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Next PZ lens test report: Tokina AF 50-135mm f/2.8 AT-X Pro DX
#1
I really wanted to love this lens... but some day I had to give up. If you find a good copy, it's certainly fun in the field, though.



http://www.opticallimits.com/nikon--nikk...a5013528dx



-- Markus
Editor
opticallimits.com

#2
Thanks for the review Markus... One remark from my side about the focus shift: In the bokeh fringing test shot I see that @f/8 the focus shifts slightly forward (behind the focused 100-15 mark). Can we still say that "These shots also illustrate that the lens does not suffer from focus shift when stopping down" ? Is it within the tolerance limits?



Serkan
#3
Sorry, that was a copy&paste error, actually that statement is from the AF-S 28 review, which I used as basis. I removed the whole paragraph, as I think focus shift is well within tolerance limits on the Tokina lens.



-- Markus
Editor
opticallimits.com

#4
I would like to take this chance to ask a question about focus shift since it is mentioned above. I understand the meaning of "stop-down" type focus shift, which is presumably caused by the spherical aberration of the lens. But there seems to exist another kind of focus shift: the focus shift when the lens is wide open. And, from what I read from the internet, this kind of focus shift can arise either from the malfunction of the camera (e.g. the foucus sensor in the camera is not properly aligned) or from the lens. My question is: how can this wide-open focus arise from the lens? If the focus sensor is perfecly algined then what you see in the OVF is what you get in the image sensor--the essense of SLR. So, what kind of problems with the lens can make the precise AF system of the camera think that the image is in focus while the image on the sensor plane is not in focus?



Thanks,

Frank
#5
Can somebody answer my question? Or, is my question too silly? <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='Smile' />
#6
[quote name='Frank' timestamp='1342760032' post='19478']

Can somebody answer my question? Or, is my question too silly? <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='Smile' />

[/quote]



I don't think it's silly...



"So, what kind of problems with the lens can make the precise AF system of the camera think that the image is in focus while the image on the sensor plane is not in focus?"



You exclude the optical problems like focus shift and field curvature and ask about other potential problems arising from the lens itself. If we eliminate the optical elements, what remains is the built-in AF motor of the lens. AFAIK, the camera body does the math for PDAF / CDAF and -incase of a lens with AF motor- tells the lens motor to move the focusing elements based on these calculations. So in general I believe the body itself is mostly to blame (either the calculations or data stored for lenses in the internal tables or even maybe the parts like misaligned AF sensor / back mirror etc...). But maybe there's the possibility that the built-in AF motor in the lens does not listen to the data provided by the body.



Serkan
#7
Hi Serkan:



Thank you for the answer.



However, I am still confused. According the article http://mansurovs.com/how-phase-detection...ocus-works, the phase-detection AF focus system works in the following ways (cited from that article):






  1. The light that passes through the extreme sides of the lens is evaluated by two image sensors

  2. Based on how the light reaches the image sensors, the AF system can determine if an object is front or back focused and by how much

  3. The AF system then instructs the lens to adjust its focus

  4. The above is repeated as many times as needed until perfect focus is achieved. If focus cannot be achieved, the lens resets and starts reacquiring focus, resulting in focus “hunting”

  5. Once perfect focus is achieved, the AF system sends a confirmation that the object is in focus (a green dot inside the viewfinder, a beep, etc)




"Once perfect focus is achieved, the AF system sends a confirmation that the object is in focus (a green dot inside the viewfinder, a beep, etc)". So I assume that when the AF sensors "sees" a state in the Figure 2 in that article the camera "thinks" that the subject is in foucus, then the camera tells the motor stop moving and let the lens elements stay in their positions, then images are taken. So, according to your answer, it can happen that for some problematic lenses, although the motor puts the lens elements in wrong positions, the AF sensors in the camera can still see a state as in the Figure 2 and then a out-of-focus image is taken. Is it right?



Best regards,

Frank



[quote name='PuxaVida' timestamp='1342770407' post='19480']

I don't think it's silly...



"So, what kind of problems with the lens can make the precise AF system of the camera think that the image is in focus while the image on the sensor plane is not in focus?"



