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Thread started 23 Jul 2008 (Wednesday) 16:22
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Chink in * focus armor?

 
LeeSC
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Jul 23, 2008 16:22 |  #1

I started * focusing awhile back when I had an XT. I absolutely loved it as I tend to be "heavy fingered" when it came to the shutter release button!

Anyway, I was trying to get some shots of my daugther yesterday with my 30D. She was on a swing pretty much coming straight/ slightly angled away from me. I pretty much figuired that one shot AF wasn't going to cut it so I switched to AI Servo. I got home and found about 80% of the shots OOF.

My only thoughts are that even with AI servo the focusing distance difference was so vast between AF capture and shutter release that there was very little chance of getting an in focus shot.

I guess in shooting circumstances like these returning the AF to the shutter release is probably going to be the best option.

Anyone with a similar problem or any ideas?


GEAR

  
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FlyingPhotog
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Jul 23, 2008 16:26 |  #2

Need a little more info:
- Amount/Quality of Light
- ISO
- Shutter Speed(s)

Are you sure it's a focus issue and not motion blur?
Did you try any shots by pre-focusing at the apex of the arc and shoot at the change of direction (top of arc but not quite moving back the other way...)?

If you post up a couple of examples (with the EXIF info intact) folks might be able to do a better disection.


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Yohan ­ Pamudji
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Jul 23, 2008 16:26 |  #3

The camera keeps focusing as long as you have the * button pressed, so unless you let go of it right before you hit the shutter release it shouldn't matter that this functionality is decoupled from the shutter release button. A kid on a swing is a challenging subject period. You could do experiments with and without * button focus, but I doubt you'd get substantially different results.




  
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gooble
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Jul 23, 2008 16:28 |  #4

I don't see how * focusing would have made this worse. Your're in AI servo so you hold down the * button and the use the shutter when needed, one at time or in high speed drive mode.

This may be more a function of the AF systems lack of ability to track properly due to it's natural limitations and the fact that a person on a swing moving quickly in and out again is hard to keep up with for any camera.

As for my 20D, it had trouble tracking anything that was moving mostly directly towards or directly away from me but lateral movements it could do pretty well.

My 40D however performed quite well when I shot a Great Egret flying straight towards me. Out of my 17 shot burst only the very last one was OOF.

Edit: forgot to add that I think the focusing algorithms do some predictive work when something is either getting closer or farther away. It sees that the subject is getting closer and assumes it will get even closer still so it knows what directions to keep moving. However, a swinger is moving in and out rapidly and as soon as the camera thinks it knows where the subject will be it changes direction.




  
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LeeSC
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Jul 23, 2008 16:35 as a reply to  @ gooble's post |  #5

My thinking was that since there was a 6 foot or so swing radius was that the AI Servo just couldn't keep accurate focus over that span of distance.

Let me fire my laptop up and I will post some pics with exif.


GEAR

  
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gooble
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Jul 23, 2008 16:43 |  #6

LeeSC wrote in post #5970469 (external link)
My thinking was that since there was a 6 foot or so swing radius was that the AI Servo just couldn't keep accurate focus over that span of distance.

Let me fire my laptop up and I will post some pics with exif.

I don't know if the distance has anything to do with it. It would be speed with which the swinger changes direction that would cause problems. If it only moved a few inches per second I don't think the camera would have any problem keeping up but I don't think speeds of 5-10 feet per second are unreasonable for a swing (depending on the size and age of the rider) and that's what the camera can't match.




  
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diredesire
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Jul 23, 2008 17:59 as a reply to  @ gooble's post |  #7

Yep, the speed of swing/lens was probably the culprit. The * focus and servo is continually tracking, if you don't let go of the * button at all, theoretically speaking you should have captured your image. I think it'd be tough to nail the shot every time either way. You may be better off manually focusing and shooting off a burst. You might even increase your keeper rate that way. In any case, it's not ideal.


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LeeSC
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Jul 23, 2008 18:29 as a reply to  @ diredesire's post |  #8

OK guys, here is what most of the "off" shots look like:

1/400
f/ 3.5
ISO 200

85 1.8 on a 30D

IMAGE NOT FOUND
IMAGE IS A REDIRECT OR MISSING!
HTTP response: NOT FOUND | MIME changed to 'image/png'


But, this one made up for all the bad ones!

IMAGE NOT FOUND
IMAGE IS A REDIRECT OR MISSING!
HTTP response: NOT FOUND | MIME changed to 'image/png'

GEAR

  
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LeeSC
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Jul 23, 2008 18:38 as a reply to  @ LeeSC's post |  #9

And to be quite honest, I can't say with 100% certainty that I was keeping the * button depressed fully while shooting.

As these are very difficult shooting circumstances, I am inclined to cite "operator error"!

Pressing the * AF then pressing the shutter release in single shot could obviously result in some misfocuses in a moving subject due to the delay between focus time and actuation. I guess this would not be the issue in AI Servo as the focus is constant.


GEAR

  
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tonylong
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Jul 23, 2008 18:39 |  #10

I'd say your shutter speed was way too slow for the motion. If you upped your ISO and upped your shutter speed I imagine you'd have a whole lot more keepers. I try to use a shutter speed of at least 1/1000s or faster for moving objects. Whenever bad light forces me to go slower I assume that I'll lose some.


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gooble
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Jul 23, 2008 18:46 |  #11

That one was nice and sharp with a cute pose though.




  
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DDCSD
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Jul 23, 2008 18:51 |  #12

I'm guessing a big part of the problem was that she was on a swing. The AF is tracking and predicting, and then all of a sudden she is going the completely opposite direction. Was she swinging very fast?

This does not look at all like motion blur to me.

Unless you are letting go of the * button, it makes no difference which button you are using to AF, the camera will act the same.


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JCH77Yanks
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Jul 23, 2008 18:54 |  #13

gooble wrote in post #5971249 (external link)
That one was nice and sharp with a cute pose though.

I agree - you got yourself a winner!


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RTMiller
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Jul 24, 2008 06:26 |  #14

For what it's worth, I had the same issue last week while shooting a little girl on a rope swing. Most shots were OOF.

DDCSD wrote in post #5971270 (external link)
I'm guessing a big part of the problem was that she was on a swing. The AF is tracking and predicting, and then all of a sudden she is going the completely opposite direction.

I tend to agree that the camera cannot predict the sudden change in direction. Good thing that people on swings is a very portion of what I shoot. ;)



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Chink in * focus armor?
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