Pixels, image size, and image resolution in Photoshop

What are pixels?

term pixel Short for "picture element," pixels are the tiny building blocks that make up all digital images. Much like how a painting is made from individual brush strokes, a digital image is composed of individual pixels.

In Photoshop, when viewing an image at a normal zoom level (100% or less), pixels are usually too small to notice. Instead, we see what looks like a continuous picture, with light, shadows, colors, and textures all blending together to create a scene much like in the real world ( picture From Adobe Stock):

A closer look at pixels

But like any good magic trick, what we see is really an illusion. And to break the illusion, we just need to look closer. To view individual pixels in an image, all we need to do is zoom in. I will choose Zoom tool From the toolbar:

Next, I'll tap several times on one of the woman's eyes to make it bigger. Every time I click it zooms in. And if you zoom in close enough, we start to see that what looks like a continuous image is really a bunch of little squares. These squares are the pixels:

And if you zoom in even closer, we see that each pixel displays one color. The entire image is really just a grid of solid color squares. When viewed from far enough away, our eyes blend colors together to create an image with lots of detail. But up close, it's the pixels that make up our digital world:

pixel grid

Note that once you zoom in enough (usually above 500%) you start to see a light gray outline around each pixel. that's it PixelGrid In Photoshop, they are only there to make individual pixels easier to see. If you find Pixel Grid distracting, you can turn it off by going to Menu Display In the menu bar, choose show , Then choose pixel grid . To turn it back on, just select it again:

Zoom out to view the image

To zoom out from pixels and view the entire image, go to List Display And choose Fit on Screen :

Now that the image has been scaled down, the individual pixels are again too small to notice, and we're back to seeing the illusion of a detailed image:

What is the image size?

So now that we know that pixels are the little squares of color that make up a digital image, let's look at a related topic, Image size . Indicates Image size Returns the image's width and height in pixels. It also indicates the total number of pixels in the image, but it is actually the width and height that we need to care about.

Image Size dialog box

The best place to find image size information is in Photoshop's Image Size dialog box. To open it, go up to picture menu and selection Image size :

in Photoshop CC The Image Size screen displays a preview area on the left and details on the image size on the right. I'll cover the image size dialog for now, we'll just look at the information we need:

pixel dimensions

The width and height of an image, in pixels, is known as with their pixel dimensions , and in Photoshop CC, we can display it next to the word Length near the top of the dialog box. Here we see that my image has a width of 4509 pixels (px) and a height of 3000 pixels:

If dimensions are displayed in a measurement type other than pixels, such as inches or percentage, click the small arrow next to the word Dimensions and choose Units pixel From the list:

This tells us that my image has 4509 pixels from left to right, and 3000 pixels from top to bottom:

Find the total number of pixels

To find out the total number of pixels in the image, we just need to multiply the width and height values ​​together. So in this case, 4509 x 3000 = 13527000, or roughly 13.5 million pixels. You don't really need to know the total number of pixels. But as you gain more experience resizing images, you'll find that knowing the total number of pixels beforehand will give you a good idea of ​​how large an image you can print, as we'll see next when we look at the image resolution.

What is the resolution of the image?

So, if it pixels They are the little squares of color that make up all digital pictures, and it was Image size It is the number of pixels in the image from left to right (width) and top to bottom (height), so what are they Picture Resolution ؟ Image resolution Controls how big or small the image will be prints Based on the current image size.

It is important to understand in advance that the resolution of the image affects only the size of the copy printed From the picture. It has no effect whatsoever when the image is displayed on the screen. I cover this topic in more detail in my tutorial on 72 dpi for web resolution, and we'll look at it again at the end of this tutorial.

Width, height and contact resolution

In the Image Size dialog box, if you look down on the word "Dimensions," you'll find Display ، Height و resolution domains. This is where we can not only view the current settings, but also change them:

Resample option

Before we go any further, if you look under the Resolution value, you will find another important option called Resample . And by default, Resample is on. We'll learn all about the Resample option when we look at how to resize images. But in short, Resample allows us to change the number of pixels in the image:

Why would you want to change the number of pixels? If your current image size is too small to print your image at the size you want, you can use Resample to add more pixels, known as shorthand . Or, if you want to email your image to friends or upload it to the web, and the current size is too large, Resample will let you reduce the number of pixels, otherwise known as shorthand .

