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Thread started 09 Mar 2013 (Saturday) 07:46
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School me on focus points

 
rrblint
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Mar 09, 2013 22:17 |  #16

CyberDyneSystems wrote in post #15696840 (external link)
The excuse to just use center AF point and recompose was stronger back when switching AF points was harder, but I did it even when I had to use the wheel. Now we have the joystick and it's lightning fast.

I agree.

But if you think the joystick is "lightening fast", you would love "Eye Control".:D


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CyberDyneSystems
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Mar 09, 2013 22:32 |  #17

Ahh yes, Mind control AF selection!

We had that back in the late 1990's - Early 2000s... It would be nice to have that.


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Mar 09, 2013 23:08 |  #18

In good light, use the single point AF and whatever point will meet your composition.

In poor light you may find that only your center cross-type point will "find" your target.

I haven't used "Auto All Point" AF outside from shooting with a P&S camera...sure, if you pay attention to where the focus "locks" you can get the shot, but why bother:)?


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Kolor-Pikker
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Mar 10, 2013 00:58 |  #19

apersson850 wrote in post #15695580 (external link)
The difference isn't about accuracy, but about sensitivity. Linear focus point are less likely to detect a useful contrast. When they can't, the aren't accurate either, but when they can, they are just as accurate. Their single line is the same as one half of a cross type point.

I guess it makes sense to call it sensitivity

No it doesn't. The thing that makes phase detection AF so quick is that although the subject is out of focus, the camera can measure both in which direction and how far off immediately. So it knows where to drive the lens and just does that. The description above fits contrast AF, but that's not what you use when you use the AF points you see in the viewfinder.

Yes, it does; how would it know the direction? Here's a bit taken from Cambridge in Colour:

"The process of autofocusing generally works as follows:
(1) An autofocus processor (AFP) makes a small change in the focusing distance.
(2) AFP reads the AF sensor to assess whether and by how much focus has improved.
(3) Using the information from (2), the AFP sets the lens to a new focusing distance.
(4) The AFP may iteratively repeat steps 2-3 until satisfactory focus has been achieved."

The AF system of a modern camera can repeat the above process hundreds of times per second, so maybe it seems like it's focusing in a single motion, but it's not, it's just happening too fast to notice. The only AF systems that can truly see the direction of focus, are on 5-6 figure lenses used for HDTV broadcast, using two AF sensors spaced physically apart.

No it doesn't. When using multiple point in One Shot AF mode, the camera will focus on the closest subject with proper contrast, not average focus out among all the points that find any contrast. So if eight points see something in the background, and sees it well, the camera will still focus on a point in the foreground, if there's one single point finding a subject there.

I was referring to cameras with A-DEP, I should probably clarify that in my post. https://photography-on-the.net/forum/showthre​ad.php?t=270557

Edit: Might as well drop the link to CiC as well: http://www.cambridgein​colour.com …ials/camera-autofocus.htm (external link)


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Mar 10, 2013 01:43 |  #20
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amfoto1 wrote in post #15695152 (external link)
There's also a technique used a lot by sports/action photographers called Back Button Focus (external link). You might want to give it a try. It separates the focusing function from the shutter release, and puts the photographer more in control of AF. It's especially useful for action shooting and long telephoto lenses. But, once learned, really can be used effectively in practically any situation.

bw!
Thanks, thanks a bunch!! All these years fighting with my shutter-release button over focus, and the answer was there all along. Thanks for pointing it out. I just set both cameras to back-button focusing. I expect my shooting to be more proficient from now on.


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Mar 10, 2013 03:27 |  #21

Kolor-Pikker wrote in post #15697208 (external link)
Yes, it does; how would it know the direction? Here's a bit taken from Cambridge in Colour:

"The process of autofocusing generally works as follows:
(1) An autofocus processor (AFP) makes a small change in the focusing distance.
(2) AFP reads the AF sensor to assess whether and by how much focus has improved.
(3) Using the information from (2), the AFP sets the lens to a new focusing distance.
(4) The AFP may iteratively repeat steps 2-3 until satisfactory focus has been achieved."

The AF system of a modern camera can repeat the above process hundreds of times per second, so maybe it seems like it's focusing in a single motion, but it's not, it's just happening too fast to notice. The only AF systems that can truly see the direction of focus, are on 5-6 figure lenses used for HDTV broadcast, using two AF sensors spaced physically apart.

And that's exactly what we have here. The quote you found at Cambridge in Colour describes contrast detect AF. That's what a recent Canon EOS can do whe using live view, and what all compact cameras use. But the main focusing system on a Canon EOS (and the only on, prior to live view) is phase detection AF. It works by having two sensors, physically apart, registering their different views of the subject. A line in the subject will appear at the same position on both sensors if the lens is in focus, but with a distance between them if it's not. Which one that's off to the left determines if the lens is focused too close or too far away.
The spacing of the sensor pairs is very small (they are actually on the same silicon chip), so it's augmented by the second image forming optics, which uses an optical trick to get the required spread.

