does continuous zooming affect auto focus?


Mustank

New Member
Aug 10, 2008
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#1
does continuous zooming affect auto focus?

I photograph a kid running towards me using 70-200 f4 is usm and 40d on ai servo and high speed continous shooting

While the kid is running towards me, I zoom out. My keeper rate is a low 10-20%.

If I use 17-55, also zoom out when subject approaches, my keeper rate is good 80%.

does continuous zooming affect auto focus?

What can I do to up my keeper rate with the 70-200?
 

Mustank

New Member
Aug 10, 2008
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#2
Indoor in school hall. Body on f4, 1/200, iso800 throughout

3 flash guns were used.
 

Rashkae

Senior Member
Nov 28, 2005
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#3
There's a huge difference in the necessary shutter speed to avoid motion blur between 50 and 200mm. Also, different lenses will have different focus characteristics.
 

Mustank

New Member
Aug 10, 2008
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#5
The non keepers don't look like motion blur, look like oof shots.

The shots that can be kept dont have motion blur
 

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Mustank

New Member
Aug 10, 2008
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#6
Somewhat Can, 1/64 at 24mm. Direct at subject. Fire continous 5-10 shots, pause for 10 secs, then wait for next kid to run. About 10% flash didn't fire

When using 70-200, flash set to ettl, 40% didn't fire
 

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Mustank

New Member
Aug 10, 2008
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#7
Next time I don keh kiang, just use 17-55 since flash's range not high

Damn wasted
 

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Shizuma

Senior Member
Mar 19, 2012
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#8
you might experience zoom-blur if your shutter speed is low enough that the continuous zoom (i assume you are still zooming when shutter is open) affects the image.
this is evidenced if you see any tell-tale streaks converging on one point in your images
 

Octarine

Moderator
Staff member
Jan 3, 2008
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Pasir Ris
#9
TS: you might want to read up about Parfocal lens and Varifocal lens
A parfocal lens does not require any changes in the focus system, therefore the chances for keepers are higher. But that's only one factor and might not be the root cause.
 

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Mustank

New Member
Aug 10, 2008
253
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#10
Today I brought the 70-200 f4 to use in swimming pool of my kids running around
Av mode: f4, iso 100. Sunny day so shutter range from 1/1000 or more
Ai servo, fast continous shooting, only center focus pt
No flash
Pass to my wife to use, she don't know camera stuff. Gave her short briefing, then let her own time own target
98% of oictures focus tack sharp!

Suddenly dawn on me that the sheer amount of today's sunny sunlight coming into the camera greatly assisted the auto focus. Now I realize why in the school hall, the f2.8 lens focus much better than the f4

I then go and read more. One guy wrote this:
LensRentals.com - How Autofocus (Often) Works

The Effect of Lens Aperture
No matter what the sensor type, however, it will usually be more accurate with a wider aperture lens. Remember, during autofocus the camera automatically opens the lens to its widest aperture, only closing it down to the aperture you’ve chosen just before the shutter curtain opens. Phase detection autofocus is more accurate when the light beams are entering from a wider angle. In the schematic below beams from an f/2.8 lens (blue) would enter at a wider angle than those of an f/4 lens (red), which are still wider than an f/5.6 lens (yellow). By f/8, only the most accurate sensors (usually only the center point on the more expensive bodies) can function at all, but even then focus may be slow and inaccurate. This is the reason our f/5.6 lenses stop autofocusing when we try to add a teleconverter, which changes them to f/8 or f/11 lenses.
Should have bought the 70-200 f2.8 ii:dunno:
 

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