AI Servo tips, or focusing on walking subjects on 5Ds and 70D

[randomuser74]

I have some older equipment, but it should be fine for this low-key backyard wedding I'm shooting. Canon 70D and 5Ds both are misfocusing in AI Servo mode when doing mock trials of walking-down-the-aisle shots. In One Shot mode with still subjects, things look sharp, so I know it's not a lens calibration/micro-focus adjustment issue. I have a Canon EF24-105mm F4 L IS USM lens, and the minimum aperture is 4 so it's not really a narrow focal plane problem. Shutter speed is 1/250 minimum, but mostly much higher. I use a single AF point and place it directly on the subject's face and try to keep it locked on as they walk. Zone AF points didn't seem to work as well. I'm back-button focusing, holding down the AF-ON button continuously and trying to fire off 8-10 shots. Most are pretty soft and not as sharp as One Shot, and some are downright blurry. I've tried Live View, but the lag is so terrible that I only get 1 or 2 shots and the walk is done, and those are blurry as well. These cameras don't have eye tracking. I've done a handful of weddings with older DSLRs and film cameras and never really had this issue, or maybe resolution was just too low to notice. Honestly, my phone camera does a better job for this one scenario! There must be some technique I'm missing. What other tips can I try or is there some setting holding me back?  I have it set to focus priority.

[Waddizzle]

"If the subject is moving, then you need to use Servo.  Up the shutter speed to at least 1/800 of a second.

I totally, 100% disagree with that advice.

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We know you do.  It shows. Your action photos are always soft.

When a subject is moving towards the camera, using One Shot creates a small time delay between when the camera lock’s focus and when the user fires the shutter.  It’s enough of a time delay to allow the subject to move out of the focus plane.

[Waddizzle]

randomuser74,

The “secret” to capturing sharp images is to capture as much light as possible (wide aperture), as quickly as possible (fast shutter speed), with as little effort as possible (low ISO).

You will invariably have to compromise on one of those “rules” during difficult shooting conditions.

I also suggest prioritizing them in that order. 

[ebiggs1]

"It’s enough of a time delay to allow the subject to move out of the focus plane."

Nonsense and why the OP got blurry images because he did use your method. I do hope he is smart enough to see the difference and ignore your advice.