Defocus Amount RE without using DoF

[thomas_bussey]

Is it possible to get an active Defocus Amount RE without actually rendering DoF?

This would be really useful as an alternative to Z-depth for post-production lens blur.

[Vizioen]

Is it possible to get an active Defocus Amount RE without actually rendering DoF?

This would be really useful as an alternative to Z-depth for post-production lens blur.

What is the difference with that of Z depth?

[thomas_bussey]

It's based on physical parameters for a start, but more importantly, it goes in and out rather than just A to B. Focal range is field between two depths that will be out of focus. Think of your classic 'tilt-shift' photography. This short DoF is not possible with Z-depth.

Currently you'd have to use 2 Z depths, one of which would have to be adjusted inside the RE parameters (as only one can rely on clipping depth), and comp them in post, which is a pain.

[Vizioen]

Aah I get it. Does this kind of render element exist in other render engines?

[^Lele^]

The defocus amount can exist only if DoF is calculated.
Calculating the expensive DoF to throw it away in favour of a fixed-focus defocus amount RE seems odd indeed, particularly as one Z-depth is enough to pull a post focus with custom ranges: all Z does is let the post app know how far from the camera a given pixel is.
What's done with it, is down to the filter working with its data.
Could you mock up the compositing you're trying to achieve, or the one that isn't working?