how to render semi-transparent objects with transparent background as a png with cycles I am rendering out some bubbles for use in a composite Photoshop image, so I need to render as a transparent png with the bubbles semi-transparent.

I tried setting render output to RGBA and checking the transparent option under film, but while the resulting png is transparent the bubbles keep the environment background colour.

The bubbles are definitely semi-transparent, because if I include a coloured object in the scene behind them, it is visible through the bubbles in the render, but I can't find a way to allow the semi transparent material to have any alpha transparency in my png render.
You can use a Holdout shader:



The holdout shader will render as actual image alpha, as opposed to giving the color of the environment on the other side of the object.

Or you could use a Transparent shader, which will allow other objects to be visible through the transparent object:



Note that transparent in Render settings > Film must be enabled, otherwise the holdout shader will appear black and the transparent shader will show the environment behind it. You did mention that you already did this, so you should be good.