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Old Jul 18, 2006, 7:05 PM   #21
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JohnG wrote:
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As NHL indicated - you're always going to lose speed in servo mode - the camera has to 'DO SOMETHING' and that something takes time - so wheras a camera may have a stated 5 fps burst mode - you may only see 4 fps when in servo mode...
Exactly! :idea:

This comes from the fixed 250ms timeout (i.e. 1/250 = 4fps max) in the AF focusing algorithm - I'll find the write-up on this later as this is due to all focusing conditions. i.e. during sunrise or sunset for example when the light are dim and the subjects are not contrasty enough to achieve fast focus and are independent of the lens in use.

Think about it @ 8fps on a 1D MrkII that's only 125ms per frame which is too fast for most AF and why the C.Fn-21 is there built-into the camera. Trust me I use AI servo too, but one-shot AF get me a higher percentage of keepers...

Like I said before, lock yourself in a dark closet and fire away in AI-servo: no single frame will be in focus, but the camera still burst every frame. In 1-shot AF on the contrary the camera has to lock focus or it won't fire at all - Try it!!!
-> In AI-servo the camera will maintain the burst rate over achieving 100% accurate focusing :idea:
(Close enough or the DOF coverage is what most folks need... Something got to give!)


Here's the article: http://photonotes.org/other/ai-servo.html

"In continuous sequence AI Servo AF shooting, shutter release timing for the second and consecutive frames in the sequence is controlled by the EOS 1's autofocusing system. Canon calls this focus priority. However, it is extremely important to note that focus priority shutter release timing does not guarantee sharp focus. It simply means that the EOS 1 camera, rather than the photographer, controls the shutter release timing for these exposures."

"Condition 5. Adequate Subject with Excessive Speed:
There are actually two conditions of this type. The first condition occurs with a subject moving toward the camera, so close or so fast that the required focusing movement cannot be executed within a preset time limit. For USM lenses, the time limit has been set at 200 milliseconds (1/5 second). For non-USM lenses, the time limit is 250 milliseconds (1/4 second). If the subject requires more focusing time than the preset limit, the exposure is taken anyway at a focusing position that is as accurate as possible. However, this type of exposure will be out of focus towards the infinity direction."

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Old Jul 19, 2006, 7:33 AM   #22
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NHL,

Just to clarify though - single shot on a moving target that is changing focal planes will certainly not give you a FASTER burst rate than Servo. This may reduce the rate from 5fps to 3 or 2 or even just 1 (again, the key being the object is changing focal planes - i.e. coming towards or away from you).
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Old Jul 19, 2006, 8:20 AM   #23
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JohnG

Agree - You're absolutely correct!

Basically it's two different shooting styles, and neither is wrong or right:

1. One-shot AF: The 1st shot will always be in focus otherwise the camera won't fire - With subsequent shots you have to release the shutter completely or the camera won't track your subject (this is only critical if the subject is moving to or away from you) - Sideway then it does not matter.
-> Yes the overall frame rate is less but the focus is spot on every time if you let go of the shutter in between shot.
If you don't release the shutter completely then the focus will be locked on to the 1st shot - which is sometime good actually (for wildlife) when a tiny object move through various varying background, but then you get to keep your burst rate up!

2. AI-servo AF: The 1st shot may not in fact be in focus because the camera is on shutter release priority:
"Because the photographer (rather than the autofocusing system) controls the timing of the shutter release, Canon calls this shooting method release priority. The release priority method enables the photographer to shoot when desired, but it does not guarantee sharp focus. In fact, the first frame of a continuous sequence shot in AI Servo AF is likely to be out of focus, especially with a subject moving either toward or away from the EOS 1."
-> The camera relies on subsequent frames to track the subject to and away from the camera, but it could run into the preset time limit that was described earlier if the conditions prevent a lens from achieving focus in time to keep up with the burst rate...

IMO everyone should try both and see which style works best for them... :idea:
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