Wednesday, July 4, 2012

July 4, 2012 In conjunction with UWOL 23, here is an explanation of FOCUS STACKING for video.

Focus stacking is a wonderful technique to allow both the foreground and background of a photo to be in focus. Although these days everyone goes "gaga" over those out of focus backgrounds, there are still people who like to see the background in focus. It adds depth and realism.

You can do this in video. It’s more difficult than with a photo, but very effective under the right conditions. The conditions are:

1. No wind (or very little). A breeze may be too much.

2. A solid tripod. Clamp down the tilt and pan. No camera movement allowed.

3. A clear view of what will be background with no foreground elements in the way, and vice versa. This takes very careful positioning of the camera.

Many shots you want to take just cannot comply with this rule. Find another shot.

4. A buffer zone. Between what will be the focused foreground and focused background, there must be a small space that will remain unfocused. This can be blue sky (the best), sand, gravel or dirt, grass, still water, rapidly moving water (like a waterfall), busy leaves, branches or bark…anything bland enough not to be noticed that it isn’t really in focus. There can be no action in the buffer.

5. Minimal movement of your foreground subject. If it moves too much it will cross the buffer zone and get visibly cut off.

OK. You have found a subject that fit’s the criteria. Now focus on the foreground and shoot some video. Next, focus on the background without moving the camera and shoot some more video, about the same length clip. If you are lucky in your composition, it may be possible to shoot a mid-range clip as well to get that tree that just isn’t quite sharp enough in either the foreground or background.

The rest is done at home. Most video editor software has garbage mattes. These allow you to blot out whole areas of scenes. They are usually used with green screens to superimpose people over landscapes but they work here too. I use Adobe Premiere which has a 4 point, an 8 point and a 16 point garbage matte. The 16 is usually needed. Load the background-focused clip. With those 16 points, outline the background trying as much as possible to keep the border you are making inside the buffer zone (step 4, above). The unfocused foreground will be black…gone.


Now load the foreground-focused clip and do a similar maneuver, outlining the foreground, and blotting out the unfocused background. Try to simulate the path of the garbage matte to the one you did for the background. Now you have two clips with only focused material showing. Superimpose them. Adjust the mattes so no black is showing. Soften the borders (remember these are in purposely unfocused areas, busy spots or edges of things where a border is not likely to be noticed), and there it is.

The technique is not perfect. A sharp eye can see where unintended unfocused material could not be matted out due to movement of the subject. In my UWOL 23 video, pause at the Siamang (the ape) clip or the pelican clip and you can see the errors around their heads. In a moving video this is not likely to be noticed.

While filming be very careful of the wind. Your tolerances of where things are placed are so close that even a little breeze waving a blade of grass that you don’t even see in the viewfinder will ruin the whole scene. Of course if you have a good wide buffer, the wind is less of an obstacle.

This technique is fun to do, and occasionally when you are out, you will run into a situation that just screams FOCUS STACKING. When it does, take a few minutes and shoot it. You’ll be glad you did.