[davidmanze]

I have typed it a few times before: the idea that the flange distance needs to be exact for AF to be on point is just plain wrong. So shims or no shims, it does not matter. What does matter? The lens making the final step less precise than the camera and user would want.

Or: some colour focussing at a different plane than others, a colour that the camera's AF sensor is sensitive to. This makes the camera see things in focus, when the user does not in the final image.

Dave, I feel reluctant to ask you a question since you rarely answer on point when I do. But I will make myself ask it anyway: Do you perhaps use different light sources for the close up situations and the further away situations?

     Please BC ask away!

                     I live in a boat with lots of windows..........nearly always sunny and most is shooting outside.  My simple starting test was on the D750 with the boats opposite which produced a super sharp image wide open...great!.......then a quick shot of a post ......and oh dear....straight to +10 and still Oh dear....+20 and errrr  derr!........lots of various shots at distance, close to...everywhere it's front focused, but fine on the D500 at +5.

 

   Anyway that game is on hold.....two more games since.

 

 The initial AF 80-200mm F2.8 push pull at 160 euros didn't materialise not a word further from the seller.......then out of the blue....another appears at the same price......with no fungus! 

  Please feel free to comment on any of my nonsense BC 

[Brightcolours]

Thanks for the answer, so since the light source does not change, that can not be a factor.

 

I am not well versed at all with the Nikon terminology concerning AF modes and settings. Assumption on my part: you are shooting in a non-AF tracking, focus needs to be confirmed AF mode? 

If so, try shooting with a continuous AF tracking mode which does not need focus confirmation before firing. Also, try shooting in MF relying solely on focus confirmation, to see if the results are better.

 

Just to understand the lenses.. not that it will fix the issue.

[JJ_SO]

I suspect the gear and "screw"-drive of the lens just has too much play.

 

It's weird that the AFMA is that widespread. Usually, with the new drives the difference between AFMA @ ∞ and closest distance is max 4-5. If the focus to the other sie of the river and close distance already reaches 20 and more, I suspect damage or wear of the focus transmission gear.

 

Nikon AF: AF-S means you push AF-ON (or half press shutter button), the lens tries to focus and stops, if the target is reached. No more correction until I press the AF-ON again.

AF-C means, focussing is still active as long as I press AF-ON or shutter button. I can set up, if the shutter releases or not if it's focussed (shutter or focus priority).

 

I don't know, dave: is there some kind of a clutch in the lens? Meaning, can you stop a movement of the focus ring, or can you turn (override) the focus ring when AF is active? Because if so, I would guess, somebody has used a bit of violence.

 

I know you're no fan of Sigma AF reliability. But I'm no fan ofd cheap lenses as they are sometimes cheap for an unpleasant reason  :unsure:

[davidmanze]

     BC. It doesn't make any difference what mode you are in and the D750 has good AF.  What we have here is a lens that accurately and reliably focuses just "in front" of the subject, it will be a great lens on the D500 and any other that just happens to remain within the AFFT.

 

 JoJu.  Yes there's a push pull slightly clunky mechanism for MF which works as it should.

   

    It looks like new and was a "one lady owner copy" box, pouch papers and all........absolutely no signs of it having done ten rounds with Mike Tyson!

 

 Sigma has never been renowned for it's focus accuracy over the years but has always provided good value for money glass, the same remains, only the glass is better than ever, unfortunately the many reports of capricious AF accuracy continue to the frustration of many, you've only to read the forums!

 

  Even though you slant the purchase price of my S/H lens as the probable reason for an AF problem that however works fine on the D500

     .....how do you align that with a lens with it's notoriously fickle AF, the Sigma 18-35 F1.8, which "is" expensive?

 

   It seems that price doesn't buy you out of the problem!

 

        The deal is struck with the Nikor AF 80-200 F2.8D!  another really cheap bargain basement lens, we will see if hiding behind that attractive price tag lurks another can of worms and whether Nikon's AF will be up to the job?

