dof behaviour with transparent material over another transparent material

hi all!
I've this issue with the dof usage:

seems that the dof of the window is relative to the back position of the focus area.. the material is the same.. have you got any ideas?
the shader that I use is this:

Code (CSharp):

1. Shader "gRally/Window"
2. {
3.     Properties
4.     {
5.         _Color("Spec Color", Color) = (1, 1, 1, 1)
6.         _Smooth("Smoothness", Range(0,1)) = 0.5
7.         _MainTex("Albedo", 2D) = "white" {}
8.         _Dirty("Dirty", Range(0,5)) = 0.0
9.     }
10.     SubShader
11.     {
12.         Tags{ "Queue" = "Transparent" "IgnoreProjector" = "False" "RenderType" = "Transparent" }
13.         LOD 300
14.         CGPROGRAM
15.         #pragma surface surf StandardSpecular alpha:premul vertex:vert
16.         #pragma target 3.0
17.         sampler2D _MainTex;
18.         half _MainTexID;
19.         half _Dirty;
20.         half _Smooth;
21.         fixed _DIRT_1_LEVEL;
22.         fixed4 _Color;
23.         struct Input
24.         {
25.             float2 uv_MainTex;
26.             half3 normal;
27.             float3 worldRefl;
28.         };
29.         void vert(inout appdata_full v, out Input o)
30.         {
31.             // appdata_full
32.             UNITY_INITIALIZE_OUTPUT(Input, o);
33.             o.normal = v.normal;
34.         }
35.         void surf(Input IN, inout SurfaceOutputStandardSpecular o)
36.         {
37.             // diffuse color
38.             fixed4 c = tex2D(_MainTex, IN.uv_MainTex);
39.             o.Albedo = c.rgb;
40.             o.Specular = _Color;
41.             if (_Dirty == 0.0f)
42.             {
43.                 _Dirty = _DIRT_1_LEVEL;
44.             }
45.             if (c.a < 0.8f)
46.             {
47.                 o.Alpha = c.a * _Dirty;
48.             }
49.             else
50.             {
51.                 o.Alpha = c.a;
52.             }
53.             o.Smoothness = _Smooth;
54.         }
55.         ENDCG
56.     }
57. }
58.  

 

thanks for your reply @Chman ... and have you got any fixes for my usage? unlike a car with some transparent windows has usually the situation where one mesh is in front of another....