Focus points

[Aim12]

If I wanted to shoot something off center, I would either focus and recompose using the center focus point, or I would choose a focus point closest to my off center subject? If I have 2 subjects I want to photograph, and both are off center, would I turn on all the focus points? What are the uses of turning on all of the focus points? If the subject(s) are moving does that affect if you would use focus and recompose? Thanks.

 

[Trever1t]

You ask some good but fairly complex questions. Let's tackle the first one;

Shooting something off center. Using AF-S mode (Nikon) so that focus locks when the shutter release is pressed half-way, you can do either, focus using the center focus point and then recompose or move the focus point over to the off-center subject.

Now this alone leaves many variables. If you are in AF-S mode and you use the center point focus/recompose, it is possible (likely) your exposure will change when you recompose. I set my shutter to lock exposure as I shoot primarily portraiture. Also, recomposing could and probably will cause you to loose focus on subjects shot with a very shallow DOF. This an issue for me at anything wider than f2.2 at normal distance (7-15') with 85.

Focus modes and methods alone are only a part of the equation.

 

[KmH]

Understanding Camera Autofocus

The camera can only focus in a single plane that is essentially parallel to the camera's image sensor, which means that even if a multiple focus point mode is chosen the camera can still only use 1 of them for final focusing.

Multiple focus points are used for some of the focus Area modes.

Another consideration is focus point type - regular or cross-type.

Tell us what camera you are using.

 

[Aim12]

Thanks for the replies. I use a t4i.

 

[TCampbell]

Thanks for the replies. I use a t4i.

When you turn on "all" the focus points, what you're really doing is telling the camera to evaluate all of it's focus points and to choose the one it wants to use. It's just looking for one AF point to get a lock, but you're letting it decide which point that will be.

The rules for Canon are that it will always pick the AF point which is able to lock focus at the CLOSEST focusing distances.

E.g. suppose you've got a subject in the left third of the frame and you want them in focus... and suppose they are 20' away from you and framed so that the left-most AF point is targeting them. ALSO suppose that in the same frame, there's a tree/shrubbery, etc. in the right third of the frame and it's only about 10' away but it is framed so that the right-most AF point is targeting it -- but it is not the subject of interest. Finally, suppose you have enabled all AF points so that the camera gets to choose the point it wants to use. Your camera will focus on the tree/shrubbery and will NOT focus on your subject of interest. This is because the camera is looking for the AF point that can lock focus at the shortest focusing distance and, in our example, the tree/shrubbery is closer.

USUALLY a photographer will tend to frame a scene so that their subject of interest also happens to be the closest thing in the scene and if something else is closer it's usually near an edge of the frame where an AF point isn't going to target it.

If your shooting circumstances match that assumption (your subject will always be the closest thing to the camera) then it's fine to let the camera auto-select the AF point. If you are framing up a shot in a way that does not match the camera's assumption then you'll want to over-ride the camera decision and choose your AF point.

Your T4i has 9 AF points and all 9 are "cross type" points (a significant improvement over previous Canon "Rebel" series bodies.)

If your subject is MOVING (particularly if your subject is getting nearer or farther and hence changing the focus distance as they move) then you'll want to switch the camera into "AI Servo" mode. This tells the camera that the subject distance is expected to change constantly and you want it to continuously track focus on the subject (it's even "predictive" in that it can tell if your subject is getting nearer or farther and it predicts where it will need to move focus based on the speed of your subject.)

The default mode of your camera is "One Shot" -- this mode assumes a non-moving subject. It focuses, locks, and then shuts down the focus system while it waits for you to take the shot (it does not continuously re-evaluate focus.)

Be careful with the shutter button when using "AI Servo" mode because that mode ALSO puts the camera into "release priority" mode (which means when you fully press the shutter button, it WILL take the shot regardless of whether anything was in focus. This is different that "One Shot" mode which uses "focus priority". In "focus priority" mode if you fully press the shutter button, it will first lock focus and THEN it will take the shot (it will not take a shot unless it can confirm that at least one AF point was able to lock focus.)

Canon did a 3 part series on the nuances of how the focus system works. I highly recommend you watch it as it will likely answer a lot of questions about how the camera's focus system "thinks".

Part 1: Part 1 of 3: A Look at The Canon Autofocus System - YouTube
Part 2: Part 2 of 3: A Look at The Canon Autofocus System - YouTube
Part 3: Part 3 of 3: A Look at The Canon Autofocus System - YouTube

 

[Aim12]

Ok that makes a little more sense. I'll take a look at the videos. Thanks.

 

[bratkinson]

In addition to determining which AF point(s) to choose from (recognizing that the closest object gets selected if multiple points available), and setting AI Servo AF mode, you also have to consider the 'thickness' of the depth of field (DOF) when taking the shot.

With two people standing side by side, for example, even with a 'narrow' (but not too narrow/thin) DOF, focusing on either person will get the other person in focus as well. But if you are shooting at, say, f2.0 with a telephoto lens from 20 feet away, your DOF will likely be too thin to get both faces in focus. Unless that's what you are trying to capture, you will have to increase your DOF by moving back, stopping down (smaller aperture/higher f-stop number), or using a shorter focal length, singularly or in combination.

There are many aspects of taking a photograph with a DSLR that you can control to achieve the desired effect. Learning what the trade-offs are when changing each them and selecting the results you want are what makes photography both interesting and an art form.

 

[StandingBear1983]

I'm using the D5100, and there are 11 focus points and 1 that is crosstype in the center. its known that its better to use the crosstype because its more acurate in most situations...is it? not always i guess though mostly yes.

Usually i just focus and recompose from the crosstype focus center point...also shoot of course single point AF-S. if i take my time and hold still i nail the focus quite good...also try not to go backwards or forwards of course with your body when recomposing...it takes some time to master but now it feels natural to me...i got used to it...and i feel that if i start moving the focus points and concentrate about that...i might loose the shot until i manage to move the focus point on the subject and shoot...specially when shooting people because then you really need to catch there expression on the second it happens.

In conclusion, it really depends what you shoot...if its a non moving subject that you can take your time then go ahead...choose the exact composition you want, then choose the exact focus point you want and shoot but otherwise i would use the crosstype center focus point.

 

[hirejn]

The most accurate way is to put the focus point on the subject vs. focusing and recomposing, which changes the plane of the sensor and may throw the focus off. For two subjects, you have to make a decision. If they're both on the same plane, an aperture of about f/5.6 should hold them in focus. If they're on different planes (one is closer than the other), you have to decide which is more important, or you have to move them onto the same plane. The same plane means the subjects would be on a line parallel to the camera sensor.

Generally photographers don't use all focus points. They mostly use single point or dynamic groups. For portraits and weddings, I use only single point and place it where I want, and if the aperture and perspective are forgiving I'll focus and recompose slightly. There's no time to focus and recompose if the subject is moving. You have to find a relatively stationary moment to do that.