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

 
Jeff's ­ Challenger ­ RT
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Mar 09, 2013 07:46 |  #1

Hello everyone! This may be kind of a dumb question, but I'm learning! I have read, watched training video's, etc about the focus points. I have a T3i and 5D. I assume the "points" talked about are the red "flashing" points seen thru the view finder. I've read some have 9, some 15 etc. My question, how do you know which of the 15 to use? I don't think I'm doing things right. I normally use the center point. I know I can change the flashing points, but what is the purpose? Should they all flash or just one? Told ya I was just learning! Thanks in advance, Jeff




  
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little ­ johny
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Mar 09, 2013 08:09 |  #2

I am a newbie too and am learning how to " frame " my picture using focus point.

IMAGE NOT FOUND
IMAGE IS A REDIRECT OR MISSING!
HTTP response: NOT FOUND | MIME changed to 'image/gif' | Redirected to error image by FLICKR

focuspoint (external link) by seyeah (external link), on Flickr

This probably not a good example because it was taken at F 4.5



  
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Kolor-Pikker
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Mar 09, 2013 08:15 |  #3

Generally, I only use the center point too, because on most older cameras it's the most sensitive one. On some of the newer cameras, with multiple cross-type points, I wouldn't hesitate to use those either.

Each focus "point" is actually a straight line, and this line detects, along it's length, whether something is blurry or not, like so:

In this example, a horizontal af point sees a blurry area in the photo:

IMAGE NOT FOUND
IMAGE IS A REDIRECT OR MISSING!
HTTP response: 404 | MIME changed to 'text/html' | Byte size: ZERO


When the camera focuses using this point, the motor moves the focus back and forth until the camera can determine the point at which the blur is the most contrasty, and thus it determines it to be detail:
IMAGE NOT FOUND
IMAGE IS A REDIRECT OR MISSING!
HTTP response: 404 | MIME changed to 'text/html' | Byte size: ZERO

IMAGE NOT FOUND
IMAGE IS A REDIRECT OR MISSING!
HTTP response: 404 | MIME changed to 'text/html' | Byte size: ZERO


The center point in most cameras is a cross-type point that can see both directions at once, while the surrounding points can only focus on detail that's either horizontal or vertical. So if the area of contrast you're trying to focus on follows the same direction as the af line, it's less likely to lock.

The benefit of having multiple points across the frame is to catch focus on subjects that are off-center, mainly for compositional reasons, although some people prefer to first focus using the center point, and then quickly shift the camera to recompose the shot. This works well enough using longer-length lenses, but can be problematic with wide-angles, because it will also cause a significant shift in focus.

Cameras like the 7D, 5DIII and 1-series have a myriad of cross-type and standard AF points across the frame that allow them to effectively track subjects moving within the frame, shifting focus duty from point-to-point. Assuming your camera also has a mode called A-DEP, multiple points can help in focusing on a group of subjects, by setting the averaged focus plane in the middle of a group.

The flashing red dots in the squares in the viewfinder are only approximations of where the focus point should be, sometimes they are a little off-center.

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philwillmedia
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Mar 09, 2013 08:19 |  #4

Jeffs Challenger R/T wrote in post #15694681 (external link)
=Jeff's Challenger R/T;15694681]My question, how do you know which of the 15 to use?

In a nutshell, by knowing what you want to focus on and selecting the focus point that achieves that and gives the composition you want.


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windlight
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Mar 09, 2013 09:38 |  #5

It's fine to use only the center point to shoot non-moving objects. If you turn on all the AF points, your camera will essentially have to guess what you want to focus. But if you are using AI Servo and shooting a moving subject, the other points can track the target to help keep it in focus. Different AF points also have different sensitivities, cameras such as the T3i and 5D only has 1 cross-type AF point at the center but newer or more expensive cameras may have more.


