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12-18-2008, 11:01 PM   #16
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QuoteOriginally posted by jeffkrol Quote
Canon says "no"
So what?
Nikon says "no" to a wet-cleaning method they are using themselves internally.

Perhaps Canon wants to align cameras in service centres instead of leaving it to users?

QuoteQuote:
Do not attempt to autofocus on an angled chart, because doing so will degrade the consistency of the camera's focusing measurement.
"Degrade"? As in permanently?
If anything, the focusing may not be as consistent as it could be, but AF focusing is never a 100% consistent anyhow. One should always do several runs. Instead of just starting from infinity, one should also start from the minimal focus distance (-> service manual).

QuoteQuote:
Keep in mind that the camera's AF sensor is comprised of multiple pairs of linear pixel arrays. If you attempt to autofocus on a single line in an angled focusing chart, only a few pixels from each active pixel array will "see" the target.
The "line" of the focus chart I'm using is actually a bar and unless you are using a wide angle lens it will nicely fill the AF sensor area.

Also, there is a focus chart where the focus target isn't a line but a black-to-white transition. All, or at least many, AF pixels will be used in this case.

QuoteQuote:
and the reference target should be perfectly parallel to the camera's focal plane.
Why? An AF sensor cannot focus on a surface without structure and just one colour (try your wall), even if it is parallel to the sensor. How shall it distinguish between such a flat version and and angled version of that? Try to AF against a wall at an angle. It doesn't work either. The AF sensor isn't confused. It appears that to all intents and purposes the angled version is equivalent to the non-angled version and I'd be more than happy to learn what is wrong about this view.

AFAIC, the quotes you provide just don't provide sufficient technical backing up to be convincing.

Here's an interesting method for those with live view: AF Micro-Adjustment tool - Open Photography Forums.

12-19-2008, 01:17 AM   #17
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QuoteOriginally posted by Lowell Goudge Quote
what bothers me about all the focusing chart tests I have seen posted here is that I have NEVER seen a photo of one where the camera (from left to right) is square on the target.
I strive to have the camera square on the target but it is not really critical.

QuoteOriginally posted by Lowell Goudge Quote
this makes the distance scales on both sides not on the focusing plane, and really has only one point (not a line or a surface) at the correct focusing distance, and then people complain about front and back focusing.
The focus chart is a plane. The area of sharpest focus is a plane (it isn't really most of the time, but that's a good enough approximation). Two planes will always intersect forming a line unless they are parallel to each other (and not on top of each other). When you shoot a plane with an angle, you'll always get a line of sharpest focus. If the camera is not square on the target then that line won't be horizontal and hence the BF/FF indication won't be the same on each side. But since a camera which is not square on the target will have one side of the focus chart closer to it than the other, you'll just get an asymmetric reading, but never a false BF or FF indication.

I believe there are probably a number of false readings of focus charts, but these stem from
  • using tungsten light
  • using a non practical focusing distance
  • interpreting normal AF variations as BF/FF symptoms
12-19-2008, 07:09 AM   #18
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QuoteOriginally posted by jeffkrol Quote
Originally Posted by Doug_Kerr View Post
Regarding the matter of using just an angled test target for AF testing, I happened to run into this from Chuck Westfall of Canon in his Tech Tips column in The Digital Journalist for December 2008:

Do not attempt to autofocus on an angled chart, because doing so will degrade the consistency of the camera's focusing measurement. Keep in mind that the camera's AF sensor is comprised of multiple pairs of linear pixel arrays. If you attempt to autofocus on a single line in an angled focusing chart, only a few pixels from each active pixel array will "see" the target. Ideally, the contrast in the reference target should cover the entire area of the camera's center focusing point, and the reference target should be perfectly parallel to the camera's focal plane.
This is one example of such poor wording that it makes the statement invalid.

So what is he trying to say here. The AF is not heuristic based, and doesn't improve or degrade the more you use it! All contrast based AF systems depend on the fact that there are differences in the contrast within the few pixels within the array otherwise they don't know what to focus on. The AF array doesn't see a selective number of pixels, it sees them all whether white or black or any shade in between. This is one of the best examples of spreading FUD that I have ever seen, maybe Doug should be in marketing!

