Focusing, and all those little red dots :)

jjd228

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Don't laugh, but I'm struggling to fully understand how the red focus dots in my viewfinder affect my shot. I read the manual and a million articles and I'm still not 100% clear. Maybe most people are ok with that, but I'm one of those nutbags that actually likes to understand how and why something works as opposed to just being content with the fact that it works :)

So... if I leave my camera (Canon t4i) set with all the defaults, when I press the button half way to lock focus, I'll get 2, 3, maybe 4 red dots that light up. ok so these are the areas of the photo that the camera is focusing on, correct? So here are some questions:

1) I can change the mode so that only one red dot shows up in the view finder (the center one). Do I want this? It seems simpler to me if I only have to worry about "aiming" that one point where I want to focus. Can someone explain the difference?

2) I read a lot about locking focus and the recomposing. This I don't get. If I lock the focus on your nose and then I point the camera down on your belly button, wouldn't this potentially give me an out of focus shot since your nose is closer to the camera than your belly button?
 

clarcorona

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Your center AF point is the most sensitive and a lot of people prefer using it over the others.

With "Focus and Recomposing" it would still be in focus unless you have a very shallow DOF and you move back and forth while trying to reposition the camera.

This is why it is sometimes better to use the other AF points.

While they aren't as sensitive as the center point it does help alleviate some of the missed focus issues that "Focus and Recomposing" gives.
 

radiorickm

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Don't laugh, but I'm struggling to fully understand how the red focus dots in my viewfinder affect my shot. I read the manual and a million articles and I'm still not 100% clear. Maybe most people are ok with that, but I'm one of those nutbags that actually likes to understand how and why something works as opposed to just being content with the fact that it works :)

So... if I leave my camera (Canon t4i) set with all the defaults, when I press the button half way to lock focus, I'll get 2, 3, maybe 4 red dots that light up. ok so these are the areas of the photo that the camera is focusing on, correct? So here are some questions:

SORTA...IT'S NOT THAT THE CAMERA IS FOCUSING THERE, IT'S THAT AT THAT POINT IT IS IN FOCUS (What I mean is you may have a primary focus point, but other point in the frame may also be in focus, and the indicators will light indicating that area is in focus)

1) I can change the mode so that only one red dot shows up in the view finder (the center one). Do I want this? It seems simpler to me if I only have to worry about "aiming" that one point where I want to focus. Can someone explain the difference?

WELL, SEE YOUR QUESTION BELOW for one. Lets say you are shooting a portrait. You place the CENTER AF point over the subject, focus, LOCK focus, recompose, and shoot. When you recompose, you run the risk of taking things out of focus, especially with large apertures (just as you a state below). So, now you can select the focus point over the eye to be active, and when you confirm focus, you'll know what the focus point is.

2) I read a lot about locking focus and the recomposing. This I don't get. If I lock the focus on your nose and then I point the camera down on your belly button, wouldn't this potentially give me an out of focus shot since your nose is closer to the camera than your belly button?

Now lets take a different scenario. You are shooting using a tree to frame your subject...maybe branches overhead.... The camera will try and focus on the closest thing. It may continually focus on the branches, instead of your subject. CURE: select single AF point center, and force it to look at your subject under that particular point.

They are all TOOLS available for you to use. There are times when ALL AF points works fine, and times when it don't. I really think you have the idea.

Have fun shooting
 

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All 9 of the T4i focus points are cross-type focus points (see link below). The center T4i focus point in more sensitive than the others in lower light situations.

1. The camera/lens can only use 1 focus point to focus with. The focus point used determines how far from the camera the point of focus is
Some camera have a focus area mode that that will compare multiple focus points before picking 1 of them that the camera then uses to set the focus.

2. Yes. However, if the camera settings allow for a sufficiently deep depth-of-field (DoF) the belly button may be in focus too.

Another consideration is that the the area of acceptably sharp focus is a plane (DoF) that is essentially parallel to the image sensor. If you tilt the camera up/down or turn side to side, that plane of acceptably sharp focus moves with the camera and stays parallel to the camera.
You can think of it as a transparent, weightless, wall that is attached to the camera.

