diff --git a/README.md b/README.md
index add6d15..8b9ee01 100644
--- a/README.md
+++ b/README.md
@@ -59,22 +59,23 @@ editly \
   img1.jpg \
   img2.jpg \
   title:'THE END' \
+  --fast \
   --audio-file-path /path/to/music.mp3
 ```
 
 Or create an MP4 (or GIF) from a JSON or JSON5 edit spec *(JSON5 is just a more friendly JSON format)*:
 
 ```sh
-editly my-editly.json5 --out output.gif
+editly my-editly.json5 --fast --out output.gif
 ```
 
 For examples of how to make an JSON edit spec, see below or https://github.com/mifi/editly/tree/master/examples
 
-By default it will use the **width**, **height** and **frame rate** from the **first** input video. **all other clips will be converted to these dimensions** You can override these parameters however. If none are specified, it will use defaults.
+When you run with `--fast` or `fast: true` it will render a much quicker low resolution preview ⏩
 
-**TIP 1:** Run with `--fast` or `fast: true` first to improve speed while testing your edit ⏩
+By default without `--fast` it will use the **width**, **height** and **frame rate** from the **first** input video. **all other clips will be converted to these dimensions.** You can of course override any or all of these parameters.
 
-**TIP 2:** Use this tool in conjunction with [LosslessCut](https://github.com/mifi/lossless-cut)
+**TIP:** Use this tool in conjunction with [LosslessCut](https://github.com/mifi/lossless-cut)
 
 ## Javascript library
 
diff --git a/index.js b/index.js
index def6860..f6dc25d 100644
--- a/index.js
+++ b/index.js
@@ -198,16 +198,24 @@ module.exports = async (config = {}) => {
   if (fast) desiredWidth = 320;
   else if (requestedWidth) desiredWidth = requestedWidth;
   else if (isGif) desiredWidth = 320;
-  else desiredWidth = 640;
 
   if (detectedWidth && detectedHeight) {
-    const calculatedHeight = Math.round((detectedHeight / detectedWidth) * desiredWidth);
-    height = isGif ? calculatedHeight : multipleOf2(calculatedHeight); // x264 requires multiple of 2
-    width = desiredWidth;
-  } else {
+    if (desiredWidth) {
+      const calculatedHeight = Math.round((detectedHeight / detectedWidth) * desiredWidth);
+      height = isGif ? calculatedHeight : multipleOf2(calculatedHeight); // x264 requires multiple of 2
+      width = desiredWidth;
+    } else {
+      width = detectedWidth;
+      height = detectedHeight;
+    }
+  } else if (desiredWidth) {
     width = desiredWidth;
     height = desiredWidth;
     // console.log(`Cannot detect width/height from video, set defaults ${width}x${height}`);
+  } else {
+    // No video
+    width = 640;
+    height = 640;
   }
 
   // User override?