I have a 20D and my wife has a 350XT. We mostly take pictures of our son, who is always in motion, so we've certainly had our share of out of focus pictures.
Researching better focusing technique on the web led us to a different focusing method and we have since had a much much higher success rate.
None of this information is new, but I thought it might be helpful to pull it together in one place and share it with any FMers who are dissatisfied with their focusing.
1) Move the focus lock to the * button
On the 20D and the 350XT (and many other Canon bodies) changing Custom Function 4 from 0 to 1 moves AF engage from the shutter button to the * button on the back of the camera.
For us this has led to greatly improved camera handling. For example, when you want to take multiple pictures of an off-center subject you don't need to repeatedly focus-and-recompose. You just focus and recompose once, and then click away.
2) Put and leave the camera in AI Servo mode. Even though it is in AI servo mode, you'll still be able to do "One Shot", as you'll see below.
3) Put and leave the camera in multi-point AF. Even though it is in multi-point, you'll still be able to use just the center focus point and focus-and-recompose, as you'll see below.
4) If the subject is moving:
- Aim the center point at the subject and press and hold the * button
- The subject can move and you can recompose as long as the subject remains somewhere within the zone of the multi AF points.
- Take as many pictures as you like, as long as you hold the * button down the AF will usually keep tracking the subject.
5) If the subject is still (or at least not moving out of the focusing plane)
- Aim the center point at the subject and briefly press the * button to focus on it. Release the * button.
- Now recompose, if you like, and take pictures.
This is equivalent to taking pictures with just the center point and "one shot" AF because the way AI Servo works when all the AF points are active is that it initially uses just the center point to lock onto the subject. After focus is locked, when you release the * button the AI Servo stops looking for movement and focus is locked as it would be in "One Shot" AF.
So to summarize: with this technique you have Center Point/One shot AF and Multi Point/AI Servo AF available to you at all times depending on whether you just press, or press-and-hold the * button.
I strongly disagree with leaving the camera in AI Servo for one primary reason -- It NEVER locks focus! It will just get you kind of close, but it will never lock. Leaving the camera in this mode is just asking for major focus errors when working with fast glass. And you think all the self-diagnosed front/back focusing posts are bad now..... think of how many there will be if people are using AI servo and not even locking focus to begin with.
I prefer using camera on burst mode, where one can also take only one pic at the time if pressing shutter button shortly. You can hold the button halfpressed and press it again and again if want to have another pic with same focus lock.
One addition: if you need to have focusing on all focusing points, put the camera to One Shot mode. The AI Servo always focuses on the center point (even when continuously pressed) but only when the subject moves to another point, is the focus based on that point.
Ben Horne writes I strongly disagree with leaving the camera in AI Servo for one primary reason -- It NEVER locks focus! It will just get you kind of close, but it will never lock. Leaving the camera in this mode is just asking for major focus errors when working with fast glass. And you think all the self-diagnosed front/back focusing posts are bad now..... think of how many there will be if people are using AI servo and not even locking focus to begin with.>>
This isn't quite accurate. AI Servo locks onto a center point focus. A common misconception is that AI Servo will "hunt" after that. In fact, being an open-loop system, the focus point does not change again unless the AF points determine that the subject has gone out of focus and even then it does not "hunt" but goes directly to the new in-focus position and stops. Finally, the whole point of moving focus from the shutter button to the * button is that by releasing the * button you lock the focus even though you are in AI Servo mode.
Arhi Esansaari writes I prefer using camera on burst mode, where one can also take only one pic at the time if pressing shutter button shortly. You can hold the button halfpressed and press it again and again if want to have another pic with same focus lock.
This is correct, but I was surprised how much easier it is to keep your finger on the * button during a burst of pictures than to keep it on the shutter button half-pressed.
I would be very interested to hear from people who try this method about how it works for them.
In my case, I shot with AI servo and drive mode. I use only the center AF point.
The AI servo compensate if the subject move, or my body moves. If you keep
the shutter half pressed, the Focus is continous. And with drive mode, you can shot pretty rapidly. But I let the camera refocus for like 1/2 sec between shots just to make sure if the subjet is moving that the focus is ok . I have found that if I choose One shot AF, in drive mode, if the subjet come toward you, it will not refocus between shots.
The amount of OOF shot is keep to a minimum for me with this technique.
350D with 24-85 Canon. Not the greatest lens, but It fits my needs at 95%.
