Convert RGB to Grayscale in ImageMagick command-line

Question

How do I convert a RGB image (3 channels) to a grayscale one, using the (r+g+b)/3 method? I look through an examples page: http://www.imagemagick.org/Usage/color_mods/#grays

Answer 1

I use convert mostly to convert colour pictures of documents into grey-scale pdf documents in order to perform OCR. My best results are using Rec709Luminance. So I recommend

convert colourpicture.png -grayscale Rec709Luminance greyscalepicture.png

Short command, nice outputs.