Joju, things are complicated. PD AF and CD AF use different ways to command the lens. So it may well be that a Nkkor lens with PD AF does a better job than a Sigma lens, because Sigma uses their own reverse engineered PDAF logic in their lenses. I do still see quite some complaints about Sigma focus issues in Nikon forums, particularly concerning the 35mm f1.4 and 50mm f1.4. Whether that is anything to go by, I do not know.
Also, I have seen quite a number of posts about certain Nikon FF DSLRs having issue with focussing correctly with artificial light. I have not read threads about that in detail, so I can't give any pointers (not even sure if the D750 is one of those or not), but a quick google should give information about that. Also I read a thread where a 5D mk III user found that the Sigma 35mm f1.4 he used focussed differently in artificial and day light.
With my camera, PD AF still focusses when CD AF has effectively become blind.
Quote:I made that PDAF vs CAF game last night and this night, after seeing a nice movie, again. How can I say? In no case the LiveView AF gave me a less good sharpness and in about 80% it was clearly better than PDAF. I didn't use LiveView often, partly to save battery and the long delay between shutter button down and picture stored. That will change now.
'Course this makes me think. "If its' so easy to miss sharpness wide open with normal AF, no matter how much this fast lens costs, no matter how much time I spent with adjusting that thing and if the LiveView of a DSLR always will be a crippled mirrorless, then why bother?". One can say, what do you expect form a third party lens? Answer: Well, LV didn't care about genuine or Sigma lens, it just nailed the focus better. If I get in the mood I'll make a sample gallery. How about those nightshots, Dave - did you compare focus with PDAF and CAF?
Well I've examined the photos and in evening light all was well, but as night set in and I stayed with the AF35mmF2D I started getting back-focus issues, the ambient light was of all possible colours, as one would expect at Chrismas time. The D750 I think seems to need -1/-2 with most lenses and the 35mm didn't have any MA, putting the AFMA to -2 might have helped though. All in all this not a sphere of imaging that shoot a lot of so, it's gone relatively unnoticed. I will try some night shots in town maybe quickly this evening to see if lens calibration will change things and compare it with the K01.
I did a couple of LV tests but it's so slow (next to useless) that with the thronging crowds I thought it better to try in similar light away from the melée.
However it has spurred me into getting a Hoodman on the D750, otherwise I absolutely cannot use LV, (glasses on/off every two seconds) I have a cheap magnetic one that is too small for the 3.2" screen but held away slightly, all the screen is visible, I need to make a fixing of some sort. I'm on the case!
txbb12, you got PM 
dave, shooting in a crowd with LV is really sloooow. Really nice shot you enclosed! I was doing my experiments at 11 p.m. and immobile objects, also with a wider angle (20/1.4) wide open. Pictures will follow.
Update!!!!
I was a bit concerned as to last nights shooting, so I have just been doing some tests with this irksome business.....
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The AF35mm F2D was way "back focusing" in PDAF I tried -1 and it was better but it needed -2 ( which may have resolved last nights errors) which with many lenses a fine tuning .......but with wides the effect is much greater.....still surprised though!
So that explains a large party of the back-focusing I was getting last night, but will require further testing at night of course.......PDAF is now fine.....
I wanted to check this against LV CD AF.......... I did a few tests distant medium and close.......and do you know what.....".Live view" looks like it's front focusing to me......I really don't get it at all, it should be spot on no?
dave, that's always happening to me as well. No matter how I test, with which software (FoCal or LensAlign), after hours of testing and adjusting I still have issues. Be it because I usually shoot on AF-C, be it because I adjusted a lens at 50× FL and the distance is just anything else (which can need a different AFMA), there is uncertainty all the way. If any DSLR user tells me, HIS pictures are nearly always spot on focused, I bet, either he's using slow lenses or stopping down.
Fast tele-lenses have a shallow DoF, hard to hit precisely. And difficult to notice while shooting. Fast wide-angles are even worse because the AF points get fat compared to the details - you never know what the AF is aiming at. Also difficult to notice while shooting. The more I look into it, the more I come to the same conclusions like thxbb12 and Roger Cicala: Occasionally a picture will be focused precisely when using PDAF. The rest ist spray and pray.
