why death of field in blender render engine don’t work
Also – a statement like “why depth of field don’t work” tells the rest of us nothing about what your actual situation is. Show examples. Attach short blend-files.
Blender comes with several integrated render engines.
“Blender Render” is by far the oldest of them and it simply does not have camera based DOF. If you want to use DOF with Blender Render, render a depth pass and add DOF in post-production. Or switch to one of the newer renderers.