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Zoom, Fixed, or Prime Lens?

During a single shot, the camera lens either zooms, or does not. Often, even though the camera has a zoom lens, it did not zoom. You can get much better tracking results if the camera did not zoom.

Select the Lens panel. Click

Fixed, Unknown if the camera did not zoom during the shot (even if it is a zoom lens)

Fixed, with Estimate if the camera did not zoom during the shot, and you have a good estimate of the camera field of view, or both the focal length and sensor (plate) width.

Zooming, Unknown if the camera did zoom

Known if the camera field of view, fixed or zooming, has been previously determined (more on this later).

If you are unsure if the camera zoomed or not, try the fixed-lens setting first, and switch to zoom only if warranted. Generally, if you solve a zoom shot with the fixed-lens setting, you will be able to see the zoom’s effect on the camera path: the camera will suddenly push back or forwards when it seems unlikely that the real camera made that motion. Sometimes, this may be your only clue that the lens zoomed a little bit (or the focus was changed, which can cause small zooms).

Important : Never use “Known” mode solely because someone wrote down the lens setting during shooting. Like the turn-signal of an oncoming car, it is only a guess, not something you can count on. Do not set a Known focal length unless it is truly necessary.

You may have the scribbled lens focal length from on-set production. If you also know the sensor (plate) size, you can use the Fixed, with Estimate setting to speed up the beginning of solving a bit, and sometimes to help prevent spurious incorrect solutions if the tracking data is marginal. The mode is also useful when you are solving several shots in a row that have the same lens setting: you can use the field of view value without worrying about sensor (plate) size. In either case, you should rewind to the beginning of the shot and either reset any existing solution, or select View/Show Seed Path, then set the lens field of view or focal length to the correct estimated value. SynthEyes will compute a more accurate value during solving.

It can be worthwhile to use an estimated lens setting as a known lens setting when the shot has very little perspective to begin with, as it will be difficult to determine the exact lens setting. This is especially true of object-mode tracking when the objects are small. The Known lens mode lets you animate the field of view to accommodate a known, zooming lens, though this will be rare. For the more common case where the lens value is fixed, be sure to rewind to the beginning of the shot, so that your lens FOV key applies to the entire shot.

When a zoom occurs only for a portion of a shot, you may wish to use the Filter Lens F.O.V. script to flatten out the field of view during the non-zooming portions, then

lock it. This eliminates zoom/translation coupling that causes noisier camera paths for zoom shots. See the online tutorial for more details. You can also set up animated filter controls using the post-solve filtering to selectively filter more during the stationary non- zooming portion.

©2024 Boris FX, Inc. — UNOFFICIAL — Converted from original PDF.