您可以通过以下方式将图片框上的选定矩形转换为图像上的矩形：

public RectangleF GetRectangeOnImage(PictureBox p, Rectangle selectionRect)
{
    var method = typeof(PictureBox).GetMethod("ImageRectangleFromSizeMode",
        System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance);
    var imageRect = (Rectangle)method.Invoke(p, new object[] { p.SizeMode });
    if (p.Image == null)
        return selectionRect;
    var cx = (float)p.Image.Width / (float)imageRect.Width;
    var cy = (float)p.Image.Height / (float)imageRect.Height;
    var r2 = Rectangle.Intersect(imageRect, selectionRect);
    r2.Offset(-imageRect.X, -imageRect.Y);
    return new RectangleF(r2.X * cx, r2.Y * cy, r2.Width * cx, r2.Height * cy);
}

注意：你可以在这里找到ImageRectangleFromSizeMode方法的源代码，并将其作为应用程序代码的一部分编写。

示例-裁剪具有SizeMode = Zoom的PictureBox的图像

例如，以下代码将裁剪图片框1的给定矩形，并将结果设置为图片框2的图像：

var selectedRectangle = new Rectangle(7, 30, 50, 40);
var result = GetRectangeOnImage(pictureBox1, selectedRectangle);
using (var bm = new Bitmap((int)result.Width, (int)result.Height))
{
    using (var g = Graphics.FromImage(bm))
        g.DrawImage(pictureBox1.Image, 0, 0, result, GraphicsUnit.Pixel);
    pictureBox2.Image = (Image)bm.Clone();
}

下面是输入图像：

这就是结果：

一个专门的类，提供一些帮助工具来确定选择的缩放因子，并将选择坐标转换为缩放的Bitmap坐标。
此 * 版本 * 仅适用于缩放图像。

ZoomFactor类提供了这些方法：
PointF TranslateZoomPosition(PointF Coordinates, SizeF ContainerSize, SizeF ImageSize)：

返回容器内Point位置的PointF转换坐标到位图内的Point位置，在容器中缩放。

RectangleF TranslateZoomSelection(RectangleF Selection, SizeF ContainerSize, SizeF ImageSize)：

返回一个RectangleF，表示在容器内创建的选择，转换为位图坐标。

RectangleF TranslateSelectionToZoomedSel(RectangleF SelectionRect, SizeF ContainerSize, SizeF ImageSize)：

返回一个**RectangleF**，表示原始位图的预选区域，该区域转换为容器内的缩放选择图像。

PointF GetImageScaledOrigin(SizeF ContainerSize, SizeF ImageSize)：

返回容器内缩放图像原点坐标的PointF引用。

SizeF GetImageScaledSize(SizeF ContainerSize, SizeF ImageSize)：

返回在Container中缩放时Image的SizeF引用。
示例用法，演示如何使用在Container控件内创建的Rectangle选择裁剪Bitmap。TranslateZoomSelection方法返回与选择区域对应的Bitmap部分：

ZoomFactor zoomHelper = new ZoomFactor()
Bitmap originalBitmap;

RectangleF currentSelection = [Current Selection Rectangle];
RectangleF bitmapRect = zoomHelper.TranslateZoomSelection(currentSelection, [Container].Size, originalBitmap.Size);

var croppedBitmap = new Bitmap((int)bitmapRect.Width, (int)bitmapRect.Height, originalBitmap.PixelFormat))
using (var g = Graphics.FromImage(croppedBitmap))
{
    g.DrawImage(originalBitmap, new Rectangle(Point.Empty, Size.Round(bitmapRect.Size)), 
                bitmapRect, GraphicsUnit.Pixel);
    [Container].Image = croppedBitmap;
}

上述行为的示例：

注意：* 示例中，Portrait中图像的预选将Width和Height反转 *
ZoomFactor类：

public class ZoomFactor
{
    public ZoomFactor() { }

    public PointF TranslateZoomPosition(PointF coordinates, SizeF containerSize, SizeF imageSize)
    {
        PointF imageOrigin = TranslateCoordinatesOrigin(coordinates, containerSize, imageSize);
        float scaleFactor = GetScaleFactor(containerSize, imageSize);
        return new PointF(imageOrigin.X / scaleFactor, imageOrigin.Y / scaleFactor);
    }

    public RectangleF TranslateZoomSelection(RectangleF selectionRect, SizeF containerSize, SizeF imageSize)
    {
        PointF selectionTrueOrigin = TranslateZoomPosition(selectionRect.Location, containerSize, imageSize);
        float scaleFactor = GetScaleFactor(containerSize, imageSize);

        SizeF selectionTrueSize = new SizeF(selectionRect.Width / scaleFactor, selectionRect.Height / scaleFactor);
        return new RectangleF(selectionTrueOrigin, selectionTrueSize);
    }

    public RectangleF TranslateSelectionToZoomedSel(RectangleF selectionRect, SizeF containerSize, SizeF imageSize)
    {
        float scaleFactor = GetScaleFactor(containerSize, imageSize);
        RectangleF zoomedSelectionRect = new
            RectangleF(selectionRect.X * scaleFactor, selectionRect.Y * scaleFactor,
                       selectionRect.Width * scaleFactor, selectionRect.Height * scaleFactor);

        PointF imageScaledOrigin = GetImageScaledOrigin(containerSize, imageSize);
        zoomedSelectionRect.Location = new PointF(zoomedSelectionRect.Location.X + imageScaledOrigin.X,
                                                  zoomedSelectionRect.Location.Y + imageScaledOrigin.Y);
        return zoomedSelectionRect;
    }

    public PointF TranslateCoordinatesOrigin(PointF coordinates, SizeF containerSize, SizeF imageSize)
    {
        PointF imageOrigin = GetImageScaledOrigin(containerSize, imageSize);
        return new PointF(coordinates.X - imageOrigin.X, coordinates.Y - imageOrigin.Y);
    }

    public PointF GetImageScaledOrigin(SizeF containerSize, SizeF imageSize)
    {
        SizeF imageScaleSize = GetImageScaledSize(containerSize, imageSize);
        return new PointF((containerSize.Width - imageScaleSize.Width) / 2,
                          (containerSize.Height - imageScaleSize.Height) / 2);
    }

    public SizeF GetImageScaledSize(SizeF containerSize, SizeF imageSize)
    {
        float scaleFactor = GetScaleFactor(containerSize, imageSize);
        return new SizeF(imageSize.Width * scaleFactor, imageSize.Height * scaleFactor);

    }
    internal float GetScaleFactor(SizeF scaled, SizeF original)
    {
        return (original.Width > original.Height) ? (scaled.Width / original.Width)
                                                  : (scaled.Height / original.Height);
    }
}