one shot vs ai servo

[93octane]

Can someone explain the difference between one shot and ai servo and ai focus??

[93octane]

I found this post elswere one question that I still have is if the camera is in one-shot mode and you focus on something and move the camera away from your face for what ever reason the camera shouldn't refocus on anything else right??? I had a chance to play with a 20D and in one shot I would hold the shutter button down half way and rotate my body 180 degrees and I could hear the and see the some focusing occur???

Canon's terminology makes this confusing.

There are two "real" focsuing modes:


"One Shot" focus means that, when you press the shutter button halfway, the camera sets the focus besed on the scene at that moment and holds that until you press the shutter release the rest of the way.

"AI Servo" focus means that if you press teh shutter button halfway, the camera makes a focus detemination and then continues to update that ("tracking" the focus iof the subject) until you press the shutter release the rest of the way to take the shot.

The "third" mode, "AI Focus", isn't really a focsuing mode but rather a "mode management plan". With it in effect (and it is the mode we have in the 300D for most shooting modes), then when you press the shutter release halfway, the system is initially in One Shot focus mode, makes a determination, and holds it - unless the camera senses movement of the main subject. Then it switches into AI Servo focusing mode and begins to track the focus of the main subject.

If we could freely choose among these modes, we might use them as follows:


One Shot focus - we have no expectation that the bear is going to charge us. We want the camera to determine the focus and then just be cool. A reason we want that might be that we have a lens with Full Time Manual focsuing (which means you can turn the focusing ring and tamper with the focus even while in the autofocus mode), and we want the camera to make a focusing determination but then we will "tweak" it manually. So we don't want the camera, after the initial focus, to play with focusing any more for that shot. (If it did. it might disagree with our "tweaking" and change the focus back to what it originally thought was right.

AI Servo focus - We expect to be working with a moving target, and want the focusing system to get on its tail immdiately when we press the shutter relase halfway, no questions asked.

AI Focus - We are not sure what is going to happen, and ask the camera to do what it thinks is best for us based on what it seesn happening.

On a stock 300D, you can't put it into One Shot or AI Servo in any of the "normal" shooting modes, so there are no buttons to be pressed. In all "normal" shooting modes, it is in AI Focus. whether we want that or not.

With the modified software ( Wasiaware) version B7.1, one can force the camera to be firmly in One Shot focus mode or allow it to be in AI Focus mode (the normal arrangement). It is the intent that we would also be able to put it directly into AI Servo mode. That doesn't work yet - well, you can put the camera in that mode, and it will start to track focus immediately when you press the shutter button halfway. You just can't fire the shot! We are hopeful that tbis egg will be fully cooked soon.

Best regards,

[tommykjensen]

One shot: The camera focuses one time when You half-press the shutter button. It does not refocus if subject moved or You recompose.

AI Servo: The camera focus continously in order to track moving subjects.

AI Focus: One Shot focus until the camera detects movement in which case it switches to AI Servo.


Personally I only use One Shot focus or AI Servo. Usually I know based on the subject which would be smartest to use (or at least I think so ;-) ).

[Jack W.]

Interesting thread.
If I have my camera on a tripod, aimed at my bird feeder, and considering that the birds tend to be in constant motion when they're chomping down that seed, would I be better off using AI servo?
Thanks.
Jack

[mjordan]

I have found that One Shot is more accurate, even for moving objects... at least with my 10D (and before with my D30). The newer cameras might be better at AI Focus, but what I found is that when I have a moving object and I'm trying to stay composed as I'm moving with it, that the focus point that decides what the focus should be is usually the wrong one from what I'm wanting to focus on. I normally shoot with just the center point as my focus and exposure point since it is faster than using multiple points and also more accurate (it focuses on what I point it at, not something that might be closer that another point sees (like a tree branch or person).

What I have had to learn to do shooting in One Shot is to release the shutter button as I follow the object. Other wise I will lock focus and by the 2nd or 3 rd shot the object has moved and it's out of focus. Yes, I still get images that are out of focus this way, but less than I was getting in AI focus.

I don't know if the 20D will do AI Focus better, but I suspect the 1D MKII and 1Ds MKII probably are a lot better.

Mike

[MrThanh]

Interesting thread.
If I have my camera on a tripod, aimed at my bird feeder, and considering that the birds tend to be in constant motion when they're chomping down that seed, would I be better off using AI servo?
Thanks.
Jack

it should be fine to use One Shot because the subject is still in the same position. if you use AI servo and something else in the frame moves, it may track that object instead.

[jimsolt]

I believe in one-shot mode, the camera will not fire if focus can't be found by the AF system whereas in AI Sservo the camera will fire -- at least in the creative modes (except for A-DEP). It won't beep and the focus light won't light either. If you are in a difficult focusing situation AI Servo might be the better choice. The downside is that apparently this mode uses more battery, but I've not found that to be a problem yet. I have a spare battery -- the planes only fly by unexpectedly once.