I'm going to ask a really dumb question at this point. I haven't used the * method for focusing so I have no experience in this. Sometimes I will lock exposure (like in AV move) by using the * and recomposing. Is this not possible to do when using the focusing methods you describe above?
edit=> No one seems to be mentioning AI Focus as a alternative to AI Servo. As I understand it AI Focus locks on a stationary object but then will track it if it moves later. This might work well for tracking a kid in motion....
toonhorse wrote:
I'm going to ask a really dumb question at this point. I haven't used the * method for focusing so I have no experience in this. Sometimes I will lock exposure (like in AV move) by using the * and recomposing. Is this not possible to do when using the focusing methods you describe above?
toonhorse writes I haven't used the * method for focusing so I have no experience in this. Sometimes I will lock exposure (like in AV move) by using the * and recomposing. Is this not possible to do when using the focusing methods you describe above?
That's correct. The * button no longer locks exposure. You can lock exposure with a half-shutter button.
ebrandon wrote:
Ben Horne writes I strongly disagree with leaving the camera in AI Servo for one primary reason -- It NEVER locks focus! It will just get you kind of close, but it will never lock. Leaving the camera in this mode is just asking for major focus errors when working with fast glass. And you think all the self-diagnosed front/back focusing posts are bad now..... think of how many there will be if people are using AI servo and not even locking focus to begin with.>>
This isn't quite accurate. AI Servo locks onto a center point focus. A common misconception is that AI Servo will "hunt" after that. In fact, being an open-loop system, the focus point does not change again unless the AF points determine that the subject has gone out of focus and even then it does not "hunt" but goes directly to the new in-focus position and stops. Finally, the whole point of moving focus from the shutter button to the * button is that by releasing the * button you lock the focus even though you are in AI Servo mode.
I challenge you to shoot a portrait session using AI Servo AF with the 85mm 1.2. It does not lock and hold the focus. It can be easially thrown off, thus it is NOT accurate for still subjects where there is a short DOF. At the camera shop I work at, we often get people who complain about shots that are out of focus. The first thing I ask is what focus mode they were using.... They were using the AI Servo when they really should be using One shot. It may seem on paper that it will lock focus and everything will be fine and dandy, but it doesn't work that way in the real world.
My fastest lens is the 50 1.8, and my shallowest DOF lens wide open is the 135 f2.. So maybe I haven't experienced such critical DOF that the differences between AI Servo and One Shot appear. If I start getting OOF shots, I will definitely try One Shot in that situation per your recommendation.
ward1066 wrote:
but if you quickly release the * button doesnt that essentialy lock it?
Kind of, but did it necessarly lock on what you wanted it to lock on? AI Servo is always trying to find motion, and when you let go of the * button, it will just stop trying to seek focus....... It won't necessarly be locked onto your subject exactly where you want it. When sports shooters shoot a quick series with AI Servo focus, it is common for the first shot to be out of focus.
The key is to use the right tool and technique for the job. That's why Canon gives you a choice between AI Servo, one-shot and MF. For my style of shooting AI Servo is useless most of the time, but I'm glad I have the option when I need it. I use MF far more often than AI Servo.
This isn't quite accurate. AI Servo locks onto a center point focus. A common misconception is that AI Servo will "hunt" after that. In fact, being an open-loop system, the focus point does not change again unless the AF points determine that the subject has gone out of focus and even then it does not "hunt" but goes directly to the new in-focus position and stops.
That isn't quite accurate. AI Servo never "locks" because it is running a routine that constantly takes new data points in order to predict the subject motion. The photographer starts focusing with the subject on the center point if an "all points mode" (this is necessary). The camera takes that as a datum to predict the next point and immediately moves the lens to the next point. It operates like a skeetshooter--it never locks on where the subject is, but "leads" the subject by directing the lens slightly away from the last point of focus to the point the subject should be by the time the shutter releases again.
If you put the camera on a tripod and use AI Servo on a perfectly motionless subject, you will observe the camera constantly moving in and out of focus as it tries to determine which direction the subject is moving and "lead" the subject. It will never "realize" the subject is not moving because by setting AI Servo, you have set the condition "The Subject is Moving" (you've lied to the camera if the subject is not moving).
I guess I'm not sure how the camera/lens continues to search for something to focus on if you're using the * button for AF and aren't pressing the button? How would this be different from having the camera sitting on a table, pointed at nothing in particular, then pretting the * button once and walking away. Is the camera/lens still going to hunt for something to focus on if it's not being told to (by some press of a button)? I'm not arguing anyones specific points, just posting some rhetorical questions that are running through my mind.
Granted, I don't use AI Servo for portrait work, but often times while shooting sports or something where I'd normally us AI Servo I'll focus with the * button, then let off, recompose (slightly...nothing dramatic) then press the shutter button. Seems to work.