I think you misunderstood my last comment JoJu!
My Live view is the problem!
Well after all this I've changed lenses 2 samples of AF35mm F2D and the AF50mm F1.8D................ and in LV I "cannot" get infinity focus... ..repeat..... I CANNOT GET INFINITY FOCUS IN LIVE VIEW......everything is front focused.... .the system that is inherently accurate is in fact inaccurate and useless!
"I'm sorry to read you having problems with your D750" ^_^ that was your line a couple of posts ago. Indeed, I didn't read carefully enough. What can I say? Mine is accurate, bitingly sharp.
Before you start a mayhem :ph34r: with " ...the system that is inherently accurate is in fact inaccurate and useless!" - have you tried to enlarge the focus point and get manually better focus? How you're engaging focus, pushing the gutter release half way or using AF-L button for that? Which kind of focus mode you're using, AF-S or AF-F? Focus field wide, normal, tracking or face detection? was it a plane surface and sort of rectangular to optical axis or did the focused surface move away?
I tried, but I can't confirm your findings, sorry man. 
Right so the deal is this:
In PDAF in good light at all my lenses within reasonable bounds focus accurately.
The one lens I didn't calibrate completely was the AF35mm F2D which showed back-focusing last night in night-shots.....Having re-calibrated it "today" it needed -2 and now focuses OK.
in PDAF. Or at least typical of how PDAF should focus.
Checking against CD AF using live view, I found the 35mm front focuses and will not focus to infinity!
The AF50mm F1.8 also LV front focuses and will not focus to infinity.
AF20mm F2.8D have not tested for LV accuracy but it will focus to infinity.
AF24mm F2.8N have not tested for LV accuracy but it also will focus to infinity.
The AF85mm F1.8 have not tested for LV accuracy but it will focus to infinity.
Two of my lenses therefore defy the theory that CD AF is accurate without recourse to PDAF painstaking mechanical calibration, both these lenses are very well centered and function well in PDAF on the D750.
So "what the hell"" is is going on there? ..... A very good question and one that I have no real answer......other than is damned ironic in a thread about CD AF accuracies!
.. I've rarely used LV on my DSLRs because of their limitations within the system, I have however used the Pentax K01 mirror-less. It had focusing issues, again it could not reach infinity, but when returned from Pentax it worked OK.
So here again CD AF turned round and bit me in the "derrière" and the problem is there is nothing to adjust.
Never mind as Nikon's implementation of LV focusing is just too painful to use anyway!
Quote:"I'm sorry to read you having problems with your D750" ^_^ that was your line a couple of posts ago. Indeed, I didn't read carefully enough. What can I say? Mine is accurate, bitingly sharp.
Before you start a mayhem :ph34r: with "...the system that is inherently accurate is in fact inaccurate and useless!" - have you tried to enlarge the focus point and get manually better focus? How you're engaging focus, pushing the gutter release half way or using AF-L button for that? Which kind of focus mode you're using, AF-S or AF-F? Focus field wide, normal, tracking or face detection? was it a plane surface and sort of rectangular to optical axis or did the focused surface move away?
I tried, but I can't confirm your findings, sorry man.
Sorry JoJu
I should have written: "the system that is inherently accurate is in fact not always so in "my" camera".
(moment of irksomeness)
I was using AFS LV trying both small and medium focus points in fact I tried all the combinations.
In fact I'm pleased that I found -2 AFMA PDAF cured accuracy in normal light at least, like yourself having bought about 8 S/H lenses I also had plenty of lens checking to do and last night's AFD must have slipped through the net! (I also had to buy two copies of the 35, one slightly de-centered so I checked for centering and forgot AF calibration)
No the point being is, nothing in this world is fail-proof not even LV focusing.
I'm a happy boy and am not worried about CD AF mostly because of it's lack of speed and my eyesight. Here below are two shots, the first in PDAF focus point is on the can, the second LV AF the can is nearly in focus but too much in front remains in focus.
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.
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