Jim

[scottbergerphoto]

AI-Servo works like this: The camera shuts down all AF points except the center AF point until it locks focus. Then, it tracks the moving object with the other available AF points. In single shot mode, all shots are release priority-the camera will fire whether or not the image is in focus. This is not true of One Shot AF. You will simply not get the shot if the object isn't in focus. In AI Servo, Continuous Mode, the first shot is Release Priority, and all subsequent shots are focus priority. There is no better choice for shooting a moving object then AI Servo. particularly when the object is moving towards or away fom you.
If you manually select a single AF point in AI Servo, the camera will only use that AF point to acquire focus and track the subject.
The AI Servo is 1DM2>20D>10D

[BigRed450]

A1-Servo is also predictive, that is, once locked on and tracking, can and will predict a focus point just ahead of the subject in order that it will be in fine focus by the time the shutter trips...
There is just simply no substitute for A1-Servo in Sports Photography. C.Fn.4-01 ( focus with * button) is also very handy for A1-Servo mode.

[the apprentice]

Hi Scott,
Help please! When I use my 20D with AF set to AI Servo I don't get red blinking square on AF points in the viewfinder! How is the camera performing AF in this mode, wouldn't be normal to display the points with correct AF in al modes? Is this a normal behavior or it is something wrong with my camera?
Regards,
the apprentice

[the apprentice]

AI-Servo works like this: The camera shuts down all AF points except the center AF point until it locks focus. Then, it tracks the moving object with the other available AF points. In single shot mode, all shots are release priority-the camera will fire whether or not the image is in focus. This is not true of One Shot AF. You will simply not get the shot if the object isn't in focus. In AI Servo, Continuous Mode, the first shot is Release Priority, and all subsequent shots are focus priority. There is no better choice for shooting a moving object then AI Servo. particularly when the object is moving towards or away fom you.
If you manually select a single AF point in AI Servo, the camera will only use that AF point to acquire focus and track the subject.
The AI Servo is 1DM2>20D>10D


Hi Scott,
Help please! When I use my 20D with AF set to AI Servo I don't get red blinking square on AF points in the viewfinder! How is the camera performing AF in this mode, wouldn't be normal to display the points with correct AF in al modes? Is this a normal behavior or it is something wrong with my camera? <SPAN style="FONT-SIZE: 10pt; FONT-FAMILY: Arial">

[richwood]

Hi Scott,
Help please! When I use my 20D with AF set to AI Servo I don't get red blinking square on AF points in the viewfinder! How is the camera performing AF in this mode, wouldn't be normal to display the points with correct AF in al modes? Is this a normal behavior or it is something wrong with my camera?
Regards,
the apprentice

IIRC that's normal.

Because it's continuously focusing, it never locks the focus, so doesn't cause the AF point to flash.

[djthemac]

AI-Servo works like this: The camera shuts down all AF points except the center AF point until it locks focus. Then, it tracks the moving object with the other available AF points. In single shot mode, all shots are release priority-the camera will fire whether or not the image is in focus. This is not true of One Shot AF. You will simply not get the shot if the object isn't in focus. In AI Servo, Continuous Mode, the first shot is Release Priority, and all subsequent shots are focus priority. There is no better choice for shooting a moving object then AI Servo. particularly when the object is moving towards or away fom you.
If you manually select a single AF point in AI Servo, the camera will only use that AF point to acquire focus and track the subject.
The AI Servo is 1DM2>20D>10D


I am trying to shoot my sister's dance competitions, with a 70-200 2.8 and due to the bad lighting i consistently find myself shooting ISO 1600 and F2.8. Up to this point i have just been using single shot mode because I want to focus on her face primarily. Should i still try to use AI servo, even with the limited DOF?

P.S.

Sorry to dig this one up from the dead! :D

[Max F]

AI-Servo works like this: The camera shuts down all AF points except the center AF point until it locks focus. Then, it tracks the moving object with the other available AF points. In single shot mode, all shots are release priority-the camera will fire whether or not the image is in focus. This is not true of One Shot AF. You will simply not get the shot if the object isn't in focus. In AI Servo, Continuous Mode, the first shot is Release Priority, and all subsequent shots are focus priority. There is no better choice for shooting a moving object then AI Servo. particularly when the object is moving towards or away fom you.
If you manually select a single AF point in AI Servo, the camera will only use that AF point to acquire focus and track the subject.
The AI Servo is 1DM2>20D>10D

Does this mean that you should have all points selected for focusing or does it even matter? Right now, i have just the center point selected and using AI Servo (which I just started using).