Sunday, July 26, 2015

Correcting Back Focus

I really liked this image when I made it on July 3. The lens, a Nikon 20-35mm 2.8 D, was purchased for under $300.00 from Fireside Camera in San Francisco. I was warned that one of the glass elements in the lens had a small scratch on it. It turned out to be a tiny and could only be seen if a flashlight was used to light it.

On this day, my goal for the day's shooting was to experiment with large shooting aperture settings using my formula for flash-fill. To limit the depth of field, I set the ISO to an equivalent of 100 and installed a 3-stop neutral density filter. The lead shot was made at F 3.5, which for all intents and purposes was wide open. Again, I was happy with the shot, until I took a closer look.

You can see that the foreground subject is very soft, while the spectators in the background are noticeably sharper. Now I KNOW I had focus properly, but obviously something was amiss. This condition, where there is a disagreement between the actual plane of focus and the locked autofocus setting is called Back Focus, or Front Focus if you prefer. In my case, there was clearly a problem, exacerbated by my nearly wide-open shooting aperture. Photographers who shoot with fast prime lenses will do well to check their lenses, since these large aperture wonders leave the shooter with a very, very narrow margin of focusing error.
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To test the lens, I used a Datacolor SpyderLensCal Autofocus Calibration Aid, a relatively inexpensive device that gives a photographer a convenient way to easily check for back or front focus. First, you'll need to set the device the Calibration Aid at the proper distance from the film plane of the camera. I found that the website of a similar device suggested that the distance between the file plane and the focusing spot should be 25 times the focal length of the lens. In my case, the 20mm short setting suggested a distance of 500 mm, or about 20 inches. Once the distance is set, you photograph the high-contrast focusing spot in the center of the device and check the inclined ruler on the side. By examining the sharpness of increments, you could see exactly where you camera was locking focus. 

When set up, you'll get a photo that looks something like this. Incidentally, I used a Nikon speedlight for illumination. After making the photo, you can check the offset by simply zooming in during the image preview. I've found that it's pretty easy to see where the plane of critical focus is. Once you've established how a particular lens responds during autofocus, you can save a custom focus setting in the camera body for up to 12 individual lenses.

This sample image shows the 20-35 lens with the focus locked on the focusing spot. It was alarming to see an out-of-focus reference point, and discovering a plane of sharp focus was behind the zero setting. Notice that the numbers between 2 and 6, located behind the focusing spot, are in sharp focus, while the 1 and the 0 are not.

You can see a short YouTube video here. Don't be discouraged, as making the adjustments gets easier each time you do it. The focus offset for my 20-35mm lens was set to -20, the most offset I could dial in. When I assigned it to Custom Setting #3, I was surprised to find that the camera was able to extract the aperture and zoom range directly from the lens itself and include those specifications in the custom setting. 

You can see the improvement it made. Just so that you know, there will always be a bit more depth of field behind the critical focal point than in front. At any rate, this would allow me to establish critical focus on my subjects eyes, exactly what one needs to do when photographing people.

The proof came when I photographed this woman during International Night here at the Adult School. While the settings were different from those in the lead photos, the focusing problems appear to have been addressed. The eyes are sharp, and there is plenty of detail in the rest of the photo.

These before and after back-focus comparison shots are very much an apples and oranges affair. But the improvement is there to see, even though the shooting conditions of the two sample shots weren't identical.
Some final caveats: You have to remember to turn the back focus setting "off" when you change to another lens. Also, this feature is only available on the "prosumer" level cameras, which means the D300 series, the D7000 series, and all of the full frame bodies.