[JJ_SO]

Dave, things like "notorious fickle" or "never been renowned for it's focus accuracy" are something you bring in like I suspected "at this price I'd be careful what to expect". It's not my experience with the brand, otherwise I would not have bought around 10 lenses from their new "global vision" line. However, I don't have any elder model from them. Can't talk about that part of reputation. And honestly, I couldn't care less, as my Sigmas have no lower keeper rate than my Nikkors. Maybe just luck? I don't see myself as a die-hard Sigma fan, I just admit they got a fat bonus of sympathy when they first came out with the 35/1.4 at half the price of the genuine Nikon yet better in each aspect, except some scales and a more rigid lens body of the Nikkor. but that was compensated by the first go it's kind USB-dock. I'm no war photog, my lenses don't need to resist bomb blasts. 

 

The 18-35 is tricky to focus with not because it's unreliable, but because often there's too much going on in the AF single frame, at the same time f/1.8 has not much of a tolerance for inaccurately chosen focus points (a little bit of movement can make the AF jump to another distance. The AF frames are rather big on a DX camera. It's like using a caliper to measure the diameter of a rope. The caliper is precise, but how's the rope?

 

How do YOU explain the lens' reliability on a D500 and unreliability at D750? Do you have other lenses to check the "screw" drive with them, or is the Sigma the only one? If your D750 otherwise has a "good AF", does it have it with "screw" drive lenses as well? I'm just asking because with that mechanism I have experience from my Pentax days a decade ago and could not tell if it was good or bad. At the time I had Pentax (ok), Sigma (ok) and Tamron (sometimes ok) lens/es.

 

D AF Lenses

Do not use AF lenses with the lens focus mode switch set to M and the camera focus-mode selector set to AF. Failure to observe this precaution could damage the camera or lens. This does not apply to AF-S lenses, which can be used in M mode without setting the camera focus-mode selector to M.

I've no idea what really happens in this case. But (prejudice warning again!) in my opinion: New bodies work best with new lenses. Old lenses developed for old bodies in front of new bodies and expecting the AF could do what it never did before (being 3 × more accurate, i.e.) is something I try to avoid. I'm not saying old lenses necessarily are bad - but AF evolved during the last decade quite a bit.

[Brightcolours]

BC. It doesn't make any difference what mode you are in and the D750 has good AF.

Are you sure it makes no difference? With shutter priority, focus does not get checked finally, but with continuous focus, focus... continuously gets checked. It would be very, very odd if there would not be a difference.

[JJ_SO]

Actually, it does make a lot of a difference in my experience. It's a very valid question, BC.

 

Focus priority decreases fps to make sure to get an in-focus shot, especially with AF-C.

AF-S for handheld pictures, at close distances and apertures wider than f/4 just doesn't work well for me. Except the distance is very far or I use a tripod, AF-S is the exception for me. AF-C, especially if together with VR/OS/IS draws noticeably more power out of the battery, but here I get a better in-focus rate.

[davidmanze]

"How do YOU explain the lens' reliability on a D500 and unreliability at D750?"

 

   Generally up till now I have very few differences in lens AFFTs between the two cameras.

  

    Sigma has to reverse engineer their AF algorithms as best they can, that IMHO plays a large role......for instance the lens information that identifies the lens itself is incomplete.....many memorized AFFT settings are confused between themselves, my K mount Sigma 105mm macro (now gone) worked fine but took on the same AFFT settings of my old Bigma..if you change the setting for one it would change the other...the two identification codes were seen by the camera as the same.

  As far as I know all Nikon's lens identify themselves independantly.

 

​  None of the third party lenses use the full features of the later Nikon AF system ....even many of the older Nikon lenses do not either, Nikon publish a list of lenses that "do" use the full facilities available.....when I find it again I will post it.

 

    Between the lens identification codes and the fact that Nikon will not licence their soft ware to Sigma or Tamron for that matter puts them at a disadvantage, as to how much that affects things in the end is another question!   

   So enter the USB dock.......it gives us ordinary folk the opportunity to make our own settings straight on to the lens....a get round for those capricious lenses which had AF quirks here and there and giving us independence from the service departments.

 

    If you don't start out with Nikon's blessing, a manufacturer is out on his own when it comes to exactly what is needed to get the job done without hiccups........and effectively Sigma and Tamron are out on their own!

 

   There are many subjects that thanks to internet have been explained, exposed and been thoroughly understood, right down to the last detail, the exact working of PDAF isn't one of those!

[davidmanze]

Are you sure it makes no difference? With shutter priority, focus does not get checked finally, but with continuous focus, focus... continuously gets checked. It would be very, very odd if there would not be a difference.

