Custom depth shader

Shader "DepthShader"
{
    Properties{
        _Depth ("Depth", Range (0.5, 50)) = 0
    }
    SubShader
    {
        Tags { "RenderType"="Opaque" }

        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag

            #include "UnityCG.cginc"

            struct appdata
            {
                float4 vertex : POSITION;
            };

            struct v2f
            {
                float4 vertexWorld : TEXCOORD0;
                float4 vertex : SV_POSITION;
            };

            float _Depth;

            v2f vert (appdata v)
            {
                v2f o;
                o.vertexWorld = mul(_Object2World, v.vertex);
                o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
                return o;
            }

            float4 frag (v2f i) : SV_Target
            {
                float distFromCamera = distance(_WorldSpaceCameraPos.xyz,i.vertexWorld.xyz);
                distFromCamera = 1.0 - 1.0/(1.0+_Depth*distFromCamera*distFromCamera);
                return float4(distFromCamera,distFromCamera,distFromCamera,1.0);
            }
            ENDCG
        }
    }
}