Now I am not picking on you Jeff, and I like the idea of the LensAlign although I think its overpriced (personal opinion, YMMV). There is also another novel way of checking autofocus using Liveview here is a link:

AF Micro-Adjustment tool - Open Photography Forums
12-19-2008, 08:13 AM   #19
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QuoteOriginally posted by WheresWaldo Quote
This is one example of such poor wording that it makes the statement invalid.

So what is he trying to say here. The AF is not heuristic based, and doesn't improve or degrade the more you use it! All contrast based AF systems depend on the fact that there are differences in the contrast within the few pixels within the array otherwise they don't know what to focus on. The AF array doesn't see a selective number of pixels, it sees them all whether white or black or any shade in between. This is one of the best examples of spreading FUD that I have ever seen, maybe Doug should be in marketing!

Now I am not picking on you Jeff, and I like the idea of the LensAlign although I think its overpriced (personal opinion, YMMV). There is also another novel way of checking autofocus using Liveview here is a link:

AF Micro-Adjustment tool - Open Photography Forums
Hey no problems. It's all just for informational purposes.
The Bart_van_der_Wolf interference test pattern is interesting and free....
What happens as one manually adjusts the focus is that at the exact optimal focus setting the background will change from uniform gray into larger (colored) aliased dots when viewed on the camera's LCD. The circles and cross hair will allow to acquire AF easily, and when calibration is optimal, moiré will be maximized on the camera's LCD.

A procedure that works for me:
- I switch to LifeView, which in its current implementation will only allow manual focus.
- Optimize manual focus by searching for maximum aliasing. This will only occur a best focus (and assuming a decent enough lens is used), otherwise the defocus will act as a low-pass filter and prevent the aliasing.
- Switch off LifeView, and watch the lens barrel's focus indicator for the next step.
- Use AutoFocus (single AF spot) to focus on the (laptop) LCD screen, and watch the direction of adjustment. That will show whether the current AF calibration setting will front or back-focus.
- Apply an adjustment via the camera menu, and repeat the procedure. Once the adjustment is optimal, there will be no difference between manual and auto-focus.

The flat computer screen will prevent misinterpretation of the focus distance, because the AF system cannot react to phase effects from subjects at other distances. That makes it quite easy to get repeatable results. And because the computer LCD emits light, it can be easily done indoors, at common shooting distances for the lens to be calibrated for.

For super tele lenses it is probably easier (for distance reasons) to use a Zoneplate type of chart outdoors.

Bart

12-19-2008, 09:38 AM   #20
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Jeff,

Have you actually tried this? I have and the moire is very evident on even Pentax's poor version of LiveView. The interesting thing is that if you have focus confimation on, when you leave LiveView if you try to autofocus a lens that is focused properly wont budge. If its off a little you will get a very short whirr from the AF motor as it adjusts. Two post it notes with a registration line will allow you to swing focus to infinity or minimum and should snap back to realign the mark if AF is working properly.

It works, I tried it. For those lenses without a focus clutch or manual AF switch holding in the lens release button will allow you to adjust focus to infinity or minimum without switching to manual focus.
12-20-2008, 04:02 AM   #21
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QuoteOriginally posted by WheresWaldo Quote
There is also another novel way of checking autofocus using Liveview here is a link:

AF Micro-Adjustment tool - Open Photography Forums
I've just discovered this recently and it is a very cool idea. For anyone using this method, however, I'd suggest to confirm the calibration setting with a standard focus chart in daylight. The colour temperature of the target LCD may influence the AF system just as tungsten light does.
12-20-2008, 08:04 AM   #22
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Well...

I love DIY projects so i'll be making my own lens align and will post here so we can save money to buy more lenses
12-20-2008, 08:15 AM   #23
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QuoteOriginally posted by vitalsax Quote
I love DIY projects so i'll be making my own lens align and will post here so we can save money to buy more lenses
Looking forward to seeing it.

02-07-2009, 07:55 PM   #24
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Any news Vic on your DIY project?

- Andrew
02-07-2009, 08:06 PM   #25
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The pattern mentioned below is different but looks cool when you bring it up on LCD screen. In my small computer room it may be hard to get far enough away to focus.


https://www.pentaxforums.com/forums/pentax-dslr-discussion/46793-moire-patter...djustment.html
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