Java croping and image?

I have a scaled down image and have a cropping rectangle to select the new cropped image on a form. But I want the crop to come from the original image. How do I convert the selected region on the scaled down image to the original image that is not scaled down? This is being done with java. I don't want to use the original image with the selection rectangle because the image may be to big for the screen if it is of good quality. Thank you in advance.