How can I reduce Bit Depth of a .tif file?

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VB

I have scan an A4 size Image with 100 dpi as '.tif' format. The file size stand 2832 KB. Now I want to reduce the file size to 50 KB.

I have already try to reduce the bit depth such as ...

dim SourceName As String = "C:\Sample.tif"
dim TargetName As String = "C:\Sample_Compress.tif"
Dim tiff As Image
Dim enParams As New EncoderParameters(2)
Dim tifCodec As ImageCodecInfo = ImageCodecInfo.GetImageEncoders(3)

enParams.Param(0) = New EncoderParameter(Encoder.Compression,
EncoderValue.CompressionLZW)

enParams.Param(1) = New EncoderParameter(Encoder.Quality, CLng(1))


tiff = Image.FromFile(SourceName )
Dim tempBM As New Bitmap(tiff.Width, tiff.Height, PixelFormat.Format32bppArgb)

tempBM.SetResolution(tiff.HorizontalResolution, tiff.VerticalResolution)
Dim g1 As Graphics = Graphics.FromImage(tempBM)


g1.DrawImageUnscaled(tiff, New Point(0, 0))
tempBM.Save(TargetName, tifCodec, enParams)
tiff.Dispose()

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Now the file size reduce to 2832 KB to 400 KB. How can I reduce to 50 KB ?