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FORUMS Canon Cameras, Lenses & Accessories Canon EOS Digital Cameras 
Thread started 02 Mar 2009 (Monday) 04:36
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Canon Focusing explained - when soft is sharp enough

 
David ­ Ransley
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Mar 02, 2009 04:36 |  #1

Thought this will come in handy to those who battle with soft images and or focusing.

http://http …da.com/forum/to​pic/241524 (external link)

It helped me with my 40D focusing and explaines why image sharpness improves if you activate the focusing system more than once. The phase difference calculation is an open loop system, which only looks once at the beginning of the process. The minor adjustment that follows is a closed loop system. If the initial adjsutment is a large one from way out of focus to sharp, then the random focus selection in the sensitivity range of the lens may result in softer enlargements. Canon says sharp is 6x9 inches viewed at 10 inches.

Hence Pixel peeping may = soft :D

It takes a while to study this system and how to get the maximum out of it.


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troutfisher
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Mar 02, 2009 05:21 |  #2

Very interesting-thanks for posting


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neilwood32
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Mar 02, 2009 06:51 |  #3

That would explain why the cameras focus better if you take the time to press the focus button twice - ive certainly found this to be true. The first gets it reasonably close and the second gives a "micro adjustment" which is very near perfect.


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JoYork
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Mar 02, 2009 07:41 |  #4

One person on this forum has reported an e-mail from Sigma stating that Canon cameras will tend to stop at the first part of the depth of focus range they reach, depending on the direction the lens must move to get there. My results have not verified that, and I don't think it really corresponds to how Canon says the system works."

I wonder if this explains the awful track record of Sigma lenses when it comes to front and rear focusing.


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iwannabe
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Mar 02, 2009 08:50 |  #5

Thanks for bringing this to our attention! I plan on changing my technique. I've been kind of wondering about the softness issue. This should help quite a bit!


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Beanie's ­ Dad
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Mar 02, 2009 10:04 as a reply to  @ iwannabe's post |  #6

Wow - that explains a lot of the unpleasant focusing surprises I've had. Now that I know what the camera's doing I've got a chance of adapting my technique to get more keepers. Many thanks for posting that!


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Mar 02, 2009 10:10 |  #7

I've read that article before though not on Miranda's site and have been double, triple tapping the AF since. It is a very good read though, almost should be put into the user manual.


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tkbslc
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Mar 02, 2009 10:16 |  #8

Canon should make an option to turn on "AF-refinement" that goes through 2-3 AF loops. You could turn it off for action shots, and turn it on for more accuracy. Seems like it would be an easy firmware enhancement.


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WaltA
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Mar 02, 2009 10:44 |  #9

Great article. Thanks for sharing.


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René ­ Damkot
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Mar 02, 2009 11:04 |  #10

David Ransley wrote in post #7436263 (external link)
The phase difference calculation is an open loop system, which only looks once at the beginning of the process. The minor adjustment that follows is a closed loop system.

That's not about a second time activating the AF, the way I read it:

FredMiranda thread wrote:
Although the camera does not take a "second look" to see if the intended focus has been achieved, the lens does take a "second look" to ensure it has moved the direction and distance commanded by the camera (it is a "closed loop" system). This second look corrects for any slippage or backlash in the lens mechanism, and can often be detected as a small "correction" movement at the end of the longer initial movements.

The only thing that's closed loop, is the lens, checking whether it actually moved the right amount and didn't "overshoot"...

Also, this:

FredMiranda thread wrote:
As a result of this tolerance (within the depth of focus or within 1/3 of the depth of focus), the camera can place the actual plane of focus at random anywhere within the tolerance range, and not necessarily at the same place each time.

indicates that your statement

David Ransley wrote in post #7436263 (external link)
explaines why image sharpness improves if you activate the focusing system more than once.

is not necessarily correct...


I know with my 1D2, if I AF repeatedly on a stationary subject, the last shot is not necessarily the sharpest.


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Mar 02, 2009 11:44 |  #11

Interesting read, thanks to the OP for posting RDKirks summary. RDkirk....I learn something new from you about every week....Thanks for taking the time to put it in layman's terms.


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mrkgoo
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Mar 02, 2009 11:47 |  #12

I typically focus more than once. Because of my scientific background, I tend to do things a few times until the iterations converge. If the same spot doesn't wildly hunt on later AF activations, it's probably pretty good - but I wouldn't say the last one was any better than the first.

It's just kind of a way to check that the first one was as precise as the last. (Precision vs. accuracy)




  
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Mar 02, 2009 11:49 |  #13

Now I,m confused.com( excuse the play on a TV ad)-so what does it mean?


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Mar 02, 2009 11:58 |  #14

it means now I know why my AF acts the way it does, not that I had to know but it helps to know, makes it more logical why, in certain situations it acts the way it does.


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David ­ Ransley
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Mar 02, 2009 14:18 |  #15

Note that each press of the focus button initiates both loops. The fact is that the second iteration of the sequence is a refinement of the first. I see this often and the second phase check and micro adjustment is just what the doctor ordered.

The AF mode selection is also informative and where predictive focusing fits in. I have had bad results with AI Servo and this article helped me with that as well.

The other thing that helps is the understanding of the f2.8 sensor and the way it works and also why the camera may select something else to focus on if you had a certain part of the image in mind.

From this article I believe that triggering the focus system more than once is worth while. It explains why very fast Aim and Shoot or Point and Shoot isn't the strong point of the system. You may just land a few soft images.


DRH

  
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Canon Focusing explained - when soft is sharp enough
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