Howdy Pentaxians,
I bought the Sigma 85 1.4 a week ago for my K7. I am aware of a lot of happy Siggy 85 owners, Canikon inclusive. I did formal testing (that is: test chart, tripod, mirror up, IR remote, etc.) and found this 1 major problem. With a lot of trials, I came up with a -5 microfocus adjustment for the lens.
The AF is rather inconsistent, considering normal use. What I mean is, it has 3 different behaviors:
(assuming subject is like 2-5 meters away, and remember, this is with -5 microfocus adjustment)
1. If AF starts from behind-subject (what I mean is, say I twisted the focus ring towards infinity before AF), the AF always locks on correctly.
2. If AF starts from very close to subject (say, the lens was already prefocused on the subject from a previous AF), the AF would lock on about 50% of the times. The other 50%, it locks on in front of the subject.
3. If AF starts from front-of subject (say I twisted the focus ring towards the minimum focus distance before AF), the AF always locks on somewhere in front of the subject (front focus). The distance would be similar to behavior 2, when it front focuses.
Here's what adds to the complication. As I add or subtract MF adjustments, the whole behavior shifts by that much, front or back depending on how you adjusted MF. Now the difference in the behaviors brings about miss focuses of a factor similar to as if I had done a 5 to 7 MF adjustment.
Also, to those interested, I did find that the AF does FF and BF depending on subject distance, noticable at F1.4, but probably ignorable at F2.0. And, I did not notice any FF or BF with change in aperture. So, this is a big win for Sigma. Or maybe it's to do with the optics of any 85mm compared to 50mm.
As a result, my pictures come out with a 1:3 to 1:7 hit:miss ratio. This is awful
Especially considering the DOF is so thin for those OOF parts of the subject and also because I know other lenses (Sigma 50 1.4, Pentax 55 1.4) were a lot more consistent. And of course for the almost $1K price tag!
Question to forum:
So, my question to the forum members is if anybody knows what is causing this, or if anybody else has experienced the same thing with the Sigma 85 1.4. I'm definitely dissatisfied with the behavior and will exchange (BH has a good policy)
I have had a Sigma 50mm F1.4 before, for a short while. But, I returned it because it did BF and FF too much depending on aperture and subject distance, which I put forward to the forum
in this post. Also, I was not too fond of the perspective, although I will have to get one for those 50mm shots. But, later
But what I have noticed as a difference between the Sigma HSM and Pentax SDM is that the SDM is very sensitive to slight changes in the subject distance, while the HSM does not decide to reevaluate until the subject distance changes enough to notice the clear difference, given the thin DOF at 1.4. Maybe this is what is showing up in my copy, only quite exaggerated.
Cheers,
Anil
Last edited by icypepsi; 03-29-2011 at 07:46 AM.