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One-shot and AI servo

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I don't know (obviously!) how AI Servo works, but since it assumes subject movement, I'd guess the algorithm includes periodic lens movements to confirm maximum contrast (sharpness), followed by an adjustment back to maximum contrast. That is consistent with a series of AI-S shots of a static subject having some out of focus. If you press the shutter at the "wrong" instant, the lens can be in process of checking or re-focusing, hence an out of focus shot.
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JerryG

My galleries at:
http://www.pbase.com/jerryg1
 
JerryG1 said:
I'm a pretty boring focuser, usually favoring one-shot mode and center-point with my 5D2.

Looking for trouble, I just did a quick test comparing one-shot to AI-servo for a stationary subject. I pointed my 100L macro IS at a telephone button about three feet away and took a series of photos alternating between One Shot and AI servo, refocusing for each shot. (I have focus programmed into the AF button, not the shutter.) No tripod, but at iso 1600 anf f3.2 shutter speed was 1/800, so I don't think camera-shake was a factor in any of the images.

All of the one shot pictures were sharp. All of the AI-Servo were slightly misfocused, the kind of misfocus I could easily see on the LCD by setting the review to maximum magnification.

I know some people use AI Servo all the time, but that does not seem like a good idea if it introduces focus errors. Has anyone else observed this? Is this sort or error expected or might I have a problem with my camera/lens?

Thanks.
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JerryG

My galleries at:
http://www.pbase.com/jerryg1
Click to expand...
never had that problem.. basically because i use the tools the camera gives me the way canon intended..
AI Servo is for moving objects.
why is this simple concept difficult?
one shot for stationary objects.
yes, you can use a screw as a nail.. but that is not what it was intended for!
 
JerryG1 said:
I put my 5D2 on a tripod and did a similar test, focusing on text about 3 feet from the camera with the 100L IS macro. One-shot yields consistently sharp images; AI-Servo consistently slightly soft.

Tried the same thing with my 40D and couldnt tell the AI Servo images from the one- shots.

5D2 is still under warranty. Think I'll take a drive to Canon this week.
--
JerryG

My galleries at:
http://www.pbase.com/jerryg1
Click to expand...
Jerry, The results you got with the 40D are what you should be getting with the 5DII. This is all pointing to your camera having poor AI servo performance, most likely a tiny alignment error. As another poster pointed out, I normally use 1 series cameras with more sophisticated AF, but I have recently acquired a 5DmkII to replace my 1DsII. I have just gone and tested my 5DII plus 100mm macro and the results are indistinguishable in one shot or AI servo when viewed at 100% on the computer screen.

The people who keep saying that one shot is for stationary subjects and AI servo is for moving ones are just repeating the old mantra - it is not wrong as such, just sometimes restricitve and not the whole story. One shot has its uses - if I was shooting something like a statue that is truly static off a tripod I would use it, but if you are standing there swaying around shooting something that lives and breathes handheld or even shooting something living that appears "still" but is probably swaying around or blowing in the breeze or whatever, is it truly a static target ? If the subject is large in the frame or you are shooting at a wide aperture the DOF will be very shallow. If the subject (or you) move just a fraction between acquiring focus in One shot and firing the shutter then the subject will most probably not be critically sharp.

Here is a static kingfisher shot in AI servo - I repeat, the mode I use 90% of the time !



Then a few moment later it dived off the post. If I had still been in One shot I would have had an OOF shot. Instead of that I panned in AI servo and got the shot.



Ophrys
 
JerryG1 said:
Three shots with each body in each focus mode. Total of 12 shots in last test.

Haven't really looked for focus point movement. As I recall, all are front focused.
Why do you ask?
Click to expand...
Because if the focus hasn't settled down to one point, that could mean something in the AF system is faulty.

For example, here's what I got with my defective 1D Mark III when focusing on a static subject with AI-Servo:



When it was replaced with a 1D Mark III that worked, here's the same test:

 
pete zeugma said:
AI Servo is for moving objects.
why is this simple concept difficult?
Click to expand...
Because it's not exactly true. Photographers take shots of things that move, then stop, then move again.

