-=HOW TO=- Test if your 10D is back focussing.

[Jesper]

I noticed my 10D has a very slight back focus, so I did The Ruler Test. I made one shot with auto-focus, using the center focus point, and one with manual focus. For both shots I focused on the "20" on the ruler (the number was exactly in the center AF square). This is what I got:

http://home.hccnet.nl/jesper.de.jong/images/backfocus.jpg

Clearly, with AF, the "19" is more in focus than the "20", while with manual focus, I was able to get the focus exactly on the "20". :(

What can I do about this? Can Canon re-calibrate the focus to make it exactly accurate? Did you have this problem and did you bring it to Canon, and did they repair it well?

[msvadi]

what lens did you use? what was the distance to the target? was the lighting okay? I was getting frontfocusing with my DRebel but it was more due to the bad light and target. Remember that AF sensor area is biger than AF points you see in the frame, so with the target like yours AF can easily choose not what you want.

Try to paint 20 line in red, make sure you have enough light and that you are not too far from the target and repeat the test.

[Scottes]

You're actually better off by putting a flat black piece of cardboard or something parallel to the sensor plane and focusing on that. Put the ruler next to that cardboard at a 45-degree angle to the sensor plane, and line the "20" up to the edge of the cardboard.

Now you'll get true focus on the cardboard, and then use the ruler to check focus.

Trying to focus on a line on the ruler is not very accurate.

[Jesper]

msvadi, I used my 17-40L at f/4, 40mm. The light was good, sunlight, with a shutter speed of 1/500 second. So bad light is not the cause of the problem. The target was quite close, maybe 1.5 feet, the camera was on a tripod and I used mirror lock-up and the self-timer.

Scottes, thanks, I'll try that...

[msvadi]

I still think that there are too many lines too close to each other. This may confuse the AF. Check this http://photography-on-the.net/forum/showthread.php?t=32115&highlight=front+focusing I think that Tom W setup is worth trying.

[defordphoto]

You're actually better off by putting a flat black piece of cardboard or something parallel to the sensor plane and focusing on that. Put the ruler next to that cardboard at a 45-degree angle to the sensor plane, and line the "20" up to the edge of the cardboard.

Now you'll get true focus on the cardboard, and then use the ruler to check focus.

Trying to focus on a line on the ruler is not very accurate.

Very, very true!! This type of ruler test is highly inaccurate.

[Jesper]

OK..... I tried Bob Atkins' focus test (http://www.photo.net/learn/focustest/).

With this test, the focus is exactly spot-on with my 17-40L at f/4! :)
No need to have my 10D checked by Canon.

Thanks for the info....

[Tom W]

Here's the setup I used to provide myself with the peace of mind necessary to keep shooting and stop worrying:

http://home.comcast.net/~trwilk3/Images/50_14_focus_small.jpg

Its kind of a combination of Bob Atkins' chart and Pekka's methodology. Using a separate object for focus keeps your focus sensors from locking on something other than what you're shooting.

Here's a retouched copy of the focus sensor size/shape compared to the squares in the viewfinder. Note that the actual sensor size is in red (this is from either Chuck Westfall of Canon or Bob Atkins' web site, I don't remember):

http://home.comcast.net/~trwilk3/Images/Focus_Point_Size.jpg

As you can see, the sensors are far bigger than the indicator squares (this is common BTW). If any part of an active sensor catches a high-contrast object, it will grab focus. That is why I used a separate object, since the camera could conceivably grab focus on any mark on a ruler.

[msvadi]

OK..... I tried Bob Atkins' focus test (http://www.photo.net/learn/focustest/).

With this test, the focus is exactly spot-on with my 17-40L at f/4! :)
No need to have my 10D checked by Canon.

Thanks for the info....

Excellent!

[JZaun]

Thanks to all of you for the replies on this thread. With all the discussion on 10D focus issues, I was wondering if I had a problem :? Just wondering. This thread led me to the Adkins test. I set it up and Like Jasper, my 10D is fine. Not only that I tested all me lens. 100-400, ( at different lengths) 100mm macro and 24-85. None were more than 1-2 division off center!!! Now I have a easy test in case I get paranoid again :D :D :D :D :D

JZ

[CyberDyneSystems]

We have a number of threads on this.. but this thread in particular looks to be a great place to start a Sticky..

Jesper.. would you mind if I edit the title... slightly to a how to sort of title?

[Jesper]

We have a number of threads on this.. but this thread in particular looks to be a great place to start a Sticky..

Jesper.. would you mind if I edit the title... slightly to a how to sort of title?
No, that's OK with me ofcourse. Good to have a sticky topic about this. Probably all the talk about focus problems with the 10D is because too many people like me are doing a "ruler test", without understanding that it's not an accurate way to test the AF.