To make small size images for digital works, do I change resolution, pixels dimension or document size?

Question

Here's the thing. I have a 300 DPI image ready for printing. I need a small version of it to use in a digital book. When I say small I mean two things: I will reduce its display size and I want it to become a small file. In Photoshop I have some options.

1. Change DPI without resample the image: It doesn't help because the file size remains the same.

2. Change DPI resampling the image: It will result in a small file size but the pixel dimensions will reduce more than I want it to. For example, an image with 2000 x 2000 will became something less than 800 x 800.

3. Don't change DPI and reduce the pixel dimension directly: Ok, now I can get it the size I want BUT I don't know if this is right thing to do. The image will keep its 300 DPI but the pixels will be reduced. Is it right?

4. Change the document size: I think the effect is the same as the third one above. Size and pixels will be reduced but the resolution won't change.

We always read that we don't need 300dpi images for e-books and web and we should use 72 instead. My ONLY concern here is to produce a small file that will cause my e-book to have an acceptable size to be downloaded. So, I don't care if the resolution remains 300 DPI, as long as the final size is reduced. So, the third and fourth option seems to to that, but I need a more advanced opinion.

Answer 1

300 DPI and 72 DPI have become a sort of short-hand for "high quality" and "low quality" or "print" an "web"... none of that is really true though. The DPI or PPI value of an image file is meta data, nothing more; it doesn't affect the quality or anything else... It is only a useful measurement when combined with a physical size.

So all that really matters is the size in pixels. Resize your image to the size that you need it in pixels and your image is good to go. If you do need your images to be 72 PPI then you can do that after resizing. So in Photoshop you would...

1. Go to Image → Image Size...

   Set your desired pixel size with "Resample" checked and hit OK.

2. Got Image → Image Size... again

   Uncheck "Resample" and change the resolution (PPI) to 72. Hit OK again.

Your image is then at the correct pixel size and PPI.

It's worth noting that Photoshop's Save for Web always saves the file at 72 PPI, so you can just change the pixel size and use that (I still use CS6 so I'm not sure how Photoshop CC's new export features work in regards to that).

Answer 2

How are you generating the e-book file? You might be able to resize the images automatically during the export process. For example, in InDesign, this would be under the "Conversion Settings" tab. (see attached screenshot).

That way, you can keep the high quality versions of your images in the layout. Then control the image resolution depending on your output (epub, pdf for printing, etc).