This happens a hundred times during a basketball or football game. Do you expect every shot taken with AI-Servo of a basketball player standing dribbling the ball to be out of focus? Do you expect every shot taken with AI-Servo of a quarterback standing in the pocket looking for a receiver to be out of focus?
 
Ophrys said:
Jerry, The results you got with the 40D are what you should be getting with the 5DII. This is all pointing to your camera having poor AI servo performance, most likely a tiny alignment error. As another poster pointed out, I normally use 1 series cameras with more sophisticated AF, but I have recently acquired a 5DmkII to replace my 1DsII. I have just gone and tested my 5DII plus 100mm macro and the results are indistinguishable in one shot or AI servo when viewed at 100% on the computer screen.
Click to expand...
That is really helpful to know!
Three more questions: Was AF point area expansion enabled in your 5D2,
Did you test with an f/2.8 or faster lens, and did you use the center point?

I ask because I found a solution to my problem and it involves AF-point area expansion (CF III, 7).

By turning AF-point expansion off, my AI-S misfocus problem totally went away. I'm guessing that is because one of the invisible auxiliary focus-points is prevailing, shifting the focus slightly. The camera was angled downward when I took the pictures, so I am seeing what appears to be front-focusing on a line of text below the main center focus point.

I am still concerned why another point prevailed when the main point had a high contrast subject to focus on. Your answers to my three questions may help.
Thanks so much, Ophrys.

Beautiful photos, btw!

--
JerryG

My galleries at:

--
JerryG

My galleries at:
http://www.pbase.com/jerryg1
 
Thanks Scott,

Your question prompted me to look for a pattern, and I found that all images were front-focusing by the same amount. That ultimately led to a cause and solution. At this point I'd be happy to call it user error, but not sure that is entirely correct. See my SOLUTION! post above.
--
JerryG

My galleries at:
http://www.pbase.com/jerryg1
 
JerryG1 said:
Ophrys said:
Jerry, The results you got with the 40D are what you should be getting with the 5DII. This is all pointing to your camera having poor AI servo performance, most likely a tiny alignment error. As another poster pointed out, I normally use 1 series cameras with more sophisticated AF, but I have recently acquired a 5DmkII to replace my 1DsII. I have just gone and tested my 5DII plus 100mm macro and the results are indistinguishable in one shot or AI servo when viewed at 100% on the computer screen.
Click to expand...
That is really helpful to know!
Three more questions: Was AF point area expansion enabled in your 5D2,
Did you test with an f/2.8 or faster lens, and did you use the center point?

I ask because I found a solution to my problem and it involves AF-point area expansion (CF III, 7).

By turning AF-point expansion off, my AI-S misfocus problem totally went away. I'm guessing that is because one of the invisible auxiliary focus-points is prevailing, shifting the focus slightly. The camera was angled downward when I took the pictures, so I am seeing what appears to be front-focusing on a line of text below the main center focus point.

I am still concerned why another point prevailed when the main point had a high contrast subject to focus on. Your answers to my three questions may help.
Thanks so much, Ophrys.

Beautiful photos, btw!

--
JerryG

My galleries at:

--
JerryG

My galleries at:
http://www.pbase.com/jerryg1
Click to expand...
Hi Jerry - No I was using centre point only with no expansion 100mm f2.8L IS macro shot wide open and IS off after micro-adjusting the pair together. If you use a smaller aperture or expand the AF you could be disguising the problem. I would still get the camera checked if I were you.

Ophrys
 
Ophrys said:
Good demonstration - was that pre and post submirror fix ?
Click to expand...
No, the first one was post submirror fix, post return to Irvine since it still didn't focus, post Irvine saying the camera worked "per spec", post Canon "Technical Support" telling me I didn't know how to use a 1D camera after using two previous models for six years.

