Title says it all. When I change my image from 72dpi to 300dpi, the image gets smaller. Can I scale the image back up and print it out at 300dpi without sacrificing quality?
Every digital image has a specific size: the width and height in pixels. The amount of information depends on that.
In digital image files, the number of pixels per inch is just a hint. It indicates a proportion that should be used for calculating the actual size of the image when printed.
If you have an image of 1000x1000 pixels and you print it at 100ppi, the final size in paper will be 10x10 inches. If you print the same image at 10ppi, the size will be 100x100 inches.
Simply changing the ppi (or dpi) field doesn't really add/remove pixel data. Your software should not delete any detail when changing from 72ppi to 300ppi. Similarly, it can't add detail.
If your software allows you to change the ppi without changing the final print size, then it surely does it by adding/removing pixels. It might add pixels by interpolation, which doesn't really add detail, but helps to avoid the big squares (pixels) that you see in badly magnified digital images.
To summarize, if you want to get image details out of nowhere by twiddling with the ppi field, the answer is: "No, you can't"
On the other hand, if you want to print a low-resolution image in a big size, you can increase the resolution by adding interpolated pixels, and then print in high ppi. This way you will avoid the ugly big squares. That said, any modern equipment should do that for you when printing in low ppi.
You can scale it up to a certain limit with acceptable quality. If the original image is very high quality you can do up-sampling with great success. The printer and image viewer will do it, but you want to control how it is done. If you are going to process the image and add effects, upsample first, so the added things will be the true output resolution.
Here is an example why you want to control how it is done. From top to bottom: 1. original 2. window auto resize 3. bicubic 4. lanzcos
I would call no. 2 downright catastrophic. 3 and 4 are very similar, with 4 preserving the contrast between the black and grey text better.
A better question would be what is an acceptable DPI for printing. The DPI measurements within an image file are for all intents and purposes an imaginary value in the digital world. An image has a fixed number of horizontal and vertical pixels. The DPI number just tells a printer (if the driver isn't used to override it) what the printed size of the image should be. It's just a way of equating how big each pixel should be rendered in the real world.
Changing the DPI has no impact on image quality unless you choose an option to keep the height and width (in a physical measure) the same, in which case it will have to interpolate new pixel information from the old which will actually result in a loss of quality as many of the original pixels will be lost in the approximation. Keep in mind that interpolation makes an image that looks smoother, but actually lowers the image quality so it should be used with care.