Fragmentarium: EXR image format using floats for RGBA with depth value stored in A channel, check the sources on https://github.com/3Dickulus/FragM you can name the "A" layer anything you like, "depth" maybe if you're targeting some image processing software that expects that name for that data layer. Maybe it can be adapted to Synthclipse, the source can at least present an example. Using EXR tiled format also removes the image XY size limit imposed by Qt and RGBA values can be 16 or 32 bit.

edit: you can also bind the focal plane to the "target" coords and control the target pos via a spline path, then depth value could be  target distance +/- range value.

oh, yes, I recall _revers_ asking me if he could use some EXR stuff from FragM  (of course I said yes)

what I mention above applies to all images in an animation sequence, not sure about the "best" way, 8 bits is a tight space.

In production I think rendering to EXR format would be preferred.

The default depth buffer in GL is 24 bits depth and 8 bits stencil, in FragM I set the depth buffer to 32 bits and no stencil so the fragment can write the full 32bit float value to the depth buffer, when the value is taken from the depth buffer it is stored as 32bit float in the EXR format Alpha channel, the EXR package includes some tools for viewing and managing the images, like, converting to 16bit png or extracting channels, these tools are in the FragM/bin folder as part of the OpenEXR package.

If the full EXR package is intalled then FragM will show a "EXR Tools" menu next to the "Help" menu, they are all commandline tools but the menu will give information about how to use them.
 ( @_revers_   you should include these with synthclipse too )

are you rendering a full color sequence, then rendering a depth only sequence, using some video processing software to add elements to the full color sequence using the depth sequence as the blend mask?