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amfoto1
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Mar 09, 2013 11:17 |  #6

OP, your T3i and 5D are pretty similar... Both have nine points shown in the viewfinder and user selectable. Both cameras only have one of the more sensitive dual-axis type points... the AF point at the very center.

Now here's where there is some difference... The 5D has 6 more points that you can't see in the viewfinder. They are called "Assist" points and can optionally be turned on or off. They are clustered close around the center point... Holding the camera horizontally or in landscape mode, there is a row of three of these hidden points immediately above the center point, and a row of three immediately below. All of these points are within the Spot Metering circle, so they are close to the center point. There is a diagram showing them on page 76 of the manual.

On both 5D and 5DII, the two Assist points immediately above and below the center point are single axis/vertical type... when the camera is being held in landscape orientation. The other four are single axis/horizontal type.

The most important thing to remember about the Assist points, which brings the 5D/5DII's total number of AF points to 15, is that they only work in AI Servo focus mode. So they are mostly intended to help when trying to track moving subjects. They don't work in One Shot mode. Also, keep in mind that you have to enable the Assist points for them to work, via Custom Function III-7. Give them a try, if you are struggling to keep action shots in focus with your 5D. I don't use them much on my 5DII, but I simply don't use that camera very much for action/AI Servo photography. I have a pair of 7Ds that I use for sports/action/AI Servo. They are much better at it than the 5D/5DII. (The 5D/5DII are a bit better for low light and stationary subjects... and also for really big enlargements.)

If all your visible AF points are flashing when you look thru the viewfinder, you have the camera set to All Points/automatically selected. This might be called a "point n shoot" focus mode of the camera and leaves a lot to chance, that the camera will focus where you actually want it to focus. Generally the camera will focus on whatever is closest and covered by one of the AF points. That may or may not be where you want it focused.

Yes, a lot of use use the center point by default... But that doesn't mean you shouldn't use the single-axis peripheral points if it makes more sense to do so. If the subject is in reasonably good light, has decent contrast and some fine detail, the single-axis points can work just fine. Some of them are horizontal axis, some are vertical. Notice the dual-axis point in the center of your camera's viewfinder is indicated by a square box and the peripheral points by slightly smaller rectangles. The rectangle indicates the orientation of the single axis. A horizontal single axis will work best if focused on a sharp line that's roughly vertical (crossing it) and vice versa with the vertical single axis points.

The above example litle johny posted is excellent illustration of this... The center point would have focused on the people behind the subject. The far right point would have focused on the windshield of the parked car. The bottom AF point would have focusing upon the blurred objects right in front of the photographer, and all the people would have been heavily blurred. The AF point used for that shot is perfect, assuming the one person walking toward the photographer is the main subject.

In One Shot mode... used for stationary subjects... the AF point(s) actually flash twice. Once when the AF starts and a second time when focus is achieved and locked on. Often this happens so fast it only appears to be a single flash of red. The second flash of red is also "Focus Confirmation", letting you know that the camera thinks focus has been achieved. There is also a green LED that lights up in the viewfinder for confirmation. And, if you have it enabled, the camera will emit an audible "beep".

In One Shot, once focus is achieved, confirmed, you can recompose the image if you wish and the focus will not change so long as you maintain half-press on the shutter release button.

In One Shot, if you want to re-do focus for any reason (you moved, your subject moved, or you just want to focus somewhere else), you have to lift pressure off the shutter release button, then reapply it to cause the camera to refocus.

In AI Servo mode... used for moving subjects... the AF point(s) only flash once to indicate focus has started. Because AI Servo runs continuously tracking the subject as the distance from camera to subject changes, you have to maintain the active AF point right on the subject, focus never locks, and there is no Focus Confirmation. You cannot recompose because if you do, moving the AF point off the subject while focusing the camera will refocus on whatever is now under the AF point.

If trying to use All Points/auto selection with AI Servo, you are hoping the camera is successful passing focus on the subject off from one AF point to the next, if the subject is moving across the image area.

