How to efficiently use Threads and locks

            Thread thread1 = new Thread(() => bitmapRenderer.UpdateBitmap(0,height/8));
            Thread thread2 = new Thread(() => bitmapRenderer.UpdateBitmap(height/8,2*height/8));
            Thread thread3 = new Thread(() => bitmapRenderer.UpdateBitmap(2*height/8,3*height/8));
            Thread thread4 = new Thread(() => bitmapRenderer.UpdateBitmap(3*height/8,4*height/8));
            Thread thread5 = new Thread(() => bitmapRenderer.UpdateBitmap(4*height/8,5*height/8));
            Thread thread6 = new Thread(() => bitmapRenderer.UpdateBitmap(5*height/8,6*height/8));
            Thread thread7 = new Thread(() => bitmapRenderer.UpdateBitmap(6*height/8,7*height/8));
            Thread thread8 = new Thread(() => bitmapRenderer.UpdateBitmap(7*height/8,height));

            thread1.Start();
            thread2.Start();
            thread3.Start();
            thread4.Start();
            thread5.Start();
            thread6.Start();
            thread7.Start();
            thread8.Start();
            thread1.Join();
              .
              .
            thread8.Join();

  lock (bmp) 
        {
              bmp.SetPixel(j,i,iterToColor(iterCount_temp));
        }