soft images after recomposing? 24-105

[iphonemaster93]

I'm about to go shoot cars at one of the biggest car events of the year in California and I don't want this event to be a hit or a miss for me, as it only happens once per year and I may not be able to go next year. With the 24-105, I find myself constantly getting soft images when I recompose. I normally zoom all the way in on a specific car part, focus with the single point AF, and then zoom out, take the shot, but sometimes as I'm zooming out, the car part becomes out of focus and the entire car, as well as the photo, becomes soft and sometimes even blurry as well. Am I just doing something wrong, or is that just how the 24-105 is? Because that never happened with my 17-55. I'm starting to think I should test a few where I move the single point AF around, don't zoom in at all, focus, take the shot, and see what happens. The event I'm going to, there will be cars doing burnouts and hard launches and I don't want to come back home with poor quality photos. Thanks! 

[cicopo]

That lens doesn't retain critical focus as you zoom to another focal length. You MUST refocus if you change focal length. Also based on personal experience at the track you must turn IS off if you pan a moving car.

[Waddizzle]

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@cicopo wrote:

That lens doesn't retain critical focus as you zoom to another focal length. You MUST refocus if you change focal length. Also based on personal experience at the track you must turn IS off if you pan a moving car.

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Yes, the IS system can get into a tug-of-war with the AF system, if either the camera or the subject is moving too quickly.  I fight that all the time whenever I'm trying to capture birds in flight. 

I've resigned to the following technique when time is short.  I typically leave the lens in MF mode, and the lens pre-focused to a known distance.  I also have the camera in continous shooting mode.  If I have time to switch to AF mode, then I will do so.  Many times it's point the camera, hold the shutter, and adjust the focus through it's range slightly.  I'll go from out of focus, to in focus, and back out.  I almost always wind up with a keeper by knowing the focus distance of the lens, and the range of adjustment.

Hopefully, cars will not be catching you by surprise, or leaving you with little time to capture an image.  There are different ways to capture images of moving cars, depending upon what type of motion blur you're looking for.

[TCampbell]

You should not "zoom" after focusing with the vast majority of zoom lenses.  

Some lenses are "parfocal" which means that if you focus the lens to any given distance and then zoom, the focus distance remains unchanged (you would not need to re-focus).  VERY VERY FEW photographic lenses on the market are "parfocal" (parfocal zooms are common in the video industry.)

For Canon, the lenses which are truly parfocal include:

• EF 17-40mm f/4L USM
• EF 16-35mm f/2.8L II USM
• EF 70-200mm f/2.8L USM (note, this is the non-IS version.  The IS version is not parfocal.)

There are several other lenses that are nearly parfocal (meaning the focus doesn't change by much... but it does change.)

[ebiggs1]

"I'm about to go shoot cars at one of the biggest car events of the year in California ..."

Can I assume these cars are not moving very fast?  If, yes, than you have all the time you need to get it right.  If it were me, I would use center focus point only.  Turn all others off.  Focus on the main most important thing and make it the main most important thing.  Use One Sot not AI-Servo (unless they are moving fast).

Also you can't use f4 or even f5.6 and expect to have a lot in focus.  Try for f8 or f11.  Use a middle of the road ISO unless you have great light then use ISO 100. 

I am not a fan of the "spray and pray" method of shooting. You really need to know what you are doing and set things correctly.  Spray and pray will fail you eventually.  And if you use RAW, like I do, you don't get many shots before the buffer fills up.  And you are shooting RAW, right?