If using the 5D with Assist points enabled, it's sort of like having an extra large center point... in theory. That way if you have trouble keeping the center point on the subject, hopefully the Assist point will pick up on the subject.

There are three things that together make for good AF performance: the camera, the lens and the user. So far all we've talked about is the camera. Lenses also can make a difference... Canon USM lenses are faster and more accurate focusing than micro motor lenses. The newer STM focus drive lenses are somewhere in between. Sigma offers many HSM lenses, similar to Canon USM. And Tamron has begun offering USD lenses, also similar to USM.

Larger aperture lenses also tend to be faster and can make for higher precision (shallower depth of field). This isn't always the case, though. Some extremely large aperture lenses (85/1.2L II, for example) are deliberately designed to focus a little slower, in order to be more precise. This is called "long throw" focus, a term that dates back to manual focus lenses that were designed similarly, and you had to turn the focus ring a long way to move the focus much, but could do so very precisely.

Extreme lenses such as macro, many of which are able to focus all the way from infinity to full 1:1 magnification just a couple inches in front of the lens, also tend to be slower focusing simply because they have to move their focusing elements a long, long way. Conversely, very wide angle lenses that have a lot of depth of field hardly have to move their focusing elements at all to achieve focus, so might be very fast focusing even with simpler focus drives.

The third major factor effecting AF performance is the user themself. You have to practice and learn the AF system in your camera, how to set it up for different situations and how it will react in a wide variety of lighting conditions, etc. In general, keep it simple... I use single point the majority of the time, usually the center one... And about 80 or 90% of my shooting is sports/action with AI Servo mode.

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.

Not sure which of the online tutorials you have viewed... I highly recommend the AF system overview series of three that B&H has posted on Youtube, starting with this one (external link). Watch all three! They are about 1/2 hour apiece.

Finally, Google "Hyperfocal distance" and read up on it. There's some explanation of this, too, in the video above. This is a technique where you don't actually focus right on the subject, but place the zone of focus to include the main subject and other secondary subjects, in combination with using various size kens apertures to control depth of field. Doing this well in the field is something you learn through practice... different focal lengths, distances to the subject and apertures render more or less DOF. Many lenses have a focusing scale on them that gives you a means of calculating a hyperfocal distance setting. Some cheaper lenses don't have this, and it's not as easily done with zooms. But you also can estimate it by eye.
Hope this helps.


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Mar 09, 2013 11:46 as a reply to  @ windlight's post |  #7

Choosing a single focus point to use to get the part of the subject that you want to be in best focus, actually in best focus, and letting DOF take care of the rest is usually the best way to go. Theoretically you survey the scene through the viewfinder and by using a joystick, dedicated AF buttons, or dials select the focus point that covers the part of the subject that you want to be in best focus, then focus and shoot.

For example in the photo posted by little johny above, he chose to focus on the face of the man wearing the parka that is just right of center, so he selected the FP covering the man's face and focused using that point. He could, however, have chosen the point covering the face of the lady wearing the brown cap if she had been his selected point of interest. Watch out for the stacked produce in front of her though, as the FP might choose to focus on that instead of her face. Some focusing and recomposing may be necessary if no focus point covers the exact point where you want focus to be. If he had wanted the Fed Ex truck(for example) to be in best focus, he might have chosen the right-most FP to be placed on the truck, focused and then recomposed the shot before releasing the shutter.

Back in the 1990s and early 2000s Canon had a system available on some film SLRs called "Eye Controlled Focus Point" selection(or ECF for short) whereby the process of selecting the FP by hand was eliminated. All the photographer had to do was view the scene, decide what part of the subject needed to be in best focus and look at it. The camera then detected his line of sight and chose the FP that covered the exact spot where he was looking. This system was much faster and easier than today's joysticks, buttons and dials and while some degree of focusing and recomposing was still necessary for subjects near the edge of the frame, it was a whole lot less trouble and much easier to use. Canon inexplicably, but probably for reasons that don't really apply now, decided to drop ECF from digital cameras when the film era ended.

