12-24-2015, 06:31 PM
(This post was last modified: 12-24-2015, 08:20 PM by Brightcolours.)
Dave, not all CD AF algorithms or lenses are created equal. With Canon, crude lens motor systems make for crude CD AF results (probably also depending on the body model). With my 450D and Sigma 18-50mm f2.8, with "crude" DC motor, results were very uneven. Sometimes very OOF, sometimes just OOF, very sometimes more or less in focus. With the Tokina 12-24mm f4 and the even cruder AF implementation, only blurry results were possible. I blame that on the big steps that lens takes as "small step" during CD AF. CD AF looks from frame to frame if contrast has increased. If contrast has increased, focus is not reached yet. If contrast has decreased after it has increased before, the camera knows focus was reached and takes a step back, then declares focus.
Now if your lenses with the in-body motor also make bigger steps than for instance the best performing AF-S lenses, it can happen that the CD AF steps just happen to step over exact focus, and the camera never will know it as all it can base its conclusions on is on what was the highest contrast it actually saw.
Just to show that my -2 focus correction was not all to do with "hot wind"
I had to go out in the town and take a few night test shots, I took about thirty of similar to shopwindow type stuff so as to have a good comparison to what I took at Montpellier.
Basically a "chalk and cheese" result every shot was sharp.
Shot 1600 ISO F2 @ 1/250 sec.
[ATTACHMENT NOT FOUND]
100% crop! So me and my D750 have kissed and made up!
Old optics with older coatings? I see the tell tale "shine" from bright parts 
...... It has all the qualities of your name fully open! 
Quote:I'm glad everything is working fine for you  and I also understand why you don't want to use LV - I have the same reasons, glasses, slow.
But here are my comparisons of last night I didn't put in both versions full size but the small 100% comparisons with focus point marks. Tried to hit the same spot with LV, mostly successful except the glasses and the jewelry.
What is the lens being tested here?...... Is every shot is that far out? Has any AFMA been applied?
What's the story here?
The story is quite simple: Although I adjusted AF for the Sigma 20/1.4 in front of a D810, the LiveView based contrast detection AF knows better in most cases or equally good and never (!) worse. There are about 3 shots where both kinds of AF came to the same conclusion. I didn't show all the samples. But I've seen enough.
It was highly unscientific - without any tripod and probably some blur came for shutter/mirror vibration. I count that as one reason more to abandon mirrors... all shots were done in about ½ hour, just by walking through the streets, take one picture with AF-C, move the AF-point to a suitable place and focus. Then I switched to LV, focused again and released. There's a note in the gallery which tells just what I said. The bicycle was trickier and here I enlarged the AF zone in LV and adjusted manually. The little jewelry tree was also tricky, I didn't see the branch in foreground in the viewfinder, but thought, I was focusing at the face plaquet under the rear branch. That was were I put the AF to in LV, again with manual adjustment. The wall painting with the soldier in a small creek, the light chains on the facia - hopeless to get that with PDAF.
In the gallery is only the first picture with PDAF. Plus a comparison of the AF zone at 100% crop. My conclusion: It's close to pointless in terms of getting the full resolution, the most possible information out of a picture if I (you or others may do better) can't rely on PDAF, adjusted as good as it gets with FoCal. Fast lenses = pricey lenses and in my case high resolving sensor, outresolved by the lens in theory. In real life crashed and smashed by a second best AF device. When I was adjusting the lenses, the spread of focus quality is visible in any FoCal protocol although the software triggers AF and shutter. If I go to check various focus points, I even see which one are weak.
As you said, dave, LV is a pain in terms of speed and of no use in street photography with a DSLR and I fully agree. Of course, DSLRs are not made for street photography with LV. Checking proper focus afterwards? Yes, why not. Unless I don't compare it with the potential of the LV versions, I was always fine with the results and thought, that's as good as it gets. True, for PDAF it is. But there's more in the lenses and I can't see it in most samples.
There's something strange for it to be so unreliable as that, many threads about Sigma's art range on Nikons. PentaxForums tested a Sigma with hopeless AF results, they spoke to Sigma , tried two more lenses tested by Sigma etc. All adjustments tried with the docking station, they could only get it to focus reliably at one distance. Others talk about problems because of their reverse AF engineering........
Anyway, it sounds like you've had it up to your ears and are leaning towards a mirror-less of some sort, a Sony A7RII would be my choice, but looking at the model upgrade speed maybe waiting another year or two will bring about the perfect Sony mirror-less!
PentaxForums link:
http://www.pentaxforums.com/reviews/sigm...focus.html
and here:
http://www.pentaxforums.com/reviews/sigm...usion.html
quotes from the latter;
Pros:
Perfectly accurate autofocus, regardless of aperture, when using Contrast Detect AF (Live View)
Cons:
Dreadful autofocus accuracy when using PDAF (looking through viewfinder)
Read more at: http://www.pentaxforums.com/reviews/sigm...z3vHNjI4bC
Well Dave, I did sort of the same experiment with a Nikkor 85/1.4G two days before. You may reason whatever you want, but I saw the Nikkor failing as well in PDAF. I'm sorry for the ones who can't adjust their findings to mine, all I can say is: I took it for granted PDAF is faster and reacts better in low light. What I saw during the last days tells me a different story..
The Sony is tempting, indeed but I have to sell a kidney for it and like you stated, I also suspect rapid firing updates. Beneath 1000 $ that could be okay, for 3500 + I'm listening to a traumatized purse  No, I looked a couple of times into the Fujis, PZ lens tests, other sources, sample pictures and I think, for a start as travel (lightweight) camera, an XE-2 will do fine.
Also, this
Quote:
Others talk about problems because of their reverse AF engineering
Others talk ALL the time. Can't help, but even if that'd be true, the Sigma lenses are still better than their Nikon pendants...
I'm not really reasoning, more baffled actually....the same with the Nikon? .......more you speak about these problems with what are in essence very expensive lenses the more I'm wary .......add to that my lack of success with LV AF and your roaring success with it........Hmmm.....
............. it leads one to think that there's something not right with this world!
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