Best way of connecting two meshes, one with many faces I am trying to model a gas mask. I've made three separate meshes and planned on attaching them to the main head mesh. Those three comprise of the two lenses and frontal gas mask filter. But I've run into a problem with that. 

I'm unsure of the best of attaching a mesh with many faces to another mesh with less faces. I don't know what the most efficient way of doing this would be.
A typical subdivision approach would avoid using meshes of different densities where possible, and rely on Catmull-Clark subdivision to create curvatures/circles..

Any (6+)-gon will make a reasonable circle under subdiv. Often made from 4 quads:


Select all quads
CtrlX Dissolve central vertex
AltP Poke single face
-CtrlShiftB Bevel central vertex
Shipped Add-On, Loop Tools > Circle the result




Then, with extrusions, and some rotations/scales around Custom Orientations, you can model something like this mask without having to reduce topology to match between densities.



If you do really need to match areas of significantly different detail, often, to avoid fiddly hand work, you can model at a lower level of subdivision than your intended result, apply one level of subdiv, and continue modeling from there.