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Camera orientation can make a difference in AF

ericbowles ericbowles

Is from: Atlanta, US
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ericbowles Moderator Awarded for his in-depth knowledge and high level skills in various areas, especially Landsc... Writer Ribbon awarded for for his article contributions to the community Donor Ribbon awarded for his very generous support to the Fundraising Campaign 2015 Ribbon awarded as a member who has gone beyond technical knowledge to show mastery of the art a Nikonian since 25th Nov 2005
Tue 15-Jan-19 03:55 PM | edited Tue 15-Jan-19 04:04 PM by ericbowles
I was at a museum this past weekend for some casual images using the Z6 and 24-70 f/4 S lens. Flash and tripods were prohibited.

One of my tests was just checking auto focus on a wall that had a strong vertical target. I had heard that there might be problems with focus - and those problems were confirmed.

The test was quite simple. Using the center AF point set on Single area mode, I tried to focus on the edge of this panel. There was a strong edge - something that normally would have made an easy target. After three good attempts, I found that focus was failing every time with this target.

Here is the scene. You can see the AF sensor outline.

Click on image to view larger version


Here is a 100% crop of the target area. You can see that it's very soft and out of focus. This was not a one time miss - it was attempted three consecutive times with all the time time necessary to try to achieve good focus. It failed every time regardless of what I did.
Click on image to view larger version


So I turned the camera to a horizontal orientation. I got the camera to focus immediately and without any delay.

Here is the entire scene with a horizontal view.
Click on image to view larger version


Here is the 100% crop of the same scene. While not perfect, you can see focus is much better.
Click on image to view larger version


For both images, I was shooting at f/6.3, ISO 6400, and 1/13 sec. the IBIS made this exposure very reliable.

The issue is caused by the on sensor AF system. There are no cross sensors - all the sensors are horizontal sensors. So if your subject runs the same way as the sensor, focus is less reliable. A wall seam like this, an edge of a book, or architectural features all could face this issue. All you need to do using back button focus is to turn the camera at an angle - focus normally - then return to your desire orientation to make the image. It's an easy workaround if you expect the problem.

Here are a couple of images from the visit. The museum was the Michael C. Carlos Museum at Emory University in Atlanta. The Greek and Egyptian areas are world class with a very good restoration department. One exhibit is an very early mummy from 4500 BC - the only one in the Western Hemisphere from this era. It was restored using medical technology such as MRI and CT scans.
If you are in the area, this small museum is well worth a visit.

Click on image to view larger version



Hard to believe this sculpture is more than 2500 years old. It looks like something from the 1920's.
Click on image to view larger version



Click on image to view larger version


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Eric Bowles
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