public static function fromMM($stringData)
    {
        if (!is_string($stringData)) {
            throw new InvalidArgumentException('Given data is not a string');
        }

        if (!preg_match('#^([0-9]+)mm$#', $stringData, $matches)) {
            throw new RuntimeException('Given focal length is not in a valid format. Need: "<number>mm"');
        }

        $numerator = $matches[1];

        return new self(
            "{$numerator}/1"
        );
    }