Autofocus problems

[RickD]

Been doing some troubleshooting on different scenarios on why my lens cannot focus.

For some reason why both lenses (Tamron 28-70mm f/2.8 and Nikon 17-55mm f/2.8) cannot auto focus on the green back ground, but if I move focus to the other objects, the camera auto focus' without any problem.

Settings @

Aperture Priority
1/10 @ f/2.8
ISO 400

Any explanation to this?

See attached.

[dminton]

The wall is not contrasty (is that a word?) enough. There is not enough contrast difference on the wall for the autofocus sensors to detect. the guitar case and the box have several light/dark places all over them that the AF sensor can "see" and use to judge focus. The wall is too flat in a light/dark difference way. Same reason that most cameras have a hard time focusing on a clear blue sky.

[srwatters]

David is right. In these difficult low contrast conditions, your best bet is to choose the center focus point and look for the highest contrast area in the same focal plane to lock onto. Of course there is always the good old tried and true method using your own eyes and the manual focus ring.

And BTW, at 50mm and 1/10 shutter, you'll be lucky to get a sharp image if you're hand holding. You might want to keep that in mind when doing this kind of test.

Have fun!

[Sentinel Chicken]

So I take it that the AF sensor looks at how sharp the contrast is at the target area? I've wondered about this ever since I noticed that some airline color schemes are easier to autofocus on when it's cloudy versus others.

[RickD]

Thanks. I understand now.

[toverman]

You might even try manual focus when a scene lacks contrast (or enough light) to easily autofocus. If your subject isn't moving, it's an easy technique to master.

[RickD]

I typically go with manual focus when the lens cannot focus. I just wanted a more technical answer to the issue.