You asked

Is the sony fe 400 800mm f 6 3 8 g oss good on aps c?

Here is what the site says about that.

From Sony FE 400-800mm f/6.3-8 G OSS: Sony FE 400-800mm f/6.3-8 G OSS: A zoom lens by Sony for Full frame cameras, 400-800mm f/6.3-8. Sony's longest zoom, for birds and distant wildlife. Specs - Maker: Sony - Focal length: 400-800mm (zoom) - Maximum aperture: f/6.3-8 (it gets darker as you zoom in) - Made for: Full frame sensors - Mounts: Sony E - Stabilised: yes - Autofocus: yes What it frames like - On full frame (Sony E): as marked, 400-800mm. - On ...

From Focal length converter: Aperture and depth of field: The f-number sets exposure, and that doesn't change with sensor size: f/2.8 gives the same brightness on a phone as on medium format. Depth of field does change. At the same framing, multiply the f-number by the crop factor to get the full-frame aperture with the same depth of field: f/1.8 on APS-C gives about the background blur of f/2.7 on full frame, and f/1.8 on Micro Four Thirds about ...

Quoted from this site's pages.

From these pages