user12777: I am currently using an FFMPEG command like:
ffmpeg -i input.mov -vf scale=1920:1080 output.mp4
to convert varying input format file to a fixed resolution MP4 output file.  One final step I need is to make the output Black and White (the input files will be full color).
I have tried both 
    -pix_fmt gray 
and
    -vf format=gray,format=yuv422p
but both just result in a black output video.
After much trawling of google, it looks like the FFMPEG exe I downloaded (for windows) wasn't compiled with the "-enable-gray" option, but I don't know if this makes any difference and don't know how to obtain a windows FFMPEG with this option enabled (building FFMPEG myself looks like a lot of hard work).
Any advice welcome!  Thanks.
PS I should add that the conversion I'm currently doing and any grayscale conversion could be separate steps if required.

user1871: Yes, you do need a ffmpeg build with gray pixel format enabled. You should get your binary from the Windows link at ffmpeg. Download the latest 32-bit static build.
You can check if your build has gray enabled by running 
ffmpeg -pix_fmts | findstr /r gray

It should list an entry for gray. 
And use format=gray in your vf chain and -pix_fmt yuv420p together for broad compatibility.

The command below is for linux. grep will work as findstr!
ffmpeg -pix_fmts | grep -i gray



12777: <p>I am currently using an FFMPEG command like:</p>

<p>ffmpeg -i input.mov -vf scale=1920:1080 output.mp4</p>

<p>to convert varying input format file to a fixed resolution MP4 output file.  One final step I need is to make the output Black and White (the input files will be full color).</p>

<p>I have tried both 
    -pix_fmt gray 
and
    -vf format=gray,format=yuv422p</p>

<p>but both just result in a black output video.</p>

<p>After much trawling of google, it looks like the FFMPEG exe I downloaded (for windows) wasn't compiled with the "-enable-gray" option, but I don't know if this makes any difference and don't know how to obtain a windows FFMPEG with this option enabled (building FFMPEG myself looks like a lot of hard work).
Any advice welcome!  Thanks.</p>

<p>PS I should add that the conversion I'm currently doing and any grayscale conversion could be separate steps if required.</p>
 1871: <p>Yes, you do need a ffmpeg build with gray pixel format enabled. You should get your binary from the Windows link at <a href="https://ffmpeg.org/download.html" rel="nofollow noreferrer">ffmpeg</a>. Download the latest 32-bit static build.</p>

<p>You can check if your build has gray enabled by running </p>

<pre><code>ffmpeg -pix_fmts | findstr /r gray
</code></pre>

<p>It should list an entry for gray. </p>

<p>And use <code>format=gray</code> in your <code>vf</code> chain and <code>-pix_fmt yuv420p</code> together for broad compatibility.</p>

<blockquote>
  <p>The command below is for linux. <a href="https://docs.microsoft.com/en-us/windows-server/administration/windows-commands/findstr" rel="nofollow noreferrer"><code>grep</code></a> will work as <a href="http://man7.org/linux/man-pages/man1/grep.1.html" rel="nofollow noreferrer"><code>findstr</code></a>!</p>

<pre><code>ffmpeg -pix_fmts | grep -i gray
</code></pre>
</blockquote>
