Scott Sewell wrote:
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.
Well, obviously if you turn the focusing system off, it's going to stop focusing. But if it's in AI Servo mode, you may well have turned it off while it was "leading" the focus predicitively--and thus it will be out of focus. The camera knows two things for certain: A) The subject is moving (you set that condition when you set AI Servo), therefore B) when the shutter releases, the subject will not be where it is at present instant. It's commanding the lens to put the focus where the subject will be in the next instant
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.
Hmm....
Saved by depth of field and serendipity, helped by the fact that you tend to sway yourself, and that sway is easily predictable.
RDKirk, I guess I'm not following ya. Forgive my slowness this evening, but I thought of this while I was out shooting a softball game tonight and actually took a shot of what I'm talking about. Let me know what I'm missing here...
This shot was taken with a 300/f2.8 on a 20d with the 20d set to AI Servo, the * button for focus C.Fn.4/3 and using the center focus point only. Using the * to focus, I put the center focus point (square) smack dab in the middle of the zero score below the Home team on the sign. I then let off on the * button, recomposed the shot and then pressed the shutter button. The image in this post is what resulted.
So, by using the * button isn't it pretty much the same as a focus preset even when the camera is set to AI Servo...as long as I'm not pressing the * button? Again, I'm not trying to be argumentative, but just trying to figure out what's going on here and what you're saying. I guess what threw me off is when you said in AI Servo the focus never locks. But isn't it in affect locked at the last AF point when not pressing the * button?
I basically use this same technique a lot in baseball when I think there might be possible action at multiple spots on the field. For example, I'll use the Focus Preset on the 300 lens for something like second base, then press the * button to focus at home plate. So, if action happens at home plate I just aim the lens and press the shutter button (and focus doesn't change). Maybe technically isn't not supposed to work, but I've got a whole boatload of baseball action using one of these two focus preset methods.
I think we're getting caught up in what the AI Servo naysayers mean when they say it "never locks"
Everyone agrees that you can release the * button, AF stops, and you can recompose without changing your focus.
What they're saying is that AI Servo won't do a good job of focusing on a stationary object in the first place because after initially focusing on that object it keeps "hunting" for changes. And on that point you're just going to have to choose who to believe -- them or your lying eyes.
For example RDKirk wrote 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 tried this experiment tonight and this did not happen. The AI Servo system was smart enough to "predict" that the stationary object would be in the same place 1/50 sec from now as it is now and not change the focus. It's true that if you move the camera slightly by pressing on the * button you'll hear AF move around, but if you're very careful not to move the camera while pressing * -- total silence.
I worked out a CSD File camset_d.csd (1DMKii) (sports setting) over a few months and its only 2K if anyone wants to try it.
It uses the * for sure, AI Servo off
This was the best setting for shots like this at 80MPG turns a half a block away coming at you. http://danrich.smugmug.com/photos/19440745-L.jpg Dimensions: 3522 x 2348
Capture Time: 2005:04:09 08:22:04
Camera Model: EOS-1D Mark II
Type: CR2 Image
Size: 7.5 MB
ISO speed: 125
Exposure time: 1/3200 sec
Aperture: f 3.5
Focal length: 200 mm
Exposure program: Aperture priority
Shutter speed: 11.63
Lens aperture: 3.625
Exposure bias: 0
The metering mode: Spot
My results are different from yours. On both my 1D and D30 (both in AI servo mode), the AF was continually "moving" when the * button is held down (I'm in CF4-3), so I'm with RDKirk with this.
Perhaps the newer generation cameras (20D or MkIIs) have more intelligent AI Servo modes, but at least for the 1D Mk I and D30, the AF will be continually "moving" if you point it at a stationary object.
ebrandon wrote:
I think we're getting caught up in what the AI Servo naysayers mean when they say it "never locks"
Everyone agrees that you can release the * button, AF stops, and you can recompose without changing your focus.
What they're saying is that AI Servo won't do a good job of focusing on a stationary object in the first place because after initially focusing on that object it keeps "hunting" for changes. And on that point you're just going to have to choose who to believe -- them or your lying eyes.
