autofocus help

[Terry McDaniel]

I recently saw an article on the the autofocus screen (not sure of correct terminology) can get knocked out of adjustment, and how to fix it, but can no longer find the article. May 7Ti just doesn't seem to focus as sharply as it used to, I know it's been dropped a time or two. So anyway, can it be adjusted by folks of less than professional skills, or better left to a pro?

 

[Wilt]

There could be TWO possible misalignment issues resulting in AF being wrong...

1. the supplementary AF reflex mirror which is mounted on the back of the main reflex mirror (which aims light to focus screen of viewfinder)
2. the AF sensor mounted near the bottom of the camera, which sees what the supplmentary AF reflex mirror bounces downward

Better let a professional do it.

 

[Capn Jack]

I doubt the problem is the focusing screen because that is just below the pentaprism, problems there affects what you see, but not what the AF system sees.

Another possibility I haven't heard mentioned anywhere is the main mirror being dirty. A DSLR camera mirror is a really a beam splitter. When composing a picture, most of the light goes to the eyepiece while a small amount goes to the autofocus system via a secondary mirror. If the main mirror gets dirty, it blocks light to the autofocus system and can cause it to miss focus, or not focus at all, under low light conditions. The main mirror can be carefully cleaned by a user. I had this problem and was able to fix it myself with an optical alcohol wipe. The main mirror intercepts most of the dirt that would otherwise get on the main and autofocus sensors and the secondary mirror.

See this link for a diagram showing the mirrors and autofocus system: https://photographylife.com/how-phase-detection-autofocus-works
The author didn't call out the focusing screen, but it is drawn just under the pentaprism.

@Wilt forgot a couple more alignment issues. One is the sensor itself. The AF and main sensors both have to be the correct distance and aligned with the lens or you will have focusing issues. If you are only using one lens, that lens may have at least one element out of alignment.

 

[SkedAddled]

Jack presents valid concerns, while it really could be a simple misalignment/adjustment of a focus screen.
In either case, professional service can determine the issue, while I have corrected the same issue
with a decent focus scale.

I put split-prism focusing screens in my EOS Rebel XTi, EOS 40D and EOS 50D.
The XTi gave no issue with proper focus, nor did the 40D. The 50D had issues
with the screen from the 40D, and that was resolved with a proper focusing chart
and proper shim placement.

Food for thought...

 

[Terry McDaniel]

Thanks for all the replies.

 

[gjl711]

Is the camera missing focus on your images? Does the image in the viewfinder look out of focus? THe two are different and have a different light path through the body. It would help to know why you are concerned.