The second is post camera replacement which Canon authorized.
 
Ophrys said:
...

Hi Jerry - No I was using centre point only with no expansion 100mm f2.8L IS macro shot wide open and IS off after micro-adjusting the pair together. If you use a smaller aperture or expand the AF you could be disguising the problem. I would still get the camera checked if I were you.
...
Click to expand...
I may not have been clear. Expansion was enabled when I had the problem. Turning expansion off made the problem go away.

I suspect the extra points in expansion mode work analogoulsy to automatic point selection using the standard nine points: the AF point with the closest, high-contrast content gets the business.

All my shots were at f/2.8 and at short range to minimize dof and not mask the problem.

I may get the camera checked in any case, but I am curious to know if your same camera and lens behave like mine with AF-point expansion enabled. I photographed a page of text in a plane that is not parrallel to the sensor. The page was flat on a table with the camera pointing downwards at about a 45 degree angle towards the page.) With my 5D2, the line of text closest to the camera but still within the spot-focus circle is what gets focused on in AI Servo.

With One Shot, the line of text directly under the visible focus point is sharpest, as one would expect.

When I turn off expansion, both focus modes work as expected: sharpest line is right at the AF point.

I appreciate your several support posts. Thanks.
--
JerryG

My galleries at:
http://www.pbase.com/jerryg1
 
Ken Phillips said:
... to use a system that expects motion for an object known to be stationary !
Click to expand...
Not really. See Scott's post below.

From what I am piecing together, AI Servo was originally developed only for moving objects. But newer incarnations are so good that a lot of knowing photogs are able to use it effectively for stationary subjects.

It may be time to revise the instruction manuals. ;)

--
JerryG

My galleries at:
http://www.pbase.com/jerryg1
 
Ken Phillips said:
... to use a system that expects motion for an object known to be stationary !
Click to expand...
You think those photographers under the basket are constantly switching between AI-Servo and One-Shot every time the player with the ball stops?

A firmware update for the 1D Mark III specifically addressed a problem we were having with focusing on static subjects in AI-Servo so it's definitely something Canon expects their cameras to be able to do.
 
I'm feeling pretty good about the health of my camera now, and about my knowledge of AI Servo focusing. Thanks to all who contributed, including the naysayers who weren't quite on board with the issues. All the comments helped me.

I found a thread from a few months ago that was also helpful to me.
http://forums.dpreview.com/forums/read.asp?forum=1032&message=37090059

The very first post kind of confirms the down sides that I learned about AF point expansion the hard way. It's here as a reference to make it easier to find.

Thanks again.
--
JerryG

My galleries at:
http://www.pbase.com/jerryg1
 
Scott Larson said:
pete zeugma said:
AI Servo is for moving objects.
why is this simple concept difficult?
Click to expand...
Because it's not exactly true. Photographers take shots of things that move, then stop, then move again.

This happens a hundred times during a basketball or football game. Do you expect every shot taken with AI-Servo of a basketball player standing dribbling the ball to be out of focus?
Click to expand...
you are a pretty crap photographer if you cant focus on a stationary object!

Do you expect every shot taken with AI-Servo of a quarterback standing in the pocket looking for a receiver to be out of focus?

i rarely have out of focus shots of moving objects using one shot.. i just am not one of those new generation of photograpers who are out of their comfort zone unless they have the ability to point and spray at 10ffs

And basketball is not really a high speed example... I do an awful lot of motorsports and aiviation photography.
 
JerryG1 said:
pete zeugma said:
...

never had that problem.. basically because i use the tools the camera gives me the way canon intended..
AI Servo is for moving objects.
why is this simple concept difficult?
...
Click to expand...
Not difficult, just dogmatic, unimaginative, uncreative, and requires an absence of curiosity.
Click to expand...
there are some things that are just not worth being curious about
JerryG1 said:
JerryG

My galleries at:
http://www.pbase.com/jerryg1
Click to expand...
 
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