My Xsi is back focusing in low light conditions, is this normal?

Yesterday I was shooting in a room that had low/yellowish lighting and found my shots to be out of focus. So I did the battery test in the low lighted room and I can confirm it is back focusing.

However when I took shots and did the battery test in a brightly lighted room that had whitish light, the focus was spot on.

Is this normal? I didn't feel like the lighting was that low. It was just sort of yellowish and not as bright because of the incadescent bulb used in that room.

What lens?

SolidxSnake wrote in post #6156888
What lens?

the EF-S 18-55 IS kit lens that came with my XSi

I don't think that lens is the best focusing lens out there (it's a f/3.5 lens, the sensors could really benefit from an f/2.8 or wider lens).

I did some more battery tests.

1. With the focal length at 18mm, max aperture, yellow/warm/low light, batteries 2ft from the lens : It will back focus.

2. With the focal length at 18mm, max aperture, yellow/warm/low light, batteries 1ft from the lens: It will focus spot on.

So why is it back focusing in case #1 ? I made sure to space the batteries far enough so the center auto focus point won't pick up the adjacent batteries.

well gary, willis says, "say what? let's see some sample images!"

Hi Gary,

Did you do this test on a tripod or handheld? Did you take into account, regarding the spacing of the batteries, that the actual focus point is 3X bigger in all directions than the AF light in the view finder indicates? What were your shutter speeds?

Kind regards, Wim

picturecrazy wrote in post #6158331
well gary, willis says, "say what? let's see some sample images!"

I did the tests before I went to work an hour ago. I will post them when I get home.

wimg wrote in post #6158373
Hi Gary,

Did you do this test on a tripod or handheld? Did you take into account, regarding the spacing of the batteries, that the actual focus point is 3X bigger in all directions than the AF light in the view finder indicates? What were your shutter speeds?

Kind regards, Wim

I used neither. I let the camera sit on top of the table along with the batteries. I don't remember the shutter speeds because I used Av mode to set the aperture at the lowest setting to get the shallowest DoF, so the camera took care of the shutter speeds for me.

picturecrazy wrote in post #6158331
well gary, willis says, "say what? let's see some sample images!"

What chu talkin' bout Willis?

I pity the fool with a back focussin' XSi!

I did some tests again today. I used 3 batteries in the for the center focus to make sure AF won't stray to somewhere else. It seems focus on the center batteries again, but there is still slight back focus.

Here is the testing configuration

HTTP response: NOT FOUND | MIME changed to 'image/gif' | Redirected to error image by FLICKR

HTTP response: NOT FOUND | MIME changed to 'image/gif' | Redirected to error image by FLICKR

garycoleman wrote in post #6162733
I did some tests again today. I used 3 batteries in the for the center focus to make sure AF won't stray to somewhere else. It seems focus on the center batteries again, but there is still slight back focus.

Here is the testing configuration


As you can see here the center batteries are in focus now. But the rear battery is more focused than the front battery. Aren't they supposedly to be equally out of focus?


I took 10 shots and that's how all they look like. I manually adjusted the camera out of focus and then let the camera auto focus for every shot. So does anyone think this slight back focus is ok?

No, as a rule of thumb, the focal point is 1/3 into the DoF. Which means, that there will be more DoF behind the subject instead of in front of it.

BTW, it looks just fine to me. No back/front focus issues that my eye can see. Also, if it hasn't been mentioned yet, the lens could be backfocusing (if there is a focus problem anyway) and not the camera.

battery in the back looks like it has more light.

SolidxSnake wrote in post #6162752
No, as a rule of thumb, the focal point is 1/3 into the DoF. Which means, that there will be more DoF behind the subject instead of in front of it.

BTW, it looks just fine to me. No back/front focus issues that my eye can see. Also, if it hasn't been mentioned yet, the lens could be backfocusing (if there is a focus problem anyway) and not the camera.

thanks, i guess eveything is fine after all

I was just trying to find any flaws with my camera before the 14 day return/exchange policy is up

garycoleman wrote in post #6162799
thanks, i guess eveything is fine after all

Looks like it to me, too. Enjoy your new Xsi...

-js

garycoleman wrote in post #6162799
thanks, i guess eveything is fine after all

I was just trying to find any flaws with my camera before the 14 day return/exchange policy is up

Hi Gary,

As others indicated, this is perfect focus. Back focus and/or front focus means that the actual focusing point is not in focus, IOW, if you focused on the middle battery, and it would not be in focus, but either the batteries closer towards you (front focus), or the batteries further away from you (back focus), you would have a problem.

I am glad everything is ok after all.

Kind regards, Wim