Join Deke McClelland for an in-depth discussion in this video The Image Size command, part of Photoshop CS5 One-on-One: Fundamentals.
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All right I'm still looking at that same image, Shepherd and small flock.tif. I'm going to go ahead and press Ctrl+1 or Command+1 on the Mac, to zoom this image out to the 100% zoom ratios so that we can see each and every pixel inside of this tiny image. Now the question becomes, let's say you want to modify the number of pixels in an image, or the resolution of an image, what do you do, and the answer is you choose, one of the most essential commands in all of Photoshop. You go up to the Image menu here and choose the Image Size command, and I like this one so much, I didn't just add a keyboard shortcut in dekeKeys, I lobbied Adobe to add a shortcut and they gave it to me, Ctrl+Alt+I or Command+Option+I on the Mac.
And up comes the Image Size dialog box. Now this dialog box is divided into three parts. First we see the Pixel Dimensions, which is the Image Size information, the Width and the Height of the image in pixels, and you can see, sure enough we've got 993 pixels wide 668 pixels tall, just as we saw before. And if you multiply those two values together, you get 663,000 and change, each one of the pixels takes up three bytes of information in memory. So you have to take that 663 multiply times three, in order to get the Pixel Dimensions, which is 1.9 megabytes again, very small by Photoshop standards.
Now drop down here to Document Size. That's how big the image is in print and only in print, and we see that it's 9.93 inches wide by 6.68 inches tall at a Resolution of 100 pixels per inch, which makes perfect sense, because if you divide 993 by 100, you get 9.93, 668 divided by 100 is 6.68. So everything is hunky-dory so far, the question is, can I live with a Resolution of 100 pixels per inch? Well, not for this small of a piece of artwork.
You need more for print purposes. And exactly what Resolution you need is the subject of a lot of conjecture out there, and you understand this topic more and more over time, as you gain experience. But basically, when you're preparing an image for a commercial output, you're concerned about the Line Screen, which is the number of halftone dots that print in a linear inch. And a typical Line Screen is 133.3 and so what you want is 2 pixels for every halftone dot, so you take your Resolution up to 267 pixels per inch.
And that's one of the big standards out there, 267, but there are no hard-and-fast numbers. It's not like a big issue, if you set it to 269 instead, that's not going to mess anything up, feel free to do that. Now the other big Line Screen out there is 150. That's the next step up. It tends to be for glossy stock and if you have a Screen Frequency of 150 LPI, as it's known, then you want to double that as well and you take the Resolution, up to 300 Pixels per inch, so that's where that popular standard comes from. If you're outputting to an Inkjet Printer, you may go as high as 360 pixels per inch, but you don't have to go that high.
Bear in mind, you can get away with resolutions for print as low as 220 pixels per inch. All you're interested in seeing is that the pixels blend together so that the picture looks snappy in print, so that it looks continuous. All right, so in our case however, we're dealing with Line Art and I want this Line Art to look super-sharp let's say, so we have all the definition in the world. So I'm going to take this Resolution value up to 600 pixels per inch. Mostly for the sake of demonstration here, but I'm really going to take that value through the roof.
Now, if you're working along with me, you need to make sure that Resample Image is turned on. That way you will be adding pixels to the image. If Resample Image is turned off, your Pixel Dimensions are no longer available and you reduce the printing size of your image as you raise the Resolution, and I'll tell you more about that later but for now, make sure Resample Image is turned on and now I'm going to have to redo things, notice that I still have my small Document Size, but this is great, because I can show you a tip.
If you run into something like this and everything goes kaflooey inside this dialog box as it has for me, then don't click Cancel and re-choose the command, press the Alt key, or the Option key on the Mac and the Cancel button changes to reset, then click and everybody's back to the way they were, including Resample Image, so you may need to turn that back on. Then change the Resolution value to 600 pixels per inch. The Width value jumps up to 5958 pixels, the height jumps up to 4008 pixels, if you do the math there and multiply Width times Height, you will find that you now have, 23,879,664 pixels, which is nearly a 24 megapixel image, so it's huge and it's going to take up three times at 24 megapixels, 68.3 megabytes in RAM.
So this is instantaneously going to become a much, much bigger image. Thanks to the fact that we are resampling, and when you add pixels to an image, it's called Upsampling, just so that you know. As you might expect if you remove pixels from an image, if you reduce the number, it's Downsampling. So the question becomes, if Photoshop is taking what used to be 1 pixel and it's turning it into 6 pixels wide and 6 pixels tall, so every one pixel is becoming 36 pixels now, thanks to the fact that we increase the Resolution value to six times its former resolution then where is Photoshop getting all this information? How in the World is it deciding how to turn 1 pixel into 36? And the answer is this option right here, the Interpolation Algorithm, and I will explain how that works in the next exercise.
Download Deke's free dekeKeys and color settings from the Exercise Files tab.
- Assembling photorealistic compositions
- Understanding image size and resolution
- Correcting the brightness and color of images
- Creating accurate selection outlines
- Retouching and healing photos
- Mastering layers and effects
- Printing and exporting to the web