Back Focus?

hi jdouglas,

in my limited experience, it is nearly impossible to evaluate back/front focus from action shots, because too much can be going wrong. I have lots that are much farther off than yours, but when I did two different types of serious back/front focus tests on my lenses and body, 4 of my 5 lenses were spot on, and one was only a very small amount off. I think the only way to determine whether this particular combination has a back focus issue is to go through the normal tests, with a tripod and an appropriate target an appropriate distance from the lens. A bit of a pain, but I don't think there is really any alternative.

Here is an image where it is easy to see the front to back focus. It is also easy to see how narrow the depth of field is. For action pictures where i want to capture more than one subject I really try to make sure I have a wide enough depth of field. could that be what happened here?

Mike

Its certainly true that the DoF is thin and a smaller aperture would have produced a better shot for the OP.

There doesn't seem to be a lot of information with this photo and for some reason my EXIF viewer isn't working. Just curious of a few things. 1.) Is this a crop of the image or is this the entire image? 2.) Is this shot wide-open at f3.8 or did you stop down to f3.5 for much greater sharpness? 3.) Do you have any filters on the lens that may be degrading IQ? 4.) Have you done any micro focus adjustment? 5.) Are you shooting with the OS on? If so, with number 1 or 2? 6.) Were you panning at the time of the shot? 7.) Have you done any controlled tests on stationary objects to rule out camera shake? 8.) Were you shooting with a shutter speed high enough to prevent subject blur?

We really need a lot more information here to help. It's always hard to nail down a problem from a single shot with so many variables. When troubleshooting it is always necessary to eliminate as many variables as possible.

Here's the EXIF I'm getting:
Camera Maker: Canon
Camera Model: Canon EOS 7D
Lens: 120-300mm
Image Date: 2000-01-01 00:00:03 +0000
Focal Length: 274mm
Aperture: f/3.2
Exposure Time: 0.0010 s (1/1000)
ISO equiv: 100
Exposure Bias: none
Metering Mode: Spot
Exposure: Manual
Exposure Mode: Manual
White Balance: Auto
Flash Fired: No (enforced)
Orientation: Normal
Color Space: sRGB
GPS Coordinate: undefined, undefined
Photographer: Jeffrey Douglas for Moving Exposure Photography
Copyright: Moving Exposure Photography
Software: Adobe Photoshop CS5 Macintosh

jdouglas003 wrote in post #15177357
Trying to figure out what's going on with 7D and Sigma 120-300 combo. Is it the camera or lens?

Not getting the results I'd hoped for.

Can anyone tell me if this is a back focus issue. Fosus point is right on the hip of the girl in the pink about to kick the ball.

Both of the other girls seem to be sharper though.

It's possible that the camera focused on the girl behind the kicking girl. The actual focus sensors cover a significantly larger area than the focus square in the viewfinder (and as displayed in DPP). It's possible for a focus sensor to lock on a high contrast edge well outside the focus square.

stsva wrote in post #15179187
It's possible that the camera focused on the girl behind the kicking girl. The actual focus sensors cover a significantly larger area than the focus square in the viewfinder (and as displayed in DPP). It's possible for a focus sensor to lock on a high contrast edge well outside the focus square.

Here is some more detailed info on this.
http://blogs.stonestep​s.ca/showpost.aspx?pid​=54

Aside from that, most of the EXIF looks good. I don't know if I would be shooting action at f3.2. At 50 ft you only have just over a foot to work with. If they are closer it gets even narrower. When I shoot action I generally shoot at f4 or f5.6 and if I can't get the shutter speed I need for that I will bump the ISO to 320.

I would get it into some controlled tests but if you can't do that, give the smaller aperture a try and see if that fixes it.

kfreels wrote in post #15179537
I don't know if I would be shooting action at f3.2. At 50 ft you only have just over a foot to work with. If they are closer it gets even narrower.

Totally agree, its a no no. The DoF should at the very least be deep enough to cover the group. Instead its so shallow that there is no room for error if indeed the AF point did indeed pick up a higher contrast on the second figure (which is very possible).