Is it better to resize an image by minus 10% or by minus 50%?

Question

I need to draw an illustration in Photoshop that will be used as a book cover both for the hardcover and the paperback edition. The hardback edition is 22 cm (2598 px at 300 dpi) high, the paperback 19 cm (2244 px), discounting margins.

As I see it, I have two options:

1. Create an illustration that fits the larger hardback cover (2598 px) and resize the image by 14 % (or to 86 %) for the smaller paperback print size.

2. Create an illustration that is larger than both print sizes (e.g. 150% or 200% of the hardcover size) and resize that image for both covers.

From reading around on the web, including answers on this site, I got the impression that resizing an image by -15% should result in a higher output quality than resizing by 40 or 50%. Is that correct? The information was often highly technical and English isn't my native tongue, so I had quite some difficulty understanding the often very detailed answers.

Answer 1

I seriously think you are worrying about some very miniscule technical difference on the pixel level that won't be visible to the human eye on print.

Downscaling an image is the same as removing information, so whether you downscale to 99%, 85% or 50% you are reducing the quality of the image no matter what. So I'm not sure what kind of "quality" you are referring to. You can make some tests to compare scaling from different sizes down to the same smaller size and see with your own eyes if you can see any differences in quality. If the pixels look good and the image is at the right resolution, there aren't any "hidden" quality problems really.

But "loosing quality" shouldn't worry you because you only need the image to be at 300 ppi in the physical size its printed. Anything above that is generally not possible to reproduce anyway because of the way printers convert pixels to halftone or stochastic screening. Besides that, even if some printer is capable of reproducing higher resolution than 300 ppi, the details might not even be visible to the average human eye anyway.

Generally when working on digital images, photographs or drawings, I would always recommend to work at a higher resolution than the final size. It just makes editing easier. Pixel based tools work more precisely and when downscaling to the final size, small pixel errors will disappear. The downscaled image also gets a crispness which I like. And by working at a larger size you have made sure that if the designer suddenly wants to change the dimensions of the book, your image is still usable.

In your case I would create the original at perhaps 150-200% of the largest needed size as you mention. Then send it to the graphic designer who will place the image in their layout program and let it downscale the image to 300 ppi on export. Or maybe they will just leave it at whatever resolution it has and let the print shop decide whether to downscale or not.

Even if you chose to just make the image 100% of the largest needed size, I doubt that you would be able to tell much difference on the final print.

Answer 2

Unless you have an explicit specification to produce an image with exactly 300 dpi, I wouldn't worry too much about the actual resolution, as long as it's more than the specified 300 dpi (but not insanely more).

Likely, you don't even need to set the exact pixel resolution in the image (i.e. those dpi): it is the job of the designer who creates the final print to place and scale the image appropriately. Scaling will probably happen anyway due to the required margins and other printing requirements. Therefore, you can send the same image in both cases.

Only in some professional workflows the printers/designers will respect the dpi (and thus physical size) you set in the image, and this is always explicily stated in the requirements. In this case, I would just set the required physical size without resampling the image (there is a special checkbox for it in the Photoshop Resize window), and wouldn't mind the resulting odd dpi (say, 324) - unless, as I said, there is another explicit requirement to have a fixed dpi.

Now I will address the common resampling question, even though it has minor relevance in your particular case. But it's always good to be aware of the potential issue, and good that you take notice of it.

I got the impression that resizing an image by -15% should result in a higher output quality than resizing by 40 or 50%. Is that correct?

This depends on the resampling algorithm used. The main issue here is that when you downsample a raster image, it usually loses subjective sharpness. To combat this, several tactics have been devised.

1. One is to resample and then add a little sharpening (e.g. unsharp mask).

2. Another is to do stepwise resampling in multiple steps of 10-20%. It can be shown (subjectively and mathematically) that this will preserve more details and will produce sharper result compared to just a single normal resample step.

3. Yet another is to apply a special interpolation algorithm that adds sharpness while resampling.

Photoshop (as well as most specialised software) offers all of these methods. If you open the Resize window, you'll see a selector for the resamplig method. Normally, it is set to "Bicubic Automatic". This means it will use "Bicubic Sharper" when downsampling, i.e. the third method above.

Now, if you "naively" heed the common advice and downsample multiple times by 15% with default settings, you'll usually get an oversharpened image with ugly artefacts. This is because you effectively combined methods 2+3.

Method 2 works, and sometimes works better than 3, but only if you use the "normal" unbiased "Bicubic" resampling method.

Of course, there may be cases where even a heavier combination (or entirely different methiods) may be preferable, e.g. line art or particularly blurry photos. But in most practical cases, nowadays, you shouldn't worry too much when resizing, say, ±50% or even more in one step with default settings (using good software).

Answer 3

My 2 cents.

I. 300PPI?

There is a huge misconception about the 300PPI resolution on images. I have no idea if you need 300PPI, you could need more, you could need less.

Your image will be most likely further transformed on some additional processes, for example, when screened, and this is also determined to print quality, lineature, and type of paper used on the cover.

The range could be from, let's say 200PPI to 400PPI. As you can see, the 14% scaling is not that important as you first thought.

II. The illustration

There are many types of illustrations. There are vector-based, they can be paintings, they can rely on textures (for example watercolor) they can be lines, they can have brushstrokes...

If an illustration relies on textures, and these textures need to be at a specific size, I would not do them at a very different scale. Imagine an impressionist painting. The texture is as important as the overall image. Scaling it down "destroys" the real deal illusion.

III. Are you sure you do not need an additional size?

If the book turns into a success... Are you sure you do not need the illustration to be printed in a poster-sized image for promotion?

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If your computer can handle bigger sizes, I would prepare the canvas at the largest size I can easily work with. I like round numbers. I like the size of photos of 24Mpx, so a nice number is 6000x4000px, or whatever proportion you need. Again it depends on the brush size, or de details, or the style.