Here's the modified command using AV1 encoding and outputting to a .MKV file:

```bash
ffmpeg -i image-%d.png -vf "curves=levels=0:0.5:1" -c:v libaom-av1 -strict experimental -crf 30 -b:v 0 -pix_fmt yuv420p output.mkv
```

In this command:
* image-%d.png specifies the input images, where %d is a placeholder for the frame number.
* curves=levels=0:0.5:1 specifies the "curves" filter and sets the exposure to 0.5.
* libaom-av1 specifies the AV1 video codec to be used for encoding the output video.
* strict experimental is required for using the AV1 codec.
* crf 30 sets the Constant Rate Factor (CRF) to 30, which determines the quality of the output video. Lower values result in better quality but larger file sizes. You can adjust this value to your preference.
* b:v 0 sets the target bitrate to 0, meaning the encoder will use the CRF value instead to determine video quality.
* yuv420p specifies the pixel format for the output video.
* output.mkv is the name of the output file in the MKV format.

Keep in mind that encoding with AV1 can be slower compared to other codecs like H.264 (libx264). You can adjust the CRF value and other parameters to achieve the desired balance between quality and encoding time.
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