For example RDKirk wrote 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 tried this experiment tonight and this did not happen. The AI Servo system was smart enough to "predict" that the stationary object would be in the same place 1/50 sec from now as it is now and not change the focus. It's true that if you move the camera slightly by pressing on the * button you'll hear AF move around, but if you're very careful not to move the camera while pressing * -- total silence. ...Show more →
If you don't believe that the AF is constantly hunting when in AI servo mode when shooting a stationary subject, try this. Do the same test explained earlier in this post, but this time put your ear up against your lens. You will hear it going back and forth trying to hunt for focus.
Just to repeat what has already been said, here are some definitions
Locked Focus: The AF system has detected focus. It is indicated in the viewfinder with a Green dot, and the camera stops trying to seek focus because it has effectivly locked, and is accurate.
This is not the same as stopping the focus in AI Servo mode. If you have it in AI servo mode and let go of the AI Servo button, it has stopped attempting to achieve focus, but not as a result of it achieving focus........ only because you told it to stop focusing.
While shooting a school trip earlier this week, with lots of fast-moving kids on a nature trail, I tried leaving the camera on servo mode for part of the day and it worked well.
But after shooting the whole day with a combination of one-shot and servo modes I'd have to say that I intend to get good at manual focus and use that from now on for similar jobs. Shooting wide at f5.6 or smaller you don't need to be spot-on with the focus, and manual would be so much quicker than waiting for focus lock.
My results shooting in AI Servo when my model stopped moving recently were poor, at best. I tried using it because I was tiring of going back and forth as my model either walked, or stopped and posed. Typically front-focused by 6" or so (150mm at 15 feet), and at f/2.8, it was enough for the images to be 80%+ unusable. So much for experiments! Regardless of what comes out in this thread, I know I won't be trying that again.
Why try to make one AF mode fit all circumstances? Use One Shot for stationary subjects and AI Servo for moving subjects. The One Size (or one AF mode) fits all, notion, will produce less than optimal results. I use AI Servo for birds in flight and I get a much higher percentage of keepers, as opposed to One Shot. One Shot AF produces more keepers with stationary subjects. Learn your camera and you can switch your AF modes without removing your eye from the viewfinder.
nutek wrote:
My results are different from yours. On both my 1D and D30 (both in AI servo mode), the AF was continually "moving" when the * button is held down (I'm in CF4-3), so I'm with RDKirk with this.
I tested with my D30 and 50mm and I saw the same thing: the 50mm's front element was constantly moving back and forth.
Are people with poor results using a 20D? I believe the origional post stated he used it on the 350D and the 20D, maybe the AI servo is different on these newer generation cameras?
Canon has designed autofocus modes of the 20D to mean the following:
One Shot: "The subject definitely is not moving."
AI Servo: "The subject definitely is moving."
AI Focus: "The subject is not moving now, but it could start moving."
It is clear to me that One Shot will be the most likely to be accurate, because there are fewer variables to contend with. AI Servo is an attempt at estimating focus, based not only on more variables, but also on assumptions. AI focus, being a hybrid of the above two, uses even more assumptions and could at best be only as accurate as AI Servo.
I contend that the best use of the options Canon has provided is to use them for the purposes they were designed for. There are many possible errors inherent in the EOS autofocus system that we can control to some degree if we understand exactly how the system works and we concentrate on good technique. I believe that we can get the most from the camera by concentrating on what we know we have the potential to control.
Ben mentioned this, but lest it get lost in the flurry it's worth repeating and calling attention to — in AI servo the camera will release the shutter (for the first shot in a burst) regardless of whether focus has been achieved. In one shot it will not. One result of this is that you can almost guarantee an OOF shot by stabbing the shutter in AI servo mode.
paulhodson wrote:
If AI servo is as good as single shot why would/do Canon provide single shot
If AI servo cannot achieve focus and most shots are OOF then why would canon provide it? Im not trying to start an argument, but I did try it at a car show this weekend and had good results. I was using pretty small apertures and maybe the results would be different wide open? heres a shot. the rest can be seen here http://wmaod.smugmug.com/Cars http://wmaod.smugmug.com/photos/24084758-L.jpg
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