class Viewer : public QWidget{
...
public slots:
	void zoomIn();
protected:
	QImage *image;
	QImage *t_image;
	QPainter painter;
	void paintEvent(QPaintEvent * event);
	qreal	zoom_value; // коэф. масштабирования
};
--------------------------------
Viewer::Viewer(QWidget *parent) : QWidget(parent){
	zoom_value = 1.0;
	image 	= new QImage();
	t_image	= new QImage();
	if(image->load("...jpg")){
		t_image->operator = (*image);
		if(!t_image->isNull()){
			setGeometry(QRect(0,0,t_image->width(),t_image->height()));
		}
	}
}
void Viewer::paintEvent(QPaintEvent *event){
	qDebug()<<"paint:";
	if(!t_image->isNull()){
		qDebug()<<" draw image";
		painter.begin(this);
		painter.drawImage(0,0,*t_image);
		painter.end();
	}
	else
		qDebug()<<" image is null";
}
void Viewer::zoomIn(){
	if(!t_image->isNull()){
		QMatrix matrix;
		zoom_value*=1.3;
		t_image->operator = (image->transformed(matrix.scale(zoom_value,zoom_value)));
		while(t_image->isNull()){
			zoom_value/=1.3;
			qDebug()<<" image is null, zoom value = "<<zoom_value;
			t_image->operator =(image->transformed(matrix.scale(zoom_value,zoom_value)));
		}
		qDebug()<<"transform image["<<image->size()<<"] to ["<<t_image->size()<<"]";
		qDebug()<<" image is OK,  zoom value = "<<zoom_value;
		setGeometry(QRect(0,0,t_image->width(),t_image->height()));
		update();
	}
}

transform image[ QSize(800, 600) ] to [ QSize(1040, 780) ] 
 image is OK,  zoom value =  1.3 
paint: 
 draw image 
// изображение увеличелось

...

transform image[ QSize(800, 600) ] to [ QSize(11029, 8272) ] 
 image is OK,  zoom value =  13.7858 
paint: 
 draw image 
// изображение увеличелось
QImage: out of memory, returning null image
 image is null, zoom value =  13.7858 

...

QImage: out of memory, returning null image
 image is null, zoom value =  0.0725382 
transform image[ QSize(800, 600) ] to [ QSize(14337, 10753) ] 
 image is OK,  zoom value =  0.0725382 
paint: 
 draw image 
// изображение увеличелось!!!

transform image[ QSize(800, 600) ] to [ QSize(75, 57) ] 
 image is OK,  zoom value =  0.0942996 
paint: 
 draw image 
// изображение уменьшилось от исходного в zoom value =  0.0942996 раз
 
transform image[ QSize(800, 600) ] to [ QSize(98, 74) ] 
 image is OK,  zoom value =  0.122589 
paint: 
 draw image 
// изображение увеличелось
итд

 painter.begin(this);
 painter.scale(zoom_value,zoom_value)
 painter.drawImage(0,0,*t_image);
 painter.end();