I have a problem with my Canon 20D’s focusing. It’s been in the local Canon technical service for a few days. They confirm the problem, and they say they checked some of the new bodies they have in stock, and all was acting the same. They say they will investigate what to do and let me know. I’d appreciate if you’d share your thoughts and a possible solution if you know any. (or someone to write to about this at Canon).
The problem is basically this; the focus sensors seems to be offset with regard to the focus marks by about one center focus width to the left.
The camera is about 10 months old. I was getting some back focused photos from time to time, but casual focus tests (like shooting a newspaper on the wall) showed no problems. I was blaming my self for reframing after focus lock and the like. Recently it started to get on my nerves and I looked deeper into the problem.
The problem didn’t occur when the target subject was relatively big compared with the focus marks, say as big as the partial metering circle. It happened when a single focus point is selected and the target is covering a small area , maybe a little bigger then the focus mark, say you are shooting the photo of someone from a distance, and you target your subject’s right eye, so that the center focus square fully covers the eye and the surrounding area. This should mean that even if the sensor size is bigger than the focus mark, most of the sensor is seeing the target and maybe a little is seeing the background, and should focus on the closer subject, which is the targeted subject. NO, it chooses the focus on the background, even if the focus mark is totally on the target and the target has enough detail to focus on.
So I ended up doing the following test to see what was going on.
1-I selected a pencil which I managed to place upright on the table. Adjusted camera to subject distance so that the width of the pencil just filled the width of the center focus mark.
2- First I positioned everything so that the background was just an empty wall.
3- I press the shutter button and the focus locks right on the target filling the focus mark. Good
4- I turn the camera to the right a little so that the target is just outside the center focus mark to its left. I pres the button, and focus locks. Good. Also proves that the sensor is actually bigger than the mark.
5-.Try step 4 with the subject just to the right of the center focus mark. I expected it to react like it did in step 4, since the sensor is bigger. I pres the button. Miss. Try again. Miss. Miss, miss. No good.
6- Try to place the target about one center focus mark width away from the mark to the left (like step 4 with more distance) .Press the button , and it focuses right on.
Conclusion;
1- Center sensor width is about 3 times the width of the center mark.
2- Center focus mark is not centered and only covers the right hand side 1/3 of the sensor.
Test with detailed background to cross check.
1- When the small target is filling the center focus mark; target missed, background is in focus, every time.
2- When target is just to the left of the center mark; focus is locked on target, no back focus. Sometimes the focus is missed, but this is normal I guess because the target is so small, but the success rate is very high.
3- When the target is just to the right of the center mark; focus is missed, background is in focus every time.
4- When target is to the left of the center focus mark about 1 center mark width; Target missed, background in focus, as expected.
I also checked with the camera turned upside down, to eliminate any inconsistency in the results due to light positions. The results are not changed.
I live outside the US and the local Canon people could not help at the moment. I am reluctant to send the camera to the US at the moment. If someone here had a similar problem and could have the camera fixed, I’ll be happy to know.
Thanks for reading.
Regards,
Husnu
