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TUTORIAL: Grading & Effects: Artificial depth-of-field

Posted: Thu, 22nd Apr 2004, 5:10pm

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Simon K Jones

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Chromanator Tutorials: Grading & Effects

Artificial depth-of-field

The term ‘depth-of-field' (aka d-o-f) refers to the area of a shot that is in-focus. Different camera and lens set-ups have a different d-o-f, resulting in very different visuals.

Wide angle film cameras, for example, tend to have a very small d-o-f, enabling filmmakers to stage dramatic shots that draw your eye towards a certain area of the screen. A small d-o-f is very useful when having lots of close-ups on your actors, for example, as it keeps the audience focused on the actor's face rather than wandering off to look at the scenery.

Consumer video cameras, on the other hand, have a very large d-o-f, which results in almost everything being in-focus almost all of the time. Consumer video cameras are designed mainly for quick-and-easy filming – perhaps for a documentary, or for a holiday video, for which you will want as much of the frame in-focus and clear as possible. However, if you want to create dynamic, dramatic visuals this can prove to be something of a problem.

Take this shot, for example:

This picture has three clear levels – the foreground bottle, the windowsill and the view out of the window. However, due to the video camera's huge d-o-f, everything is in perfect focus! Using Chromanator's garbage matte and effects tools, it is possible to artificially alter the d-o-f to make it far more interesting.

First I need to place two copies of the same clip on the timeline, one on top of the other. For each ‘focal area' I'll need a separate version of the same clip. In this example I'll separate the foreground bottle from everything else.

It would be possible to create separate focal areas for the windowsill and the background as well – just add more sections!

First up I need to draw a garbage matte section around the bottle. As this is the foreground object, I need to make sure it is the higher object on the timeline. In the Object View I add a new garbage matte by switching to the Key -> Garbage Matte section and clicking the button.

After drawing carefully around the bottle and locking the shape, I need to invert the shape so that everything except the bottle is removed.

  • Directly underneath this clip is an identical clip, so at first it will appear as if nothing has changed.

Now I go back to the Project View, select the background object (the one that is lower on the timeline) and return to the Object View. The tool I need is in the Grading toolset.

  • Applying an artificial blur is easy. Simply select the Box Blur tool and adjust the blur amount as required.

The background will now blur, while the bottle stays in perfect focus.

  • The bottle remains in focus because the higher layer, consisting of the cut-out bottle, is layered on top.

By altering the blur amount on different keyframes, you can even simulate racking the focus back-and-forth, by adding and removing the blur amount on both layers at once. Careful feathering and positioning of the garbage matte may be required, but the results can be very effective.

Click here to download an example clip.