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In this movie, I'll introduce you to the Image Size command which allows you to change both the image size and the resolution of a Photoshop document. I'm looking at that same low res artwork. However, this time it's a flat file without any layers or layer comps. Now let's say I want to change the pixel dimensions or the resolution. I'd go up to the Image menu, and choose the Image Size command, or you can press the keyboard shortcut Ctrl+Alt+I or Command+Option+I on the Mac. Notice that this dialog box is divided into three parts; at the top we have the Pixel Dimensions, our Width of 918 pixels, and a Height of 632 pixels.
The two values are linked to each other because the Constrain Proportions check box is turned on. Were I to turn that value off, my link would disappear so that I could change each value independently. But, that's not what I want to do. So I'll just go ahead and leave that check box on. This Megabyte value at the top here tells you the size of the image in memory. So when Photoshop opens an image in RAM, it opens the image uncompressed always, there's no way around that. And how this value is calculated, just in case you're curious, is you take the total pixel count of the image, in our case about 580,000, and you multiply that value times 3, because every full color RGB pixel consumes 3 bytes in memory.
Next we have the so-called Document Size options. This should probably read Print Size because that's what all of these values are about. The size of the image when you either print it or place it inside of a print file such as one created in Illustrator or InDesign. Notice in our case, the image measures about 9 inches wide, and about 6 inches tall because the Resolution is set to 100 pixels per inch. Also, notice right now that we have a link between the Width and Height values, again because Constrain Proportions is turned on, but we don't have a link to the Resolution.
So I could change the Resolution value to something like 300 pixels per inch. That will not change the Width or Height value in our case, but that will increase the number of pixels inside the image dramatically, and it will increase the size of the image in RAM as well. That's because Resample Image is turned on, and resample means to invent new pixels. So in our case, we would be upsampling the image because we're adding pixels to it. If we are reducing the number of pixels, that's called downsampling and you'll hear those terms more in the future.
However, I could turn off the Resample Image check box. And if I do that, then my Pixel Dimension values become dimmed and all of my Document Size values are linked together. And that's because with this check box off, you are not going to change the number of pixels inside the image. The image size, that is, the Pixel Dimensions, will remain exactly the same, just the resolution and the size of the printed image will change. And then notice in our case, because we've taken the Resolution value up and we're packing more pixels into a linear inch, the result will be a much smaller image.
It will print at just 3 inches wide and a little more than 2 inches tall. If I were to reduce the number of pixels, let's say I take it down to 72 pixels per inch, then the printed size of the image expands to nearly 13 pixels wide and nearly 9 pixels tall. And that's how you work inside the Image Size dialog box. In the next movie, I'll show you a practical application of this command.
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