (in this case at the 7-foot distance).
With +10 applied, this camera body's AF focus sharpness is spot-on within the approximate 1-to-10' zone... but lens-focus at infinity remains highly flawed, back-focusing significantly!
Attempting to correct the infinity-error though Sigma's Dock by applying "minus" values to the Dock's specific 4th infinity zone (move the infinity focus point nearer) eventually brings that zone of focus into reasonable range... but sharpness at infinity remains unacceptable.
Softness at Infinity remains the serious and significant issue; noticeable softness at f/1.4 through f/4, doesn't clear up reasonably until f/5.6 at minimum, f/8 better.
Contrarily, close-range zones (1-10 feet) are relatively razor sharp even wide open, f/1.4, same at f/2, f/2.8
I'm inclined to guess, strictly a guess... that there is an internal lens-design defect; possibly limited to only this specific lens? but possibly the issue may be more widespread; could it be that the Sigma 35mm's focus capability is problematic at infinity? i.e., a factory-design defect?
A bit more troubling if not disconcerting: Sigma's website offers virtually nothing for tech-support, nor a forum where issues might be discussed or compared, even answers gained. There is a website tech support phone number but it's extremely buried in sub-sub menus; it's now a weekend and of course their USA office is closed. Same aspect about the tech-sheet included with the Dock-product: virtually no information available, nor suggested website.
At this point I remain at a loss for a solution. Defective specific lens, or is this a widespread focusing issue at infinity?


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PostPosted: Sun Jul 28, 2013 3:21 pm 
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Sorry to hear your woes, okie!
In my experience there is no optimal focus-adjustment for all shooting distances (nor all focal lengths of a zoom, or apertures)!
That is due to the varying degrees of aberrations that you get from a lens when changing focus.
I personally would strive for optimal focus-adjustment at (or near) infinity, as this imho the most glaringly visible error. So if you correct for that you're probably off with a higher probability of satisfying sharpness in your photos. I also think that "infinity"-adjustment should work pretty well for distances up to 50..100x focal length. In case of the 35mm Sigma it should be good from 7 feet and beyond. Why? Because optical performance should not change that much between 100 x focal length and infinity. But if you shoot at shorter distances than 3 feet I've the feeling that the performance/aberration has changed enough to invalidate your adjustment for infinity.
So what you're seeing is possibly proof that a large-aperture lens cannot fully be AF-optimized for all distances.
-->But if there is no setting where you can achieve sharp infinity focus then the lens is most probably defect.
------
Sorry to be so vague, but I find AF-performance a very delicate and highly complicated beast.
This is why I do my Siemens-charts and "Unremarkables" test-shots with manual focus only.

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PostPosted: Sun Jul 28, 2013 8:49 pm 
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Manual focus that you suggest as always used in your testing is equally flawed and problematic particularly though any viewfinder. Using Live View (if it's available) is more accurate, at least more predictable, but even Live View isn't precise in achieving the "perfect" focus point, it's simply more consistent.
AF as it exists within cameras varies appreciably, one camera type to another, some far worse than others; Canon's 5D MkIII and their premium-priced model (1DX?) appear to have the latest and best focusing systems; consistency is the biggest gremlin. Auto-focus, AF as we know it I suspect is the proverbial "dirty little secret within" the camera industry; okay for average shooters and beginners, but not well suited for critical use. Not as precise as most people assume


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PostPosted: Mon Jul 29, 2013 4:37 pm 
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When I said "manual focus" I meant live-view based MF, okie.

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