How reliable using all AF points?

tecboy · Dec 4, 2014

I have 19 AF points on my 70d. When I set all AF points, sometimes I get slightly soft images. I use single AF point most of the times.

dennybeall · Dec 4, 2014

Multiple focus points- the camera picks the one to use, one focus point and you pick the one to use. On multipoint something will be in focus just maybe not what you wanted.

runnah · Dec 4, 2014

Depends on the camera. I know the new 7dmk2 has very good multipoint AF. Some of the older cameras aren't as good.

I've tried it on my 5dmk3 and have had pretty good success with stationary object. I am to scared to try it on sports.

JacaRanda · Dec 4, 2014

It will be focusing on the closest object(s), or maybe the object with the best contrast.

hamlet · Dec 4, 2014

At first, i couldn't get reliable results with my af, but i've pretty much nailed down my af focusing system. There are tricks you can learn that only be acquired by using your camera.

elemental · Dec 4, 2014

dennybeall said:
Multiple focus points- the camera picks the one to use, one focus point and you pick the one to use. On multipoint something will be in focus just maybe not what you wanted.

Exactly. Your soft photos probably have a sharp subject somewhere, it just may not be the one you wanted. I've never had any success with multipoint AF systems (admittedly, I don't shoot cameras as advanced as the 7Dmk2), so I've gotten in the habit of either using the center AF point and recomposing or choosing the AF point manually. Recomposing works fine in many situations where you're working with deep enough depth of field to absorb the shift in focal plane.

bratkinson · Dec 5, 2014

As an example...in a used car lot, there's 10 used cars available to choose from. Which one would your wife choose?

The shiny one, of course...

But you'd pick the Lamborghini over in the corner that needs a wash and has a couple of dings in it...

Unless I know in advance which AF point the camera is most likely to pick, I will usually force center AF point only, or, choose a specific AF point to be selected via camera settings. Yes, when there's a good, clear subject 'up front' to choose from (such as a portrait), even full Auto with a gazillion AF points will pick the right subject. But a somewhat diagonal line of small group of people...the camera will likely pick the closest person or object...

curtyoungblood · Dec 5, 2014

You've only given us a small piece of the equation. There are a lot of factors that have an effect on this, and something else could be causing soft focus. Are you using one shot or servo? Are you having problems when you're shooting action, still subjects, or both? How are you using your single focus points (are you selecting the one closest to where you want to focus or focusing and recomposing)? Are you using very wide apertures? Are you only using cross-type sensors?

The AF system in today's cameras are getting pretty complicated, and there's not really a one setting for every situation anymore.

KmH · Dec 5, 2014

The camera can only use 1 focus point to focus.

Setting up the camera to use all 19 focus points means the camera decides which 1 focus point to use.
The camera is a stupid, follows a rote program, machine that cannot think.
Consequently, any time you put the camera in an auto mode you let the camera do some of your your job as a photographer for you.
Don't be surprised when the not-a-human-and-can't-think camera sometimes gets it wrong when it follows a rote program that can't adapt to all situations.

curtyoungblood · Dec 5, 2014

I think there are certainly circumstances where the camera can do a better job than I can picking an autofocus point. Specifically, I get many more photos that are in focus (and get in focus quicker) using all the focus points on my 5diii when shooting football. The camera is far better at tracking a subject than I am at keeping that subject under a specific focus point. This may come down to a lack of technique, but I have more success letting the camera select the focus points.

SCraig · Dec 5, 2014

How reliable? Not very in my opinion. I've NEVER, not one single time, had usable results when I let the camera choose the focus point so it's yet another so-called "Feature" that I never use.

TCampbell · Dec 6, 2014

As other have indicated, the camera will evaluate all the AF points, but really it's just going to pick ONE. The question is... which one?

When allowing the camera to use all the AF points, Canon's algorithm is to use the AF point which can lock focus at the CLOSEST focusing distance.

Suppose you have a group of people who are all standing at slightly different distances and you frame the shot. You might want to maximize focus on the whole "group". The camera won't do this... it'll evaluate all the AF points, find the single point that allows a focus lock at the closest distance, and that's the focus it'll go with. Those people who are a bit farther away will be a bit soft.

If you were taking such a group shot, you'd want to increase the depth of field and specifically pick a subject somewhere around the middle of your group (perhaps just slightly forward) to lock focus.

Allowing the camera to auto-pick it's focus point really only works when your subject of interest (the element you want focused) is the CLOSEST thin in your scene... and there are no distracting foreground elements which are closer (at least none that are positioned at an AF point.) Even this can be risky if depth of field is narrow... I've seen shots with subjects seated, for example... where their knees and feet are nicely focused... unfortunately their face and eyes are not.

The safe thing is to put the focus point exactly where you really want focus.

robbins.photo · Dec 6, 2014

Well as others have mentioned a lot of things can affect the sharpness of the final image. DOF and shutter speed are big factors of course. But in general for slow or stationary shots I prefer single point so I select the focus point. For faster moving targets I will often use 9 point.