You exclude the optical problems like focus shift and field curvature and ask about other potential problems arising from the lens itself. If we eliminate the optical elements, what remains is the built-in AF motor of the lens. AFAIK, the camera body does the math for PDAF / CDAF and -incase of a lens with AF motor- tells the lens motor to move the focusing elements based on these calculations. So in general I believe the body itself is mostly to blame (either the calculations or data stored for lenses in the internal tables or even maybe the parts like misaligned AF sensor / back mirror etc...). But maybe there's the possibility that the built-in AF motor in the lens does not listen to the data provided by the body.



Serkan

[/quote]
#8
I think the critical thing is that the AF sensor does not actually see anything in this process. Based on the data tables and the info AF sensor received, camera body sends the data to the lens for the movement.



I guess even for each lens coming out from the same production line there would be tolerances with regard to AF accuracy. Otherwise AF micro adjustment option on the bodies would be meaningless...
#9
[quote name='Frank' timestamp='1342775556' post='19482']

Hi Serkan:



Thank you for the answer.



However, I am still confused. According the article [url="http://mansurovs.com/how-phase-detection-autofocus-works"]http://mansurovs.com...autofocus-works[/url], the phase-detection AF focus system works in the following ways (cited from that article):



  1. The light that passes through the extreme sides of the lens is evaluated by two image sensors
  2. Based on how the light reaches the image sensors, the AF system can determine if an object is front or back focused and by how much
  3. The AF system then instructs the lens to adjust its focus
  4. The above is repeated as many times as needed until perfect focus is achieved. If focus cannot be achieved, the lens resets and starts reacquiring focus, resulting in focus “hunting”
  5. Once perfect focus is achieved, the AF system sends a confirmation that the object is in focus (a green dot inside the viewfinder, a beep, etc)



"Once perfect focus is achieved, the AF system sends a confirmation that the object is in focus (a green dot inside the viewfinder, a beep, etc)". So I assume that when the AF sensors "sees" a state in the Figure 2 in that article the camera "thinks" that the subject is in foucus, then the camera tells the motor stop moving and let the lens elements stay in their positions, then images are taken. So, according to your answer, it can happen that for some problematic lenses, although the motor puts the lens elements in wrong positions, the AF sensors in the camera can still see a state as in the Figure 2 and then a out-of-focus image is taken. Is it right?



Best regards,

Frank





[/quote]

Hi Frank,



I am not Serkan, of course, but I'll give this a try regardless <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='Big Grin' />.



With PDAF, the camera doesn't actually try to focus at all, that's just the theory. It tries to calculate a best match for the amount of lens extension or focus element movements (stepper motor increments) required for the lens to focus correctly based on the starting position of the lens. Furthermore, there is no active feedback loop with PDAF, although focus hunting in difficult cases comes fairly close.



A lens really only reports back that is has executed the command provided by the body successfully (or not), i.e., it has activated the stepper motor the required number of increments in the direction as indicated by the camera.



Essentially, the camera AF system has a special profile for a specific lens, or a default profile. The lens electronics need to know how to react to stepper motor increment increase requests. If a lens does not occur in the camera AF lens database, the lens manufacturer has to either adapt (and reverse engineer) the stepper increment requests for the lens mapped to an existing lens, or to the default. Do note that the signal given by the lens to the camera really only indicates that it could successfully execute the requested command as generated by the camera AF system, e.g., move the lens x steps forward <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='Big Grin' />.



This means that there are three potential problems possible here, where there still appears to be an in-focus signal, as the lens executes the command requested by the camera:

1) The camera vs lens tolerance for increases is not within tolerance, f.e., the body really is at -4 and the lens at -4 (generally causes by mount tolerance variations), for a total of -8, while tolerance is, e.g., -5, resulting in OOF images all of the time (even if at smaller apertures they may still appear in focus)



2) The lens tolerance does not conform to the camera AF system lens profile, whether default or specific, which may result in variation in achieving focus. It may be out all the time, or only at certain specific requests. Generally speaking, one would expect it will be out all of the time in this case, although images again may appear in focus due to DoF.



3) The lens profile as implemented in the lens doesn't conform to the lens profile of the camera AF system. Focus will likely be all over the place, although in repeatable fashion, with different focusing points at different distances or even with different incremets asked for by the AF system. The exact variations may be repeatable, but they may also be not, depending on the movement the lens has to make.



Of course, the lens (or camera AF) electronics may be faulty too, but I have excluded that from this discussion.



A problem like #1 is typically an OEM problem, and generally can be fixed by the manufacturer. #2 may be an OEM problem, but also a 3rd party lens manufacturer problem. #3 typically is a 3rd party lens manufacturer problem.



Besides the above, there still are lens AF problems possible due to optical aberrations, specifically spherical aberrations, which may cause focus shift varying with distance and aperture (less with focus further away and less with smaller apertures), and things like astigmatism etc., which may throw out the PDAF system especially at larger apertures. WA and UWA lenses also may give problems, because for PDAF the beams required for focusing should not cover a too large area at a fairly large AoV, because it will have problems matching these. The latter is inherent to the PDAF system. This is also why the AF point coverage with PDAF systems never covers the entire image in the VF. Do note that the "extreme sides of the lens" for interpreting AF is incorrect; this should be extreme sides of the (specific) AF sensor, or rather, the two elements (or more, but always in pairs) capturing the split image part used for AF calculations. Another optical problem potentially causing focus problems with PDAF is field curvature, but there are a few more anyway. When one thinks about it, it really is amazing how well it generally works, if you ask me <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='Big Grin' />.



BTW, I have experienced problems in all the 3 categories mentioned.

#1 generally is the easiest to fix, by the manufacturer, or by oneself with MFA, provided, in the latter case, a body actually has MFA. I have had this happen with a bunch of large aperture lenses, especially.

#2 can generally be fixed by the manufacturer, 3rd party or not, but may require some time, especially if the lens electronics do not correspond properly to the camera AF system lens profile. I've had problems with Sigma lenses specifically in this regard.

#3 is the hardest problem to fix, and can really only be done by the OEM or 3rd party manufacturer. I reckon an example of this is the Canon 50L, which requires a special profile IMO to work correctly at all distances and apertures. The problems with this lens, if not manufactured and assembled by the smallest of tolerances, were very repeatable on an individua lens basis, however. Canon improved the lesser PDAF systems from the 50D and 5D II onwards, so there were certainly less problems with the newer bodies from thereon, but I reckon they also improved the lens profile and lens manufacturing and assembly to get better results.



Kind regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....
#10
Hi Wim:



Thank you very much for a very detailed answer.



Let me put my question in another way: Suppose that we have a lens which has focus shift problem when the lens is wide open and the problem is caused by the lens (e.g., wrong information communication with the camera); and we have a perfect DSLR camera which has perfectly aligned and calibrated AF sensors and a perfectly aligned OVF and a perfect focus screen so that what we see in the OVF is what we get. Then, mount this problematic lens on this perfect camera. Now let us do the following tests (the lens is kept wide open):



1. Manual focus the lens and judge the focusing state through the OVF by a "super" naked eye; until we see the image in the OVF is perfectly in focus then we press the shutter to take the image. Will focus shift happen in this case? (If focus shift happens then I will be surprized since I expect that "what we see is what we get".)



2. Manual focus the lens and judge the focusing state through the OVF by the "focus confirmation" function, e.g. the green dot in Nikon DSLRs. According to the article I cited in another post in this thread, the green dot in the OVF turns on only when the state in the figure 2 in that article is detected by the AF sensor. When the green dot turns on, press the shutter to take a picture. Will focus shift happen in this case?



According to your answer, for a given pre-focus state (e.g., the state in figure 1 in that article) the camera calculates the distance and the direction that the lens need move its elements to get precise focus (figure 2 in the article) and passes this information to the lens, then the lens just follows the order from the camera. If the lens misunderstands the information that the camera passes to it, then focus shift can occur. But in the above two cases I do not let the lens do AF, I do the focus job with my hand <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='Big Grin' /> So I guess here I am essentially asking "Can the wide-open focus-shift problem be solved by MF the lens?" (of course it is assumed that the shift is caused by the lens not by the camera.)



Best regards,

Frank
  


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