diff --git a/conversion.go b/conversion.go
index b7eac66..405d26d 100644
--- a/conversion.go
+++ b/conversion.go
@@ -2,6 +2,7 @@ package main
 
 import (
 	"archive/zip"
+	"bytes"
 	"fmt"
 	"io"
 	"os"
@@ -24,24 +25,35 @@ func makeConcatDemuxFile(manifest UgoiraManifest, frameDescFile *os.File) {
 	frameDescFile.WriteString(fmt.Sprintf("file '%s'\n", lastFrame.File))
 }
 
-func callFFmpeg(frameDescPath, outputFilePath, workDir string) error {
-	// TODO: allow custom ffmpeg flags
-	const ffmpegArgs = `-hide_banner
-						-loglevel warning
-						-f concat -i %s
-						-an
-						-fps_mode vfr
-						-q:v 10
-						-c:v libvpx-vp9
-						-b:v 1M
-						-lossless 1 %s`
+func callFFmpeg(frameDescPath, outputFilePath, workDir, ffmpegArgs string) error {
+	const ffmpegBaseArgs = `-hide_banner
+							-loglevel warning
+							-f concat -i %s
+						    -fps_mode vfr`
+	// Default codec arguments, for a VP9 WebM
+	const ffmpegDefaultArgs = `-an
+						       -q:v 10
+						       -c:v libvpx-vp9
+						       -b:v 1M
+						       -crf 5 %s`
 
-	cmdArgs := fmt.Sprintf(ffmpegArgs, frameDescPath, outputFilePath)
+	var ffmpegArgsTemp string
+	if len(ffmpegArgs) > 0 {
+		ffmpegArgsTemp = ffmpegBaseArgs + " " + ffmpegArgs
+	} else {
+		ffmpegArgsTemp = ffmpegBaseArgs + " " + ffmpegDefaultArgs
+	}
+
+	cmdArgs := fmt.Sprintf(ffmpegArgsTemp, frameDescPath, outputFilePath)
 	cmdArgsSplit := strings.Fields(cmdArgs)
 	ffmpegProc := exec.Command("ffmpeg", cmdArgsSplit...)
+
+	var stderr bytes.Buffer
+	ffmpegProc.Stderr = &stderr
 	ffmpegProc.Dir = workDir
 
 	if err := ffmpegProc.Run(); err != nil {
+		fmt.Fprintf(os.Stderr, "FFmpeg stderr:\n%s\n", stderr.String())
 		if exitErr, ok := err.(*exec.ExitError); ok {
 			return fmt.Errorf("ffmpeg exited with non zero code: %d", exitErr.ExitCode())
 		}
@@ -86,7 +98,7 @@ func unpackUgoira(ugoiraPath, outDir string) error {
 	return nil
 }
 
-func ugoira2video(manifest UgoiraManifest, ugoiraFileName, vidOut string) error {
+func ugoira2video(manifest UgoiraManifest, ugoiraFileName, vidOut, ffmpegArgs string) error {
 	tdir, err := os.MkdirTemp("", "u2vwd-")
 	if err != nil {
 		return err
@@ -108,7 +120,7 @@ func ugoira2video(manifest UgoiraManifest, ugoiraFileName, vidOut string) error
 		return fmt.Errorf("Could not unpack Ugoira: %w", err)
 	}
 	makeConcatDemuxFile(manifest, fraFp)
-	if err := callFFmpeg(fraFp.Name(), vidOutAbs, tdir); err != nil {
+	if err := callFFmpeg(fraFp.Name(), vidOutAbs, tdir, ffmpegArgs); err != nil {
 		return err
 	}
 	return nil