Instant Zoom Tutorial

Jeffrey Glen Jackson
(c) 1998

Problem Description

A number of consumer video cameras have a feature called instant zoom (there are probably some other terms for it too) where, at the press of a button, the image instantly zooms to 1.5x or 2.0x. This is done via software in the camera, not by optics. If you're like me, you despise this feature, but sooner or later, you're going to get footage from someone who thought it was the greatest thing since the donut hole.

Here is how I dealt with it when it happened to me. The short version is that I used moving path to turn the instant zoom into a gradual zoom. That sounds like it ought to be easy, but in reality, its fraught with peril.

Ideally, all you have to do is cut the clip at the point the instant zoom happens, make another cut a few seconds earlier, and apply a moving path that keeps the center centered, but which increases the pixel size gradually from 100% to 150% (or 200% or whatever). There are to problems that may keep this from working however.

The first is that that the instant zoom might occur between the fields of one frame rather than between the fields of adjacent frames. On my camera, all cuts occur between fields of a single frame. I've used others where they occurred between frames, and I seem to recall one where any give cut could be either way. Unfortunately, all consumer editing software is frame accurate at best. We can't have effects apply only to the last or first field.

The second problem is that consumer editing software is very buggy. I've seen MSPro not apply effects frame accurately, but actually extend the moving path one field too many, or one field too short. Dealing with this is done with the same techniques I'm going to present for solving the first problem, but because this problem is so unpredictable, you'll just have to experiment and tweak to get it right if you run into it.

TECHNIQUE 1: Deinterlace the frame containing the instant zoom.

Effectively you just throw away one of the two fields, replacing it with the other field. Loosing a 60th of a second of video is generally not noticeable (30fps is still smooth to the human eye). There are several ways to do this. Once is to make a one frame clip and mark it frame based with the deinterlace option selected. In at least one version of MSPro, this doesn't work because it will make every clip from the same source file deinterlaced. Another alternative is to export the frame as a TIFF, load it into PhotoShop and deinterlace it there. This way we can choose which field you want.

TECHNIQUE 2: Slow Motion

Export from the beginning of the slow zoom to after the instant zoom frame at 50% slow motion. This deinterlaces everything. Now create a project where you apply moving path. This will be field accurate since every frame represents on field of the original video. Export this at 200% fast motion to re-interlace it and speed it back up to the original speed and drop this into the original project. Alternatively, just export the one frame with the cut at 50% speed as a image sequence, the select the one holding the desired field and use it as a one frame clip as in technique 1.

Personally, I use technique 1 usually, but if you don't have a paint tool like PhotoShop to do the deinterlace, technique 2 will do the job.