Nov 24, 2014, 08:38
you're correct Ed, the electronics of the camera operates the lens motor to bring it into focus. Unfortunately for a given motor movement the lens does not end up correctly focused and stops before or after (front/ back focused.) In very rare cases it is the camera body - easily found out if every lens focuses incorectly by the same amount in the same direction.
* Easy enough to do a rough check without fancy stuff.
* Use a largest aperture for minimum D0F;
* suggested to be about 50x distant so with 50mm lens put camera at 2.5m;
* use a target with good contrast and good focus point (for wuick test middle letter of car number plate is not bad);
*the target should be about 45 deg to line of sight;
* use single focus point on camera;
check the resulting image on a PC (camera screen is too small.) the fall off of focus should be even in front and behind the focus point. If ists sharper to the front or back then you may have front of back focus. Its on a lens by lens basis so if you suspect a problem then a little bit more effort and precise target is needed to assess and adjust. Also need to use several exposures to check it wasn't a one-off.
Since I found out about it I have checked every lens I intended to purchase. Saved me grief after the event on 2 occasions. Contrary to what some may imagine I'm not at all obsessive about it - got caught once, and now regard it as part of a comprehensive 'test drive' before I shell out my hard-earned spondoolies - especially as lenses don't often fall into the cheap category and a week's average wage doesn't necessarily buy much of one.
* Easy enough to do a rough check without fancy stuff.
* Use a largest aperture for minimum D0F;
* suggested to be about 50x distant so with 50mm lens put camera at 2.5m;
* use a target with good contrast and good focus point (for wuick test middle letter of car number plate is not bad);
*the target should be about 45 deg to line of sight;
* use single focus point on camera;
check the resulting image on a PC (camera screen is too small.) the fall off of focus should be even in front and behind the focus point. If ists sharper to the front or back then you may have front of back focus. Its on a lens by lens basis so if you suspect a problem then a little bit more effort and precise target is needed to assess and adjust. Also need to use several exposures to check it wasn't a one-off.
Since I found out about it I have checked every lens I intended to purchase. Saved me grief after the event on 2 occasions. Contrary to what some may imagine I'm not at all obsessive about it - got caught once, and now regard it as part of a comprehensive 'test drive' before I shell out my hard-earned spondoolies - especially as lenses don't often fall into the cheap category and a week's average wage doesn't necessarily buy much of one.