the lens is so screwed up that the number of available AF adjustment values is not sufficient, or.the error is consistent but depends on the aperture (however it would then be fixable for one aperture choice),.he used inadequate procedures to obtain the AF adjustment values (I'm referring to the AF adjustment values that are to be transferred to the lens via the Sigma dock),.Potential reasons as to why the reviewer did not succeed to address the AF issues with the lens include In other words, one can correct them with appropriate AF adjustments (the Sigma dock allows you to do that for a number of distance/focal-length pairs). However, all other such issues that I know of, are constant when you fix the focal length and the subject-camera-distance (and the aperture). There are also a number of optical properties of lenses, e.g., focus shift (the shifting of the focus plane due to a change in aperture caused by spherical aberration) that can misguide the PDAF module. This will produce a constant misfocus as the module then calculates the correct focus for a sensor that would be set back by the same amount. Just imagine the PDAF module sitting a miniscule fraction too far back. Is something in the optics spoofing the sensor to believe it has achieved focus when it is only near focus?The reasons why PDAF can yield misfocus on the sensor are manifold. My guess it they are keeping tight-lipped about it while they attempt to pump the water out of their intermittently-leaky boat. What information does the camera send to the lens? What information does the lens send back to the camera? How is it that the camera sees the image and tells the lens what to do, yet releases the lens from its focusing duties with the image out of focus? Is something in the optics spoofing the sensor to believe it has achieved focus when it is only near focus? Is the camera releasing the lens in-focus, but the lens motor keeps spinning (run-on), stopping passed where focus should have been? Is it just a timing issue between the two? Is there a protocol that lenses use when communicating with the camera? Someone at Sigma knows the answers to all of these questions. Where does PDAF happen? I assume it happens in the camera's processor, which then controls the focusing motor in the lens. Now, thirteen months later, the 18-35 is available for Pentax, the lens dock is available, and the focus problems persist across platforms. During that while, I switched to Pentax but I did not stop wanting the lens. It was recommended to wait for the USB Lens Dock so focus calibrations could be made, so I waited a while. The in depth review on this site also wasn't of a single copy but two - the second supplied by Sigma after the initial failure of the first sample to focus in Phase Detect.Īt this point I would be very very cautious of this lens until more is known.When I first looked into getting this lens a year ago (for Canon EOS), reports were that IQ was beautiful, but AF wasn't accurate. The fact that multiple copies of the lens across multiple brands have had the problem is troubling - the fact that other lenses from Sigma have had the problem also is even more so. I will not have time today to dig them back up, but as your own point is that a single sample doesn't indicate a problem - so too it doesn't indicate a lack of problem. I was totally unaware of all of these problems until reading the review here - but then my searches on the web revealed multiple problems with Canon tests (and possibly Nikon but I didn't read details on any yet). The in depth review on this site also wasn't of a single copy but two - the second supplied by Sigma after the initial failure of the first sample to focus in Phase Detect.Īt this point I would be very very cautious of this lens until more is known. What’s interesting, most of these misses were registered at 18 mm.'I was totally unaware of all of these problems until reading the review here - but then my searches on the web revealed multiple problems with Canon tests (and possibly Nikon but I didn't read details on any yet). The number of misses in studio conditions reached less than 4% which is a very good result. Here's what lenstip (testing on Canon) had to say: 'When it comes to the accuracy of the autofocus, our assessment is very positive. As Class A has pointed out so well, isolated cases do not indicate a rule.

Focus problems sigma 18 35 1.8