There is a thread and a group of photographers dedicated to to the idea of the return of a new and improved version of ECF located here:

LINK to Official Eye Controlled Focus thread

I invite you or anyone who is interested to join us there. If you have questions, please ask them there rather than hijacking the OP's thread here.

This thread is quite lengthy, so read as much or little as you like. If you have a question, feel free to ask it, don't worry about whether it's already been asked or not. It probably has, but we don't mind answering it again for newcomers to the thread who do not wish to read the whole thing. We welcome anyone and everyone interested to join us.


Mark

  
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John ­ from ­ PA
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Mar 09, 2013 11:49 |  #8

Canon also has some good tutorials. Go to http://www.learn.usa.c​anon.com …5d_markii_tutor​ials.shtml (external link) and select "AF Options for Still Capture." Note that there are two so watch both. Even though that site is for the 5DII, the same procedures pretty much apply to most of the Canon line; just the number of points vary. There are tutorials for the 60D as well, you may find them applicable to the T3I.




  
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Mar 09, 2013 12:06 as a reply to  @ John from PA's post |  #9

amfoto1 wrote in post #15695152 (external link)
WALL OF TEXT
Hope this helps.

I just want to say thanks for taking the time to type all of that out. It was very insightful for me to help further understand modern DSLR focusing! Very helpful and thanks!

bw!


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Mar 09, 2013 14:13 |  #10

Kolor-Pikker wrote in post #15694736 (external link)
Generally, I only use the center point too, because on most older cameras it's the most accurate one. On some of the newer cameras, with multiple cross-type points, I wouldn't hesitate to use those either.

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.

When the camera focuses using this point, the motor moves the focus back and forth until the camera can determine the point at which the blur is the most contrasty, and thus it determines it to be detail.

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.

Finally, multiple points can help in focusing on a group of subjects, by setting the averaged focus plane in the middle of a group.

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.


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Mar 09, 2013 14:45 |  #11

I am slightly confused and worried if I'm doing the right thing - I predominately use - one shot AF and manual selection: 1 pt AF. When I am looking through the viewfinder and depending on what I wish to focus on I will use any of the focus points even sometimes going out right to the edge of the bank of focus points. Having read the above I'm now not sure if I should be only using the centre AF point?


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apersson850
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Mar 09, 2013 17:58 as a reply to  @ 0.0f's post |  #12

Use the point where the subject is.


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HaroldC3
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Mar 09, 2013 21:35 |  #13

I have a problem with leaving it on all focus points because then I can't really determine where the camera is going to focus. Thus I just use the center focus point and recompose if needed.


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CyberDyneSystems
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Mar 09, 2013 22:03 |  #14

0.0f wrote in post #15695643 (external link)
I am slightly confused and worried if I'm doing the right thing - I predominately use - one shot AF and manual selection: 1 pt AF. When I am looking through the viewfinder and depending on what I wish to focus on I will use any of the focus points even sometimes going out right to the edge of the bank of focus points. Having read the above I'm now not sure if I should be only using the centre AF point?

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.


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CyberDyneSystems
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Mar 09, 2013 22:08 |  #15

HaroldC3 wrote in post #15696747 (external link)
I have a problem with leaving it on all focus points because then I can't really determine where the camera is going to focus. Thus I just use the center focus point and recompose if needed.

I agree with half of what you say.

- rarely is it a good idea to let the camera chose which AF point to use.

- However, you should be switching AF points to get the best results as opposed to sticking with center point only and recomposing.

Look, this is a new skill like any other, learning to use the tool to it's best advantage, taking the time to be in control of the hardware. At first switching around the AF points may seem awkward or cumbersome, but this is how everything works in life.

persevere, get it down, and you be rewarded! It ends up being much easier and much more consistent than constantly bobbing and weaving trying to use center point and recomposing.


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School me on focus points
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