Thus you have an AF system working according to the same principle as you describe above in an EOS 1000D as well, in spite of cost being significantly lower.


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Mar 10, 2013 04:25 |  #22

CyberDyneSystems wrote in post #15696840 (external link)
No, you are doing it right!

Switch AF points and use the one that is covering the area of interest.

Forget "center point only" .. use the AF point that works for the composition.

Best example, when shooting in portrait, a full body shot, the center AF point is on the persons stomach.
So use the far off side AF point and put it on the subjects face.

The excuse to just use center AF point and recompose was stronger back when switching AF points was harder, but I did it even when I had to use the wheel. Now we have the joystick and it's lightning fast.

All that said, yes, there are times when you want the center high precision or cross type af points,. but for the majority of shooting, I feel it's better to swap AF points rather than recompose.

Thank you for confirming this.


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Kolor-Pikker
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Mar 10, 2013 04:44 |  #23

apersson850 wrote in post #15697296 (external link)
And that's exactly what we have here. The quote you found at Cambridge in Colour describes contrast detect AF. That's what a recent Canon EOS can do whe using live view, and what all compact cameras use. But the main focusing system on a Canon EOS (and the only on, prior to live view) is phase detection AF. It works by having two sensors, physically apart, registering their different views of the subject. A line in the subject will appear at the same position on both sensors if the lens is in focus, but with a distance between them if it's not. Which one that's off to the left determines if the lens is focused too close or too far away.
The spacing of the sensor pairs is very small (they are actually on the same silicon chip), so it's augmented by the second image forming optics, which uses an optical trick to get the required spread.

Thus you have an AF system working according to the same principle as you describe above in an EOS 1000D as well, in spite of cost being significantly lower.

Okay. I've checked a few more sources and I think I got it now, looks l've been mixing a couple of things up. I'll probably edit my post later when I'm not busy.


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Mar 10, 2013 15:10 |  #24

Alveric wrote in post #15697249 (external link)
bw!
Thanks, thanks a bunch!! All these years fighting with my shutter-release button over focus, and the answer was there all along. Thanks for pointing it out. I just set both cameras to back-button focusing. I expect my shooting to be more proficient from now on.

This is the way to go IMHO. Now your wearing big boy pants! :)


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Mar 10, 2013 15:13 |  #25

tonylong wrote in post #15697030 (external link)
In good light, use the single point AF and whatever point will meet your composition.

In poor light you may find that only your center cross-type point will "find" your target.

I will concede this.
I stress using the correct AF point to get one in the habit of using this valuable feature.

with all options, there are times when focus recompose becomes the best or only solution.
I do not advocate that this possibility be a reason to use it exclusively however.

example: I end up using it when shooting at f/8 Max aperture (500mm plus 2x) as the center AF point becomes the only one I can use.


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Mar 11, 2013 08:55 as a reply to  @ CyberDyneSystems's post |  #26

Thanks everyone for the responses, and internet links. This has been very helpful to me. Now, just gotta keep practicing! I appreciate everyone's time! Thanks again, Jeff




  
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Mar 11, 2013 13:15 |  #27

Huge chunks of helpful posts here, what a through examination. Esp @amphoto.

There is one place where the centre point is king, in very low light, or where there's little contrast for the camera to snag. If your camera struggles at the edges, shift to the middle point, that one usually can find something to pull on. Then recompose.


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djrosen
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Apr 15, 2013 15:24 |  #28

Thank you to everyone for the very helpful tips about how to get the best focus using a single point directly on the desired subject. I have a followup question about what exactly the camera is doing when multiple AF points are lit (red). I have a T4i and have sometimes shot with automatic AF point selection in One Shot Focus mode. When I view my images in Imagebrowser I select to show AF points used. Sometimes there are multiple points that are red. Also, one red point maybe on a very near subject (person) in the foreground and another red point on a distant object such as a mountain in the background. I called Canon tech support to ask how is it possible that the camera focus on multiple points at such different distances? He said that it averages the information from multiple points, which depending on the aperature and DOF, could still yield an image where subject and background are both in focus, or one may be blurred. I know there are a lot of very knowledgable photographers here, and I would appreciate your thoughts about how the camera uses the information from multiple points at the same time to come up with the proper focus. With that being said, I do realize that is always best for the photographer to manually select a single desired AF point.


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Apr 15, 2013 15:29 |  #29
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What focus/drive/metering mode are you using when you see this behaviour?


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djrosen
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Apr 15, 2013 15:51 |  #30

I have noticed multiple points being used in One shot autofocus mode, single shot, and evaluative metering mode and AF point auto selection.


Dan

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School me on focus points
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