Again, we'll learn all about downsampling and downsampling when we look at how to resize images. For now, to see how resolution affects the print size of the image, uncheck Resample to turn it off:

Change the print size, not the image size

Once Resample is turned off, you will notice that the scaling type for the Width and Height values ​​changes. Instead of displaying the width and height in pixels like I had a moment ago, I'm seeing them now in inches . And instead of being told that my image is 4509 pixels wide by 3000 pixels tall, I'm now told that it's 15.03 inches wide by 10 inches tall:

In fact, if you click the measurement type box for either Width or Height, you'll notice that the pixel is now grayed out and unavailable. This is because, with Resample turned off, we cannot change the actual number of pixels in the image. All we can do is change the size of the image that will print Print size is usually measured in inches (or centimeters depending on where you are in the world):

How does image resolution work?

Resolution controls the print size of an image by setting the number of pixels to be squeezed into each inch of paper, both vertically and horizontally. This is why the precision value is measured in pixels per inch or dpi .” Since an image has a finite number of pixels, the more pixels there are together on the paper, the smaller the image will be printed.

For example, the resolution value is currently set to 300 pixels/inch. This means that when I go to print the image, 300 pixels wide, 300 pixels high, it will be compressed into every square inch of paper. Now 300px may not seem like much. But remember, it's 300% of each supply و Height. In other words, it's 300 Once 300, for a total of 90.000 pixels per square inch:

How to determine the print size

To find out what size an image will print, all we need to do is divide its current width and height, in pixels, by the resolution value. If we look again at the Dimensions section at the top, we'll notice that my image is still 4509 pixels wide:

If we divide 4509 by the current precision value of 300, we get 15.03. In other words, the width of my image when printed will be 15.03 inches, which is the exact value shown in the Width field:

Going back to the Dimensions section, we see that my image's height is still 3000px:

If we divide 3000 by the current resolution of 300, we get 10. Which means the height of the image, when printed, will be 10 inches, just as it appears in the height field:

Changing the resolution changes the print size

If we change the resolution value, the number of pixels in the image does not change, but the print size does. Note that if you lower the resolution from 300 pixels/inch to 150 pixels/inch, the pixel dimensions remain the same at 4509 pixels by 3000 pixels. But the width and height both increase. Since I'm only going to squeeze half the number of pixels per inch onto the paper, both horizontally and vertically, I double the width and height:

Changing the print size changes the resolution

Since all we change is the print size, changing the width or height will change the resolution. In fact, when the Resample option we looked at earlier is turned off, all three values ​​(width, height, and resolution) are mapped together. Changing one of them will automatically change the others.

If you lower the Width value to 10 inches, then to keep the image's aspect ratio the same, Photoshop automatically changes the Height value to 6.653 inches. And to fit the entire image into the new, smaller print size, the pixels had to be tighter, so the resolution value was increased to 450.9 pixels/inch:

Does image resolution affect file size?

A common misconception about image resolution is that it somehow affects the file size of the image. Many people think that before you email an image or upload it to the web, you should reduce its resolution to make the file smaller. This is simply not true. Since changing the resolution does not change the number of pixels in the image, it will have absolutely no effect on the file size.

If you look next to the words Image Size at the top of the dialog box, you'll see a number, usually shown in megabytes (M). In my case, it's 38.7 million. This number represents the size of the image in your computer's memory. When you open an image in Photoshop, the image is copied from your hard drive, decompressed from whatever file format it was saved in, and then put into memory (RAM) so you can work on it faster. The number displayed in the Image Size dialog box is the actual, uncompressed size of the image:

Lower resolution vs file size

It is easy to establish that image resolution does not affect file size. Just keep an eye on the size while changing the resolution. As long as the Resample option is turned off so that you don't change the number of pixels in the image, no matter what you choose for the resolution value, the file size on top will always be the same.

Here, I've lowered the resolution from 300 pixels/inch all the way down to 30 pixels/inch. With so few pixels crammed into an inch of paper, the print size increased to 150.3 inches by 100 inches. But even with this much lower resolution, the size of the image in memory remains unchanged at 38.7M:

Higher resolution vs. file size

And here, I've increased the resolution up to 3000 pixels/inch. This reduces the print size to just 1.503" x 1", but again has no effect on the file size, which is still 38.7M. The only way to reduce the file size of an image is to either reduce the number of pixels in the image (using the Resample option), save the file in a format that supports compression (such as JPEG), or both. Simply changing the print resolution will not change the file size: