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03-27-2014, 10:49 AM   #1
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What influence does the lens have on AF accuracy vs camera?

Hi,

I am pretty new to DSLRs and my engineer's brain is intrigued by the technology behind them. The latest topic I fail to understand is how a lens can influence AF accuracy. There seem to be quite a few lenses that are known for inaccurate AF.
My understanding is that the camera detects out of focus conditions and adjust focus accordingly. Now how can the lens mess this up? I can see how some lenses are slower than others but in the end it should be the camera that makes the decision. Or am I misunderstanding how things work?

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03-27-2014, 11:18 AM   #2
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Make a search for Focus Shift. This is one of the reasons a lens can mess up focus accuracy. Other than that I canīt think how a lens can mess with AF accuracy. AF speed does depends much more on the lens than on the body. A long focus throw, Type of AF motor, brightness of the lens, all play a part in AF speed.
03-27-2014, 11:31 AM   #3
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For starters, Maxxx, lenses are not equally bright and focusing by contrast detection will be less accurate.

In various situations you're better off manually focusing.

Last edited by clackers; 03-27-2014 at 11:37 AM.
03-27-2014, 11:46 AM   #4
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My understanding is that if the lens isn't set correctly, it can affect focus in several ways:

1) Badly design lenses will improperly split the light going to the phase detection module, causing incorrect focus.
2) Lenses with in-lens motors may overshoot or undershoot as they are activated to focus.
3) Lenses with very low wide-open contrast (very fast lenses) are near impossible for CDAF systems to focus.

03-27-2014, 11:47 AM   #5
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And if a lens isn't adjusted correctly it can focus ahead of or behind the target. This is why the newer cameras allow you to micro-adjust focus and can remember different lenses.
03-27-2014, 11:48 AM   #6
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QuoteOriginally posted by clackers Quote
For starters, Maxxx, lenses are not equally bright and focusing by contrast detection will be less accurate.

In various situations you're better off manually focusing.
That makes sense.

But when I read lens reviews some of them seem to have a reputation for inconsistent focus (e..g. read the Tamron 70-200 reviews on Amazon). And I wonder how a lens could cause this.
03-27-2014, 11:50 AM - 1 Like   #7
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QuoteOriginally posted by maxxxx Quote
That makes sense.

But when I read lens reviews some of them seem to have a reputation for inconsistent focus (e..g. read the Tamron 70-200 reviews on Amazon). And I wonder how a lens could cause this.
Based on focus shifting, as another user had said. What is Focus Shift?

Lenses are focused using their widest aperture.

03-27-2014, 11:52 AM   #8
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A lens may bias the AF system due to mechanical considerations. Think hysteresis and mechanical lash. There is also an element of compliance to specification for the AF drive. Longitudinal chromatic aberration may also bias the AF system. I am not sure, but I believe that most per-lens AF calibration is an attempt to account for the last two points.

As for how the PDAF system works...the system does not detect an OOF condition so much as it finds the best phase match from its array of sensors. In other words, it seeks until it finds. There used to be an excellent applet out there that gave a very easily understood demonstration of AF in action...now if I could only find the link

In regards to carrlangas' and others comment...yes, focus shift with stop-down does happen and yes, it may throw off critical focus. In practice, this is usually acceptable within the precision of most AF systems** and is mostly a problem with critical manual focus cases. I have personally seen it when doing stop-down work using a split image and certain legacy lenses.


Steve

* PDAF is the AF system used for viewfinder photography with an SLR. CDAF (based on detected contrast) is the AF system used for live view. CDAF is intrinsically more accurate and more precise than PDAF.
** The AF points for most PDAF systems have a working precision equivalent to a f/5.6 aperture. The better systems have at least some of their sensors optimized to f/2.8. The Pentax K-3, for example, has three f/2.8 center points. Previous Pentax models have f/5.6 detectors only. Accuracy of PDAF systems for critical focus with faster lenses is sort of a cruel joke as a result. Before leaving the subject, it might be good to note that the precision of microprism focus aids in most manual focus SLRs have a similar f/5.6 limit.

---------- Post added 03-27-14 at 12:02 PM ----------

QuoteOriginally posted by stevebrot Quote
...now if I could only find the link
Found it! It takes awhile to load, but it is only too cool when it does. The accompanying lecture notes are also one of the best descriptions I have read as well.

Autofocus: phase detection

Here is the corresponding applet and discussion for CDAF:

Autofocus: contrast detection


Steve

Last edited by stevebrot; 03-27-2014 at 12:16 PM.
03-27-2014, 12:13 PM   #9
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QuoteOriginally posted by JinDesu Quote
Based on focus shifting, as another user had said. What is Focus Shift?

Lenses are focused using their widest aperture.
Thanks for that article - i've heard the term below but never found a good explanation before.

So how does a lens manufacturer calibrate a lens for focus - at a wide angle lens setting or zoomed out to its largest tele focal length?

I've read, and by my own testing - the DA 50-135 seems to be sharper at the wider end than the tele end. The DA 55-300 has a reputation for just the opposite - sharper at the longer end.
03-27-2014, 12:26 PM   #10
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QuoteOriginally posted by philbaum Quote
So how does a lens manufacturer calibrate a lens for focus - at a wide angle lens setting or zoomed out to its largest tele focal length?
Roger at his LensRentals blog has a lot of information that can answer at least part of your question.

M
03-27-2014, 12:41 PM   #11
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This is a great discussion. thanks for all the links!
03-27-2014, 12:42 PM   #12
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QuoteOriginally posted by philbaum Quote
Thanks for that article - i've heard the term below but never found a good explanation before.

So how does a lens manufacturer calibrate a lens for focus - at a wide angle lens setting or zoomed out to its largest tele focal length?

I've read, and by my own testing - the DA 50-135 seems to be sharper at the wider end than the tele end. The DA 55-300 has a reputation for just the opposite - sharper at the longer end.
Yep, Lensrental actually has a lot of info that pertains to this as well - including manufacturing variation.
When I set my Tamron 70-200 to 0 micrometer adjustment at 135mm, it back focuses by 10 micrometers at the short end, and it front focuses by 70 micrometers on the long end. Part of this massive discrepancy is likely mechanical damage (something went weird with the lens a while back), but it always had slight front focus on the long end since the day I bought it. Therefore it came with mild manufacturing issues and the damage accentuated the difference.
03-27-2014, 01:09 PM   #13
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QuoteOriginally posted by philbaum Quote
the DA 50-135 seems to be sharper at the wider end than the tele end. The DA 55-300 has a reputation for just the opposite - sharper at the longer end.
Not mine! The DA*50-135 has good enough IQ throughout that I haven't distinguished one end as better from the other, but I'd lean toward the long end being better. In any case, it's very good at the long end.

The DA55-300 had nicer IQ between 55 and 135mm. This is why I sold it - because it served no purpose if I could replace it with my DA*50-135 to get even better IQ. Then I use primes at 200 and 300mm.


Granted, the lenses I replaced it with cost considerably more. But the IQ difference is substantial - especially at the longer focal lengths. Nevertheless, I'm still a fan of the DA55-300, because it's good for the money.
03-27-2014, 01:21 PM   #14
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QuoteOriginally posted by JinDesu Quote
When I set my Tamron 70-200 to 0 micrometer adjustment at 135mm, it back focuses by 10 micrometers at the short end, and it front focuses by 70 micrometers on the long end.
This always sort of puzzles me since the same lens when manually focused will be accurate 100% of the time.

What are the manufacturers or service techs adjusting when you send a lens in? Are they tweaking the drive response or just replacing potentially worn parts?

I am curious too as to how are you measuring and applying your adjustments? Are you saying you have a way of measuring 10 ĩm (1/100 mm) back focus at the focal plane and that you have some way of adjusting your lens so that this is no longer the case?


Steve

Last edited by stevebrot; 03-27-2014 at 01:27 PM.
03-27-2014, 01:23 PM   #15
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QuoteOriginally posted by carrrlangas Quote
Other than that I canīt think how a lens can mess with AF accuracy. AF speed does depends much more on the lens than on the body. A long focus throw, Type of AF motor, brightness of the lens, all play a part in AF speed.
Getting closer to the original topic here, the body has a huge influence on AF speed, especially with a screw-drive lens. It will also affect accuracy, but this may not be as obvious in use (especially before viewing the photos).


For most shooting the differences may not matter much, but for demanding AF applications like shooting fast action, it can make all the difference. The focusing motor - wherever it's located - is a big factor. So with a screw-drive lens, the body matters a lot. The last 2 generations of Pentax bodies do much better. In fact, I got very frustrated last weekend with my K-5 inside a gym, while my K-5 IIs (and the K-3 I briefly borrowed) really came through. And unlike competing systems where they emphasize the performance of their in-lens motors, with Pentax I got better results with the screw-drive lenses.
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