 

 

   I never use the release setting in the menu BC! I see it as a way to get more OOF images.

 

    AF-C is continually on as long as you hold down the BBF button where AFS just does the one time.....( I know I don't need to tell you that)...with stationary subjects I have never noticed any difference in AF accuracy.

[JJ_SO]

Forgive me, dave, for trashing some of the fairy tales you just posted  B) :

 

Generally up till now I have very few differences in lens AFFTs between the two cameras.

U-hum. That makes me even more curious, why the lens works well with one and shitty with the other body?

 

Sigma has to reverse engineer their AF algorithms as best they can, that IMHO plays a large role......for instance the lens information that identifies the lens itself is incomplete.....many memorized AFFT settings are confused between themselves,

 

It still doesn't explain the different quality of cooperation between your two camera bodies. Also, this reverse engineering often is used to point out how bad a lens will be because it's not "the real thing". But if they do reverse engineering for decades, don't you think they know sometimes more about Nikon lenses than most managers, designers and users do? And don't you see the chance that the reverse engineerers are perfectly able to avoid some flaws? I remember very well a damaged focus drive of the 85/1.4 G or the uneven working VR of the pretty expensive 300/4 PF E. None of my Nikkors ever gets a better VR result than Sigma's OS - and Tamron's also working very well. Genuine lenses are in no way any guarantee for flaw-free products, just saying.

 

As far as I know all Nikon's lens identify themselves independantly.

 

As far as you know - that's the same as I know, too, so until one tells the opposite, we can agree on that.

The Sigma 24-105/4 Art is always confused with the Micro Nikkor 105/2.8. The latter wins in the register of AFMA-  but until I mount it, the cameras (this is ongoing since D800... and Sigma didn't fix it with a firmware update) recognizes a 24-105/4 lens! You know how I work around? Super simple. I use the dock to adjust the Sigma to the same +7 or so. Earlier I used paper labels with the AFMA value on them sticked to the inner side of the front cap, works also. These two lenses have very different purposes, so it's simply no problem just a bit of an inconvenience.

 

None of the third party lenses use the full features of the later Nikon AF system ....even many of the older Nikon lenses do not either, Nikon publish a list of lenses that "do" use the full facilities available.....when I find it again I will post it.

 

That statement stands a bit lonesome without some evidence backup. Which magic AF features are not supported? One would be enough. Remember, the new feature "focus stacking" or "AF adjustment in camera" (D500 / D850)? None of my not genuine lenses made any problem with using these functions. So once again: Maybe reverse engineered, but with some skills... And they don't have to reverse engineer only Nikon, but also Canon (or do they have a license to use their AF-techniques?). So, Sigma users could get the best out of two systems as originals. I don't see that as bad as you apparently do.

 

Between the lens identification codes and the fact that Nikon will not licence their software to Sigma or Tamron for that matter puts them to a disadvantage, as to how much that affects things in the end is another question!

 

No, it doesn't. Either you have "competing" lenses which head towards the same memory bank - then paper labels or adjusting one lens to the other come into play. Or one lens is identified  as another - as long as it's consitent, who cares?


 

So enter the USB dock.......it gives us ordinary folk the opportunity to make our own settings straight on to the lens....a get round for those capricious lenses which had AF quirks here and there and giving us independence from the service departments.

 

In the sample before (24-105/4 or 105/2.8), the USB-dock gets me better focus reliability at all focal lengths and distances than setting one AFMA for the whole lens. It's more work, but the result is usually better than what I get out of the (fast) Nikkors.


 

If you don't start out with Nikon's blessing, a manufacturer is out on his own when it comes to exactly what is needed to get the job done without hiccups........and effectively Sigma and Tamron are out on their own!

... and in business since a couple of decades. If they were not good at what they do, they already went the Soligor, Tokina, Hanimex, Yashica, Minolta, Rollei path - and became unknown by youngsters.

 

There are many subject through the internet have been explained, exposed and been thoroughly understood, right down to the last detail, the exact working of PDAF isn't one of those!

Neitehr is the "exact" working of CDAF or dual pixel or whatever - these techniques don't work better if you can explain them exactly - you just need to learn and exercise what they do and when they are in a weak or strong zone .