Résolution / définition image

You have run into the least-understood topic in all of design. It's not you, it's the terminology.

The first problem is that the word "resolution" has more than one meaning. These meanings are frequently confused, which leads to nonsense advice that just adds to the confusion. It might help if, as you read the following, you sketch out the information on a notepad.

In this context, there are two definitions  of "resolution" that you need to be concerned with:

1) the total number of pixels in the image ("4 Megapixels," for example, or "1920 x 1080 px");

2) how densely the pixels will be placed on a page when they are printed, usually shown as "ppi." (You will see "300 ppi" recommended for printing, for example.)

The important thing to know is that definition (1) applies to any digital image and does not change unless you manipulate the image to increase or decrease the total number of pixels (by enlarging, shrinking, or cropping in Photoshop, for example). There is no "per inch" or "per cm" in this definition. It is an absolute value.

The image metadata may have a number in pixels per inch (InDesign calls this "Actual ppi"), but that is literally just a number. It has no effect on how large the image appears on a screen. 500 pixels have a certain size on a given screen, regardless of what the metadata says. (In this case, the screen has a fixed number of pixels per inch. The image complies with the screen because it has no choice.)

Definition 2 ("pixels per inch") in InDesign applies only to print. The ppi number in the metadata is there to translate the total number of pixels into a print dimension in inches or centimetres. If you print the image from a photo viewer app, this is the size it will be. InDesign calls this "Actual ppi," which is very misleading.

More helpfully, InDesign tells you "Effective ppi," which means: "If you print this image at the size you have specified in this document, there will be [x] pixels for every inch of paper." If you have accidentally or deliberately distorted the image, InDesign will show a different "effective ppi" for height and for width, which are how many pixels will be printed per inch of paper in each dimension. This is a ratio, not a fixed value, but it is important if your layout will be printed: too low a "ppi" will look fuzzy or pixelated on the page.

Dots per inch refers to how densely a printing device (inkjet, laser, offset) puts ink onto paper. A desktop inkjet or laser printer may be able to put 1200 dots per inch or paper, where a very large format printer may only do 100 per inch and an imagesetter (which makes printing plates or negatives using a laser) may put 2800 dots per inch. This term is often confused with "pixels per inch," but they are not the same thing at all.

It should come as no surprise that "image size" also has two definitions. One is pixel dimensions and is similar to definition (1) of "resolution." An image that is 4000 pixels wide by 5000 pixels tall contains 20 millon pixels, so its size is "20 Megapixels."

The other common definition of "size" is: the amount of space the image will take up on a storage device like a hard drive. We measure that in bytes, kilobytes, megabytes, etc. Our 20 Mpx image might use anything from 15 MB to 1.5 GB of storage space, depending on the type of file: jpeg, raw file, TIFF, PSD, or some other format.

I hope all that is clear and useful. Good luck!

La résolution d'une image est le nombre de pixels dans une dimension physique, par exemple 300 pixels par pouce (ppp) ou 120 pixels par cm. En français, on utilise improprement la notion de DPI (dots per inch) qui correspond réellement à ce qu'on devrait nommer PPP (pixels par pouce). La taille en octets dépend du nombre de pixels mais aussi du format de fichier dans lequel l'image est enregistrée : certains formats permettent de compresser le poids d'une image.

Ce qu'il est important de savoir, comme l'a écrit @rob day c'est que si une image est réduite (par exemple à 50 %) dans InDesign, sa résolution double. À l'inverse, si une image est agrandie (par exemple à 200 %), sa résolution est réduite de moitié. Le nombre de pixels d'une image reste constant, mais c'est leur répartition dans une dimension physique qui change.

On a longtemps dit (faussement) que la résolution de sortie d'une image devait être de 300 pixels par pouce pour être imprimée correctement, mais en fait, pour la plupart des travaux d'impression courants, une résolution de 255 PPP suffit ; et comme un pouce équivaut à 2,54 cm on peut dire qu'une résolution de 100 pixels par cm permet d'obtenir un résultat correct. Il suffit donc de connaître la taille d'une image en pixels pour savoir à quelle taille elle sera imprimable : une image de 4000 pixels permettra d'être imprimée à 40 cm, une image de 200 pixels ne permettra de produire qu'une image imprimée de 2 cm, etc.

Hi @e.l , Image resolution is referred to as Pixels Per Inch (PPI). When you place an image in InDesign its resolution and scale info is listed in the Link Info panel when the image is selected—the Actual PPI, Effective PPI, pixel Dimensions, and Scale:

Actual PPI is the image resolution saved with the image. The above image was saved with an output dimension of 8" x 5" so its Actual PPI is 400—the pixel width divided by the output width as inches: 3200/8 = 400.

Effective PPI is the output resolution after scaling. The 8" wide image is scaled by 60%, so its output width is now 4.8" and its scaled output resolution is 667 PPI—3200/4.8=666.66.

The link’s Image Size in Photoshop: