Originally posted by Digitalis Do you think the criticism of the AF accuracy of this particular lens is unfair?
Systematic focusing errors (i.e., consistent front/back focus) is a PDAF phenomenon that occurs with all lenses and which can be fixed by servicing, or in the case of some Sigma lenses, by oneself.
So we are only talking about erratic focusing which appears to be more frequent with the 18-35/1.8 than with any other lens. I have a hunch that the uniqueness of an f/1.8 zoom has some bearing on the frequency with which erratic focusing can be experienced.
Bear in mind that the PDAF focusing in your camera may only give you accuracy equivalent to f/5.6 unless the f/2.8 areas can engage.
So overall, I sympathise with everyone who has erratic focus issues with the 18-35/1.8 but I don't see how based on singular experiences, some internet reports, and ignoring happy users, it can be concluded that
- all copies of the lens must have the same issue, and
- only the lens is to blame, but the camera, which is the ultimate arbiter of when focus has been achieved, is not.
Originally posted by Digitalis Do you think it is too much to ask for a brand new lens to be able to be calibrated to focus properly with at least 95% accuracy?
It would be good if all brand new lenses focused with that degree of accuracy, but the reality is that regardless of brand, this is not the case.
Earlier in this thread, I pointed to a user who tried five (5) different copies of the DA* 55/1.4. They all exhibited erratic focusing with his camera. Shall we now conclude that all copies have this trait and refer to Pentax as a brand with no QC? Your call.
There are many more examples for shoddy Pentax QC. It happens. We either live with it or somehow make manufacturers understand that good QC can reduce cost rather than increase it.
Originally posted by Digitalis Even in broad daylight this lens will inexplicably mis-focus, typically within the 2m to infinity range.
Yes, but that does not diminish its optical excellence when focus is achieved.
You can continue to play that one-note song "the AF is not reliable" all day long but you yourself acknowledged to some length how exceptionally well the lens performs in other aspects.
Originally posted by Digitalis I did have high hopes, I really did. I was willing to drop all my baggage over sigma products, and see the ART series as something new.
The ART series is something new and the optical excellence of its models is unequivocally recognised. It is your prerogative to turn a blind eye to the high praise these Sigma models receive from reviewers and users, but that does not make your opinion the truth.
The fact that there seems to be an unfortunate number of 18-35/1.8 copies that exhibit erratic focusing does not mean that your mom's idea of Sigma as being a shoddy company is still valid.
I said this before: The combination of you owning many Sigma lenses (including the 18-35/1.8) despite the fact that financially you could own any competitor lens instead (as demonstrated by your ownership of very expensive lenses) with your relentless Sigma bashing creates a bit of a logical tension.