Focus is too long with 3 cm at 90cm , with any of autofocus points selected; setting that exclude false focus ( just a relatively small -but thick- cross, on gray background). Very bad for Canon EF 85mm f1.8 with depth of field of 0.5 cm in this situation. I suspect light having to travel diferent distances to CMOS / autofocus sensors. Same, but less noticeable with EFS 18-55mm. How can i increase the lenght that light travel to autofocus sensors?
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Re: Canon 400d constant backfocus
Backfocus problems will be magnified when using wide aperture lens with shallow depth of field, nice lens btw. Camera's adjustments for manual and autofocus are inside mirror cage behind mirror, and you dont want to risk getting dust on sensor or damaging sensor. Qualified camera repair shop or sending to canon for adjustment would be my suggestion.
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The 400D uses flash assisted focusing. In low light conditions the auto focus cannot get a focus lock so it pops the flash a few times so the lens has a chance to focus. You will notice this will go away when in the manual focus mode (the switch on the side of the lens).
The only way around this problem (as people in a photo think the photo has already been taken and looks away when the camera actually takes the picture) is to use a hotshoe mounted TTL Canon Speedlight (430ex or 580ex) which use an infra-red focussing beam.
I had the same problem with my 400D. It refused to focus. On manual focus I could take pictures though, but not on Auto focus. What happened was I cleaned the mirror with a piece of soft cloth, the spring hooked onto the cloth and came loose.
I took my lens off, checked the mirror, found that the little spring connecting the mirror and the little glass part above the mirror was loose. Took a lot of patience and hours to get it back in. I used a pair of tweezers 2 x toothpicks and a short piece of fishing line (thin line). Go have a look at another 400D how the spring should be clipped in. I used the fishing line to hook the " L " shape of the spring back in after I lined the back part of spring up, since there is no space next to the mirror and the body of the camera.
Like I said. This takes a lot of patience and be prepared to sit for 3 to 4 hours.
(facing the LCD screen towards you, as if you're going to take a photo) there's a button in the upper right-hand corner that will allow you to AF to a certain point (9-pts total) or the center, or all 9-points, maybe you have it set to one point and it's off center?? definitely sounds like a camera setting probelm just a thought
I have never heard anyone mention a backfocus problem with this lens. In my experience, most of the backfocus issues that I have worked with to help people resolve have panned out as user error and/or a misunderstanding of the AF modes and performance of the camera. You sound like you know what's up, so that may not apply to you...you may have a real issue. The best thing for you to do before proceding to having is service is test the lens thoroughly on both bodies to see if this is a consistent, reproducable problem or just an anomaly.
This is a Digital SLR
With SLR cameras you can only use the viewfinder
because the image comesi nto the lens and up to the viewfinder via a mirror.
When to take a pic the mirror flips up[usually with a crash] and the image goes to the CCD-
afterwards you will get an image briefly in the LCD.