Understanding Camera Autofocus
 
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jjd228

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Thanks for the great replies everyone.

Help me with this scenario:

I have a human model standing against a wall and I just need a full body shot from maybe 20 feet away, with their face sharply focused. If I use the default settings and put the center spot over their nose, then focus lock, 2 or 3 red points might light up, but neither of them might be the nose. Do I take the shot, or do I move the camera while holding the button half way so that one of the points that lit up are now over the nose?

That's what I'm not clear on. It seems to me that if I want to focus on the nose, but the nose point doesn't light up, then I have to change something until the nose point lights up. Does that make sense?

So this is why I asked about changing the settings so that I only HAVE one red dot available. If I do this then I can make sure the area I want is always "lit up."
 

cynicaster

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So this is why I asked about changing the settings so that I only HAVE one red dot available. If I do this then I can make sure the area I want is always "lit up."

This is what I do, if for no other reason than it's what I'm most comfortable with.

Using the center focus point in conjunction with the focus/recompose method will work just fine in the vast majority of scenarios. For your 20 feet away full body shot, you'll be fine.

Just be careful when using a very shallow DoF--such as the common 50mm f/1.8 prime lens wide open at close range. Focus/recompose is likely to screw you over in this situation.

EDIT: one other thing--be careful about focusing on the nose as you've mentioned. At 20 feet away it likely won't matter, but up close, depending on what settings you're using, focusing on the nose might actually render the tip of the person's beak in sharp focus and their eyes in soft focus, which I'm guessing is not what you typically want. :D
 
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jjd228

jjd228

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So this is why I asked about changing the settings so that I only HAVE one red dot available. If I do this then I can make sure the area I want is always "lit up."

This is what I do, if for no other reason than it's what I'm most comfortable with.

Using the center focus point in conjunction with the focus/recompose method will work just fine in the vast majority of scenarios. For your 20 feet away full body shot, you'll be fine.

Just be careful when using a very shallow DoF--such as the common 50mm f/1.8 prime lens wide open at close range. Focus/recompose is likely to screw you over in this situation.

EDIT: one other thing--be careful about focusing on the nose as you've mentioned. At 20 feet away it likely won't matter, but up close, depending on what settings you're using, focusing on the nose might actually render the tip of the person's beak in sharp focus and their eyes in soft focus, which I'm guessing is not what you typically want. :D


LOL thanks. Ok, so now this whole "focus/recompose" thing. Help me out here. You say "using the center focus point..." Do you mean line up the center dot on their beak before focusing (before the dot turns red)? If so, then when I lock focus, if the nose point (center dot) does NOT light up, are you saying to move the camera so that one of the spots that DID light up are on the nose?

Sorry I just want to be clear.
 

kathyt

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I don't generally use the nose as a focal point, but you first have to consider your depth of field before you can determine if your image with be in focus or not close up. If you are shooting at say f5.0 and focused directly on the nose the face would be in focus. If you were at f1.2, the tip of the nose would be about the only thing in focus. Use a stagnant object to practice on and check them out to see what your results were with each setting.
 

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AT 20 feet, there will be ample depth of field even wide-open to get a single person in good focus. And yes, your description of focus/focus lock/and recompose in post #7 is accurate. Focus/focus lock/recompose works well when there is a small amount of depth of field; when it does NOT work is when there is a teeny-tiny amount of depth of field, such as when shooting from very close distances, or with very long telephoto lenses,or with high-magnification macro gear, or when using really wide-aperture lenses like say the 85/1.4 or 1.2 lenses shot wide-open from close range. Suffice it to say--there are MANY situations in which depth of field is quite skimpy, and when focusing becomes a very critical, difficult challenge.

At close distances, depth of field is pretty shallow; the wider-open the lens, the less depth of field there is. Now, the thing is this: a standing person at three feet distant, with the camera back parallel to the standing person's torso--their HEAD and face is actually a little bit farther away than their belly button; and their feet are also a bit farther away. Depending on the lens focal length, and thus the angle off view, the edges of the frame can be 3 to 5 inches FARTHER away than the center of the frame, at typical indoor distances. Because of the physics, well, search for the article, "Why Focus and Recompose Sucks".

