SetAspectRatio.cs

using UnityEngine;
using System.Collections;

public class SetAspectRatio : MonoBehaviour {

	public Vector2 aspectRatio = new Vector3(16, 9);

	void Start() {
		float targetAspect = aspectRatio.x / aspectRatio.y;

		float windowAspect = (float) Screen.width / (float) Screen.height;

		float scaleHeight = windowAspect / targetAspect;

		Camera camera = GetComponent<Camera>();

		if (scaleHeight < 1.0f) {

			Rect rect = camera.rect;

			rect.width = 1.0f;
			rect.height = scaleHeight;
			rect.x = 0;
			rect.y = (1.0f - scaleHeight) / 2.0f;

			camera.rect = rect;

		} else {
			float scaleWidth = 1.0f / scaleHeight;

			Rect rect = camera.rect;

			rect.width = scaleWidth;
			rect.height = 1.0f;
			rect.x = (1.0f - scaleWidth) / 2.0f;
			rect.y = 0;

			camera.rect = rect;
		}
	}
}