主界面为显示框，两个下拉框，一个是所有相机，一个是相机支持的分辨率

系统根据UI界面自动生成的部分不再描述，以下为其他部分源码

widget.h

#include <QWidget>
#include <QMouseEvent>
class QCamera;
class QCameraViewfinder;
class QCameraImageCapture;
class QCameraInfo;
class QCameraViewfinderSettings;

namespace Ui {
class Widget;
}

class Widget : public QWidget
{
    Q_OBJECT

public:
    explicit Widget(QWidget *parent = 0);
    ~Widget();

private:
    Ui::Widget *ui;

    QCamera *camera;                   //相机
    QCameraViewfinder *viewfinder;     //取景器
    QList<QCameraInfo> cameras;        //相机列表
    QList<QSize> mResSize = {};        //分辨率列表
};

main.cpp

#include "widget.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Widget w;
    w.show();

    return a.exec();
}

widget.cpp

#include "widget.h"
#include "ui_widget.h"
#include <QCamera>
#include <QCameraInfo>
#include <QCameraViewfinder>
#include <QCameraImageCapture>
#include <QList>
#include <QFileDialog>
#include <QDebug>

Widget::Widget(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::Widget)
{
    ui->setupUi(this);

    cameras = QCameraInfo::availableCameras();
    if(cameras.count() > 0) {
        camera = new QCamera(cameras.at(0));
    }
    viewfinder = new QCameraViewfinder(this);
    camera->setViewfinder(viewfinder);
    viewfinder->resize(600, 350);
    camera->start();

    //添加初始化的相机选项
    for(const QCameraInfo &cameraInfo:cameras) {
        ui->comboBox_cam->addItem(cameraInfo.description());
    }
    ui->comboBox_cam->setCurrentIndex(0);

    //添加初始化相机的分辨率选项
    mResSize = camera->supportedViewfinderResolutions();
    for (const QSize &msize:mResSize) {
           ui->comboBox_format->addItem(QString::number(msize.width(), 10) + "*" + QString::number(msize.height(), 10));
    }
    ui->comboBox_format->setCurrentIndex(0);
    QCameraViewfinderSettings init;
    init.setResolution(mResSize[0]);
    camera->setViewfinderSettings(init);

    connect(ui->comboBox_cam,QOverload<int>::of(&QComboBox::activated),[=](int index){
       camera->stop();
       delete camera;
       camera=new QCamera(cameras[index]);
       camera->setViewfinder(viewfinder);                   //设置取景器
       camera->start();

       //删除前相机的所有分辨率内容
       for (int i = 0; i < mResSize.size(); i++) {
              ui->comboBox_format->removeItem(0);
       }
       mResSize.clear();
       mResSize = camera->supportedViewfinderResolutions();
       for (const QSize &msize:mResSize) {
              ui->comboBox_format->addItem(QString::number(msize.width(), 10) + "*" + QString::number(msize.height(), 10));
       }
       ui->comboBox_format->setCurrentIndex(0);
       QCameraViewfinderSettings reset;
       reset.setResolution(mResSize[0]);
       camera->setViewfinderSettings(reset);
    });

    connect(ui->comboBox_format, QOverload<int>::of(&QComboBox::activated), [=](int index){
            //重新设置摄像头参数
            QCameraViewfinderSettings reset;
            reset.setResolution(mResSize[index]);
            camera->setViewfinderSettings(reset);
        });

}
Widget::~Widget()
{
    delete ui;
}

运行效果图