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Archive 2012 · 7D AI-Servo ... What is the Settling Time?
  
 
akin_t
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p.1 #1 · 7D AI-Servo ... What is the Settling Time?


My understanding of AI-Servo:

In AI-Servo, the 7D is continuously focusing ... So if the subject moves, the 7D compensates for this by changing the focal point of the lens accordingly.

Now my question is:

How quickly does the 7D compensate for movement made by the subject? If my subject moves ... How much time do I have to give the 7D before I am sure that my subject is in focus again?

It would have been easy to tell if I had a focusing screen, or if AI-Servo had a focus confirmation mechanic.



Feb 21, 2012 at 03:16 PM
tmalone893
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p.1 #2 · 7D AI-Servo ... What is the Settling Time?


I don't have a scientific answer but from my experiences, if you are locked in on your subject the focus is pretty much always spot on. Now if something jumps in front of your subject or you lose track of it, it may take 1/4 to 1/2 second if the contrast is good.


Feb 21, 2012 at 03:27 PM
msalvetti
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p.1 #3 · 7D AI-Servo ... What is the Settling Time?


It depends. Is the subject moving towards you, left to right, or somewhere in between? How fast? How much light do you have? What lens?

The idea with servo is that the subject to camera distance could be constantly changing, so you don't really have time to allow the AF to settle. Otherwise I would think you can use One Shot.

In general in good light with a fast focusing lens (especially one that's f/2.8 or faster so you activate the high-precision points), I've found that my 7D has little trouble keeping up with a moving object. Exceptions might be extreme examples, like a race car or aircraft moving straight at you.

As the lighting degrades, so will the AF performance (i.e. don't expect the same AI Servo performance shooting indoor sports as you would get outdoors in sunlight).

That said, in a 8 fps burst, the tougher the conditions, the number of frames in sharp focus will go down. With my 7D, often the first couple of frames are not in focus, particularly indoors. The 7D is still really good, and you probably would need to move to a 1-Series to do better.

Mark



Feb 21, 2012 at 03:38 PM
akin_t
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p.1 #4 · 7D AI-Servo ... What is the Settling Time?


I usually just stick to One Shot, I was only curious is all.


Feb 21, 2012 at 03:50 PM
 

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Ian.Dobinson
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p.1 #5 · 7D AI-Servo ... What is the Settling Time?


akin_t wrote:
My understanding of AI-Servo:

In AI-Servo, the 7D is continuously focusing ... So if the subject moves, the 7D compensates for this by changing the focal point of the lens accordingly.

Now my question is:

How quickly does the 7D compensate for movement made by the subject? If my subject moves ... How much time do I have to give the 7D before I am sure that my subject is in focus again?

It would have been easy to tell if I had a focusing screen, or if AI-Servo had a focus confirmation mechanic.


OK there are a couple of things that go hand in hand that it depends on.
1: how fast is the AF drive in the lens.
2: How fast/good is the brain in the camera.

1 is the biggest factor in keeping up with a constantly moving target, 2 has a bit more to do with picking up the subject in the first place and re aquisition if the target moved off the AF point.

as was mentioned above you also have the scenario off something jumping into the AF zone . you have an AF tweek in the menu to deal with this. if its set to fast it will try and AF on the other thing as soon as it gets in the AF zone , where if its set to slow it will try an ignore it for a bit.

But essentialy the 7D af system is good. Mine may not quite be as good as the one in my 1D2n but its very very close, on the other hand its miles better than the one in the 40D i had which was crap at AF servo even if I could keep the AF point on target

as a guide on the 3 bodies using the same lens (80-200L) which is not the best for servo AF anyway. I did the test on a dog running toward me . (quite repeatable as she'll run a run )

40D . the lens is very nervous , you can feel the AF drive adjusting and readjusting in the hand . and it can never really keep up.

1D2n. much more assured. feels like a much better lens in the hand . not so much readjusting .
AF kept up quite well (still not on a par with the fastest AF lenses)

7D close to the mk2n . a little bit more nervous in hand but did seem to keep up quite well.

as for the In focus shots id put it at about 10% 40D 60-70% mk2n 50% 7D all at the Fatest FPS
if I slow the FPS down (something that is handy on the mk2n as its still about 5fps) the hit rate increases to about 90% on the mk2 and 7D (7D slows down to 3fps) while the 40D only gets slightly better 20-30% .

with a faster AF lens I would expect the above to improve by quite a bit.




Feb 22, 2012 at 07:49 AM
Jude Perera
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p.1 #6 · 7D AI-Servo ... What is the Settling Time?


Thanks Ian for the informative answer.

Jude



Feb 22, 2012 at 11:38 AM
Alan321
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p.1 #7 · 7D AI-Servo ... What is the Settling Time?


There can be no simple and complete answer to this question.

These cameras use predictive focus in AI Servo mode, which means that the camera is assessing multiple actual focus readings and then predicting where the subject will be if you were to press the shutter button now, allowing for the time delay between pressing that button and the image actually being captured. All of this varies not just with the speed at which the subject approaches the camera but also on any changes to that approach speed. i.e. if the subject is accelerating towards you then it is harder for the camera to get the focus right, and even if it does get it right it will probably take longer because it has to figure out the rate of acceleration.

The slower the frame rate the more mirror-down time there is between shots during which time the camera can take more focus readings. This gives it more data to work with and should improve the AF tracking. Some cameras need more time than others.

On the 1-series cameras you can set an option that prevents the camera from shooting if the subject is not expected to be in focus. i.e. focus priority vs frame rate priority. This effectively makes the frame rate variable, only slowing down as much as is required. Your 7D may have this feature too but I do not know. This feature makes it harder to know how long is needed for the camera to get the focus right but in one sense also makes it irrelevant. However, it is not so irrelevant if you are missing irreplaceable shots of an action sequence because the focus is just a little less than perfect. In many cases a smaller shooting aperture masks small focus errors with the extended depth of field.

- Alan

[edit:] I forgot to mention that to get the best out of predictive focus it is recommended that you track your moving subject for a little while before shooting, if you can afford to. Half a second to a second seems to be enough.

Also be aware that if you release the focus button fully (whether it be the shutter button or the * or whatever) no matter how briefly then the predictive focus info is scrapped and calculations begin again next time you press it. This is bad if you released it accidentally but is good if you recognized that the camera was tracking the wrong subject (or part of it) and needed to be told to start again.

Always have the starting AF sensor on-target before you begin the AF or else it can track the wrong target.




Feb 23, 2012 at 09:52 AM
abqnmusa
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p.1 #8 · 7D AI-Servo ... What is the Settling Time?


7D focus is quite good in servo mode

use these guides for settings:

Birds as Art 7D guide:
https://store.birdsasart.com/shop/item.aspx?itemid=285

Doug Brown's article:
http://www.deepgreenphotography.com/2009/12/setting-up-your-new-canon-7d/



Feb 24, 2012 at 04:42 PM





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