+ or - microadjust this image?

[BenPonti]

Still trying to achieve perfect focus! Took this image this morning in slight shade but still fairly bright conditions. I notice looking at it that the sharpest focus seems to be on the two mealworms below the robin. I have marked the position of the spot focus. If I'm right, does this suggest that the lens is front focusing? If so, do I + or - microadjust and how much would the experts suggest might be appropriate. Same set up as before, 50D with 100-400mm lens.
Iso 800
f7.1
sh. 160
http://img.photobucket.com/albums/v115/BenPonti/IMG_4286.jpg

[Daniel Cadieux]

Hi Ben, I can understand your concerns...but to determine front or back focus you should do some tests on static objects and in controlled environments. There are so many variables to can come into play with a moving subject such as your Robin that you cannot be sure if the issue here is equipment or simply operator error.

Check out Artie's latest blog entry for an example on how to check for lens sharpness:

http://www.birdsasart-blog.com/

[BenPonti]

Thanks for that. I'd seen and tried the first test with reasonable success but what I can't find out anywhere is clearly explaining whether you need + or - for front focusing for example and how much to start off trying. Do you know?

[Andy Wai]

[...] what I can't find out anywhere is clearly explaining whether you need + or - for front focusing for example and how much to start off trying. Do you know?
You might want to check out the Chuck Westfall method below:

http://digitaljournalist.org/issue0812/tech-tips.html

Because it's based on comparing three sets of results, the need to remember which way's which is removed...

By the way, the test target should be flat and perpendicular to the camera as much as possible. Commercial tools like LenAlign goes to great length to ensure these. With a real world 3D subject, the AF system might do complex and unpredictable things. Aligning with one subject might completely screw up all others.

Andy

[BenPonti]

Thank you Andy, I'll have a go at that tomorrow, weather permitting!