We often hear about the size of photos / Digital Images, about whether one image “weighs” more or less than another, or whether one photograph has a higher or lower resolution than another… weight, size, resolution… are concepts that are used in an imprecise way, and we tend to forget their true meaning.
In this article we will try to clarify each of these concepts, as well as how they relate to each other and discovering photo size.
The size of Digital Images
Digital images are made up of elementary image units called pixels. We cannot use conventional measures of length (mm, cm…) as an indicator of image size because, as they do not have physical entity, a digital photo can be reproduced at different sizes on different devices (screens, printers, etc.).
Therefore, the best indicator of photo size is simply the number of pixels that make it up. As all pixels in an image have the same shape and size, we will indicate the size of a digital image like this:
For example, this image has a size of 24 pixels (6 x 4):
Although they are related, image size should not be confused with file size, which is the space (usually expressed in Kb or Mb) occupied by the computer file containing the image information, and is what we colloquially refer to when we talk about the “weight” of an image.
Resolution of a digital photo
The resolution is defined as the number of pixels per unit of length.
It is important to note that when we talk about resolution, we are not referring to any “physical” property of the image: it is merely a decision made by the operator, which we attach to the image information.
Indeed: we have an image and DECIDE that 100 pixels will fit in 1 cm, for example. The resolution will be 100 pixels/cm. If we DECIDE that 50 pixels will fit in 1 cm, the resolution will be 50 pixels/cm. In that second case, the resolution is lower but the image is the same, the “change of resolution” does not affect the size itself, that is, the amount of pixels that make it up, but rather how it will be printed.
That is, in reality, resolution is just information that we attach to the image when we are going to print it. Resolution does affect the print size. How? Let’s take a look at it.
We have defined resolution as the number of pixels per unit of length. In the previous examples, we talked about pixels/cm, however, the standard unit of measure for the resolution of a digital photograph is pixel per inch or ppi.
Okay. Let’s suppose we have a digital photo with a size of 1200 x 600 pixels, at a resolution of 200 ppi; that means that in one inch (2.54 cm), we can align up to 200 pixels. To fit the 1200 pixels corresponding to the width of our image, we will need:
1200 px / 200 px/inch = 6 inches = 15.24 cm.
And the same for height:
600 px / 200 px/inch = 3 inches = 7.62 cm.
Now we decide to change the resolution to 300 ppi.
Does this change the image size? NO, IT DOESN’T!
Does it change the print size? YES, IT DOES! Let’s see:
Now, we want to “draw” 300 pixels in one inch, therefore to fit the 1200 pixels of width of the image, we will need:
1200/300 = 4 inches = 10.16 cm.
And for the height:
600/300 = 2 inches = 5.08 cm.
The image has a higher resolution, but it will be reproduced at a smaller size (before 15.24×7.62 cm, now 10.16×5.08 cm).
This can be deduced from the definition of resolution and these examples.
HIGHER RESOLUTION MEANS SMALLER PRINT SIZE, AND VICE VERSA.
Many times, resolution is mistakenly used as a synonym for QUALITY of an image. This is a significant mistake that we must avoid because as we have seen resolution is more like “pixel density,” it gives us an idea of how sharp the details are in an image, but nothing more.
The concept of quality is more qualitative and is usually understood as the sum of resolution, size, contrast, and other technical aspects.
Digital Images Sizes for Printing
How big can I print a photo? Or is the size of my photo big enough?
To answer these common questions, you need to know the resolution of your photo and the available print sizes.
Resolution refers to the number of pixels captured in your photo. The number of pixels will determine the ideal and maximum size you can print. This page provides a guide to the resolution requirements for the most common print sizes.
The most common frame sizes and photographic prints are described in inches. This page provides a one-stop shop for the most common print sizes and helps you navigate the differences between inches, cms and ISO.
And that’s the end of the post. Our goal has been to make clear the difference between image resolution and size, concepts that are often confused (just search for “image resolution” on Google, and you’ll see how much confusion there is about it). In this article, we’ll show you how to manage the size of an image in Photoshop, the most popular photo editing program.