When a camera has multiple AF points active, the computer and the programming are doing most of the focusing work. Face recognition is pretty easy now, for computers; they can easily recognize both skin tones, and faces, and so modern AF systems tend to be able to detect, then analyze faces, and use the camera's knowledge of the settings to set a decent focusing distance, based upon a lot of available data that has been analyzed.
 
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jjd228

jjd228

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ok so it seems that "manual af point selection" is the way to go. So why would anyone ever choose a point other than the center point if it's going to be the only one that "lights up" in the view finder?
 

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ok so it seems that "manual af point selection" is the way to go. So why would anyone ever choose a point other than the center point if it's going to be the only one that "lights up" in the view finder?

As explained above, if you want to put your subject say, 1/3 of the way in from the left side of the frame, then your center point won't be on them. So you either have to put it on them focus, then move your camera to the right, or just use one of the AF points on the left and focus right as you take the shot.

The focus and then move method is more convenient, but if you have a razor sharp depth of field, you may end up with an out of focus subject, because rotating your camera to recompose actually tilts your focal plane a little bit, and it can cause what was in focus to not be in focus anymore. Diagram:

$0NCZx7D.jpg

In such a situation, you would probably be better off using a side AF point and not moving the camera in between focusing and shooting.

Alternatively, you could focus, recompose, then LEAN BACK a few inches or so depending on how far away you are, then shoot. I have done this sometimes in order to have the convenience of center point but without the missed focus... but it is kinda sketchy and not as reliable as the side AF point obviously.

$dGPRMmr.jpg
 
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jjd228

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ok so it seems that "manual af point selection" is the way to go. So why would anyone ever choose a point other than the center point if it's going to be the only one that "lights up" in the view finder?

As explained above, if you want to put your subject say, 1/3 of the way in from the left side of the frame, then your center point won't be on them. So you either have to put it on them focus, then move your camera to the right, or just use one of the AF points on the left and focus right as you take the shot.

The focus and then move method is more convenient, but if you have a razor sharp depth of field, you may end up with an out of focus subject, because rotating your camera to recompose actually tilts your focal plane a little bit, and it can cause what was in focus to not be in focus anymore. Diagram:

View attachment 48216

In such a situation, you would probably be better off using a side AF point and not moving the camera in between focusing and shooting.

Alternatively, you could focus, recompose, then LEAN BACK a few inches or so depending on how far away you are, then shoot. I have done this sometimes... but it is kinda sketchy and not as reliable as the side AF point obviously.

Got it! Thanks!
 

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ok so it seems that "manual af point selection" is the way to go. So why would anyone ever choose a point other than the center point if it's going to be the only one that "lights up" in the view finder?

Pretty simple: Imagine shooting a horizontal photo, with a moving bicyclist located off-center, and positioned one-third of the way to the right of the frame. Behind the bicyclist is a distant scene. The bicyclist is approaching the camera at a slight angle, from 40 feet away, up to as close as 15 feet. CENTER-point AF will "back-focus" on the distant scene, far,far BEHIND the bicyclist. It is in that type of situation when a group of 9 or 11 out of 51 AF points (talking Nikon cameras here), jogged with the 4-way conmtroller button to be placed on top of the bicyclist's image in the finder will work wonderfully.

With a camera that has say, a wide-area, 11-point AF system, shifting a group of four, active AF points that are working as a group, over to the right hand side of the frame would work quite well.

Same scenario repeats in many types of shots; desired subject is placed OFF-center, so the "active" AF spot needs to be shifted so it coincides with the actual, desired focus target's location.
 

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Rudy Winston of Canon delivered an excellent lecture on the Canon autofocus system. The whole lecture is on YouTube. If you watch these three videos, you will know everything there is to know about how your camera's autofocus system works. Just the first ten minutes will teach you a lot. It's very approachable and I highly recommend that you watch it:
 
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