Hi there,
I was making part of a camera, and here is the picture.
My question is how do I keep the edges sharp after using sub-division surface modifier of the edge shown in red circle. I want to keep the edges sharp like a default cube. but I also the front part to be curved, how do I do that?
Edge loops. Ctrl+r allows you to insert edge loops. Just tighten those corners and you’ll be fine.
Maybe this will help as well https://youtu.be/nBkwodrQq_4
The linked tutorial gives outdated information.
Cutting the shading is better done with auto smooth instead of edge split modifier. Creasing alone is not a very good way to support subdivisions as it often needs support loops anyway to get a better shading, and creasing doesn’t export.
Non-destructive way to add support loops is to use the bevel modifier with 2 loops, profile 1, which puts one loop on each side of the edge that is beveled.
it depends on your desired geo wich type of workflow to choose. Like ja12 said a combination of creasing and autosmooth might work.
Even placing the bevel after the subd mod could be the quickest sometimes but highly depens on the desired geo and its mesh topology and what are you goin to do with it after. If you got a mesh wich you are going to animate its most times better to refine mesh with additionally loops or if in need of keeping non destructive use Bevel weights.
Angle depended options give here often results which are hard to control while animating, because angles/normals steady changing…
You can also select that edge and press SHIFT E to control the edge crease. This has the added bonus of being easily editable and undo-able. It also does not add any extra geometry. You can also adjust it in the properties panel to the right hand side of the 3d viewport.
Thank you everyone for your replies. Thanks a lot. 
Regards
Prachu
It’s kind of amazing how so many people don’t know how sub-division creasing works. 
Creasing is something that works best with render engines like RenderMan not so much with ones like Cycles or Blender Internal. You need to use a pretty high render sub division level above 4, 6 or 7 are about ideal (of course this will make you render times explode) for it to work nice.
Make a couple of cubes with sub-division modifiers and different crease values like 0.5, 0.7, 0.9 and 1 to test out what I man.
Renderman when it was using micropolys( I don’t know if it still works like this with the newer PBR stuff) gave you a highly subdivided surface without longer render times.
For Cycles style render engines I would suggest supporting edge loops or using the bevel modifier.
It really depends on the model and how much of a sharp edge you’re looking for.
