You asked
What is the sony fe 100 400mm f 5 6 8 oss good for?
The Sony FE 100-400mm f/5.6-8 OSS is a light, affordable 400mm zoom for full-frame Sony E cameras, at about 650 g. Its main appeal is reach without a heavy lens.
It suits sport from the sideline and larger animals, where 100-400mm covers most of what you need. Because the aperture drops to f/8 at the long end, it's happiest in bright light. It's stabilised, which helps when you're handholding at full zoom.
On an APS-C body such as the Sony a6400, it frames like 150-600mm, which makes it a strong starting point for birds. For more demanding bird and sport work, see the wildlife and sport guide. You can also save a lot by buying used.
Written by Claude from this site's pages.