Depth of Field and VFX

Depth of Field and VFX

This is in response to a question from a reader about depth of field and how it relates to visual effects.

Models
With miniatures you need enough depth of field to hold focus from the front of the model to the back of the model. This means a lot of light and a stopped down lens. Lack of depth of field is one of the key things that give away the look of the model since in real life a large area would be photographed from a further distance and (possibly in sunlight) so depth of field wouldn't be a problem.

Greenscreen/Bluescreen
If you're shooting a greenscreen then you'll typically want to make sure all the foreground people and objects are in sharp focus. This is easily and frequently overlooked, especially by directors of photography. (And at times by the VFX supervisor) If you have a sunlit, exterior background image then it's likely to be shot at f8-f16. This provides a reasonable amount of depth of field. But when the DP lights the stage he probably won't be lighting to same intensity levels. If the lack of depth of field is apparent then it causes two problems. If the back end of the foreground is soft then you're forced to blur the background even if there's something important to see. It's impossible in real life to have only a mid-section that's blurred. If the director really needs to see what's back there then you're forced to try to sharpen the edges and back detail of the foreground. The end result will never appears natural. The other problem is the audience senses that this is unnatural (i.e. they only see this in vfx shots). An 'exterior' scene in bright sunlight with a normal or wide angle lens should be in focus throughout a normal shot.
You'll see animated films where they have cheated the depth of field. I find it best if you want your vfx or animation to appear natural is to use the guidelines and restrictions that a normal movie has. (depth of field, camera movement, etc)

Markers
The other depth of field issues for VFX people is soft tracking markers and very soft edge mattes. If you're on a stage shooting a greenscreen with a long lens then the markers may be so out of focus as to disappear. This is a big problem, especially if you're shooting a character from the waist up who's moving and jumping around. Since the markers are invisible, you have no easy way of distinguishing the camera motion from the character motion. Someone will have to manually work on that shot by eye until it looks reasonable. This can be very time consuming and require a number of takes.
Note that LED markers tend to hold up better for out of focus shots. These are markers using key ring lites (possibly modified) you see in the store. The point source of a red LED holds up better than an X piece of tape.

Mattes
The soft edges of the greenscreen matte (or a place you want the roto matte) will require delicate settings of the key. Any blur (from depth of field or motion) will cause some of the background to bleed through that area as if it were partially transparent. The blurred area then becomes more contaminated with the greenscreen. When rotoing a blurred edge it's sometimes a subjective question where the blur stops. If you include all of the blur then you'll be including some of the original background. If you clip off too much of the blur then it will look incorrect in the composite unless you blur the edges of the matte similar to the original. Fortunately software like Commotion could deal with natural motion blur so it was less of an issue. If you shot someone slightly soft in front of bright points of light (city, Las Vegas, etc) then in the edges of the blur you would have points of light that likely aren't in the new background. In these cases you may have to clamp down or paint out the offending lights.