How large can I print an old illustration?

Question

quick question. I've got a bunch of illustrations from the 1400's-1500's, and I would like to know how to figure out how large I can print this on art paper.

The purpose is for art prints, they're all multiple sizes and most came to me in the size of a book page. I am aware they should be around 300ppi and I should divide W&H by PPI to get the optimal size, however, I've seen much larger versions of these images on prints (an image that was 3000x2500 pixels on a print of about 40x60 cm) which looked good.

Right now I've been just resizing the images with PS, keeping the PPI at 300 and increasing the size in cm (at cost of file size), however, I have no way to corroborate how good or bad these prints will look and how large should I actually be printing them at.

Thank you all for your help!

Answer 1

The 300 ppi is a norm derived from colour printing with offset, where the images had to be transformed to grids of separate points. Iirc, those grids were closer to 150 dpi, which corresponds to 300 ppi (1 coloured point + 1 space). Also, the result was expected to be viewed from normal reading distance (~30cm), where the human eye just can't distinguish the individual dots.

That means you should aim for 300ppi for photo prints looked at from ~30cm on smooth paper, and progressively lower when the viewing distance increases. As soon as there is any texture in the paper, you can go to somewhat lower ppi (Some say you can go as low as half the recommended ppi on canvas, which has a lot of surface texture).

If you increase the resolution in photoshop to make larger prints, quality will go down (details will start blurring), as you just don't have the information there to go to the higher pixel counts. But if you have to do that, some extra sharpening after the enlargement can help.

I understand you already have the files you have to use, but starting from the originals, you can get sharp prints larger than the original by e.g. scanning at a higher resolution. But this will also enlarge any defaults present in the originals.

And of course, to judge the quality of the resulting prints, you can just print a standard 4"×6" from an interesting part of your final image file.

Answer 2

There are different topics here.

Forget the PPI for a moment.

1. What is your starting file size in pixels? What is the resolution of your scans, or photos?

This is the main issue here. Of course, if your original files are big enough you can print them big enough.

If they are not, you probably need to make a new scan or use a good DLSR, good illumination, lens and setup to have a good image.

A 10 - 24 Mpx is a typical resolution these days and can give you a good result depending on the photo of course.

2. Do you really need to increase the pixel count?

Right now I've been just resizing the images with PS, keeping the PPI at 300 and increasing the size in cm (at cost of file size)

Regarding this point, I only recommend that you resample at the exact 200% in pixels using some bicubic interpolation and apply some sharpening. In my opinion, this maximizes the way the resampling uses the original pixel information.

There are some programs that might be useful using a different approach, but you need to try them first and see if it works for the type of image you have. They can potentially get rid of important details like textures, so you probably want to stick with a normal bicubic one.

3. What are the viewing distance and final print size?

In my opinion for an image that is going to be viewed at 1m, and has 1m on the larger size, 100ppi is good enough. The pixel is going to be around 4 pixels per mm (3.9). Try to accomplish this using the original resolution. If it is too low then you can try resampling it.

The image you mentioned 3000px/60cm gives you around 5 pixels per mm.