Is my setup backfocusing?

jm94085

Hey friends...so I've been having this feeling for a while that something is off with my AF system and tried a quick down and dirty test and was hoping to get some feedback as to how bad you interpret what I think I'm seeing (you all seem to have better eyes for this than I do).

Anyway the setup is a Canon 40D with a 24-70 2.8L lens. I shot two series of photos the first at f/2.8 @ 24, 35, 50 & 70mm...the second set at f/5.6 @ 24, 35, 50 & 70mm.

The focus point is on the center (chocolate) box, right in between the two "LL"s of the word "JELLO". The boxes to the right are stepping away and the ones to the left are stepping closer.

Distance from the sensor to the focal point is about 3 feet.

I appreciate your feedback!!

1. f/2.8 @ 24mm



2. f/2.8 @ 34mm



3. f/2.8 @ 50mm



4. f/2.8 @ 70mm



5. f/5.6 @ 24mm



6. f/5.6 @ 34mm



7. f/5.6 @ 50mm



8. f/5.6 @ 70mm




Thanks,
John

ziggy53

I suggest that you repeat the process with more separation of the subjects. It is fairly easy for the AF to get confused regarding the desired subject when the surrounding objects are too close.

If these were shot at 3', that is the closest that you can get reliable AF (IMO), so you "done good". (The EF 24-70mm, f2.8L USM, MFD is around 1.25' so you were around twice MFD.)

Since the Canon 40D has a Live View function, I also suggest performing AF normally, switch the lens to manual mode, and then engage live view to confirm critical focus. You get rather instant results that way (without even having to capture an image).

Also try the AF on a more distant object to see if the problem is confined to closer focussed objects. (My EF 135mm, f2L USM is rather bad at close focus AF, for instance.)

To directly answer your question, yes, these images do seem to show back focus, moreso the wide angle images. I would still suggest doing the above things to try to resolve what is really going on.

jm94085

Hi Ziggy...thanks I will retry with your suggestions and see what I come up with.

Also with regard to your suggestion of adding more separation to the objects in the photos...do you mean adding more depth, width or both?

(also, it should be noted that I was shooting with a single center AF point)

Cheers,
John

Scott_Quier

You might also try some thing like this...

If offers a good, clean, no-guessing focus target and then the ruler (which is lined up with the vertical board) is there to help you decide which way the camera/lens combination is focusing.





you might also find useful information in this post and in this post.

jm94085

Very cool...thanks Scott. Man that is one heck of a setup in "this post" :-)

ziggy53

jm94085 wrote:

Hi Ziggy...thanks I will retry with your suggestions and see what I come up with.

Also with regard to your suggestion of adding more separation to the objects in the photos...do you mean adding more depth, width or both?

(also, it should be noted that I was shooting with a single center AF point)

Cheers,
John

Mostly add more separation and space width wise. The center focus "dot" is really more ethereal and nebulous, with a region surrounding the dot of decreased significance the further it gets from the center. The better you define the subject, the better the AF section can be at discriminating subject from background. As you zoomed in, the effective subject became larger and more significant in the view and the longer focal length tests do seem more accurate, possibly as a result.

Manfr3d

Hard to tell ... because I can't take of my eyes off the vanilla jell-o! :eat