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I make personalised story books for children. My books use a photo of the child's face inserted into the illustration to make them the main character in the story.

When my customers send me the photo of their child, I try to explain to send me a hi-res photo and not to download from social media, but often they send very low-quality photos, which will not be accepted by my printer who needs at least 300DPI for these 8 x 10 inch books (the child's face is around 3x4cm when printed).

I could get my website to only allow a certain filesize, but I know that isn't always a true measure of whether the photo will look good. For example if the photo is a close up of the child's face, it can be a smaller filesize than where the child is in the background of the photo.

Can anyone explain the parameters I need to consider to make sure the face prints correctly (especially if I were to have this coded so my order form would check the image's suitability), and how best to explain this to my customers?

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  • Don't mention DPI/PPI at all. Tell them precisely the number of pixels. If you want something suitable for 3x4 cm, tell them 354 x 472 pixels. Raphael's point about some sort of info re: cropping is a good idea. Heck, put up a blank white image that size and call it a "template" they can paste the face into. – Yorik Mar 1 '16 at 20:34
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Simply in general you can not. However the 300 dpi (or PPI to be exact) is far less fixed than people tell you*, 200 DPI could be ok. Certainly, better than nothing. You could try to expand the image. The printer, kind of, does this if you dont do it. But by doing it yourself you can sometimes achieve better solutions.

* It is lies to children of sorts.

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  • Re: lies to children: I once had to pick up an image from an older project I had used at full bleed 8.5 inches square. It was at this point, after publication, that I noticed it would only have been 300dpi at about 3 inches. It looked fine in the book, no idea why. Kind of a "wtf" for two reasons. – Yorik Mar 1 '16 at 20:28
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This is where you need to build validation on when you receive source files. Whoever built your site can implement a validation uploader in the contact form unless you built it. If you built your own site or use a template site hosting solution you could do a Q&A before you send it to the printer with ImageMagick or if you use Mac use Sips. If the source files are being sent through FTP depending on how you pick them up you could even script a review script in shell. One thing I always explain to client when I receive files is I can always scale down but I cannot scale up.

Sips Example

sips -g dpiHeight -g dpiWidth

ImageMagick Example

convert image -depth 300 -units pixelsperinch resultimage
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Ok. As Joojaa say, you do not need the 300ppi (the correct unit is ppi, not dpi)

I would say that you can use inclusive lower resolution images in extreme cases if you are printing them directly. Agree that 200 ppi will do fine.

If the child's face is about 4cm that is 354x472 pixels or 236x315 px at 200ppi which is really small!

But that resolution (300, 200 ppi) is not the resolution you need to ask to your customers.

You need to ask in terms of image size so that is easier to understand. Use Megapixels, which most people is familiar with. For example, a low end camera of 6 Megapixels, makes a photo of 3000x2000, if you post an example image of the typical frame you need, you have an excelent resolution and a more standarized framing.

Most newbie parents frame like this:

With a 6Mpx camera you have a head after cropping like 3000/6 = 500px, which is good size.

But if you ask this framing, there is no mistake.

As I did on this post, explain in more simple terms the file size you need, not technical units, which are confusing for most people.

(P.S. you can always apply some shaprening on the image, so the details of that small face shows.)

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