Join Michael Ninness for an in-depth discussion in this video Resize vs. Resample, part of Photoshop CS5 Essential Training.
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Okay. So you've got an image off your Digital Camera. You've got a real nice, fancy DSLR and shoots 18 megapixels, and you bring it into Photoshop and you're like, I just need to create a 4x6 print of this and print it out on inkjet printer. How do I do that? How do I change the size of my image? Typically what you do is you go to the Image Size dialog box that's located under the Image menu, so Image, Image Size. And let's talk about this dialog box because it's potentially confusing for a lot of people actually, even talking to people who've been using Photoshop for years and they still are kind of confused by what this dialog box is all about.
So let's make it simple. I'd break it into two parts. There's the Upstairs and the Downstairs, Upstairs easy, Downstairs hard. Now, why is the Downstairs harder than Upstairs? Because Downstairs has one more thing that you have to think about and that's Resolution. We'll talk about that in just a second. The other potentially confusing thing about this dialog box, that actually has two modes. It can be used to resize an image or it can be used to resample an image, and there's a difference between resize and resample.
The default is to have this turned on to Resample. I'm going to turn that off. Note that when I do that, the Upstairs becomes unavailable. I can't actually change the number of pixels in the file when Resample is turned off. Which means all I'm really doing is changing how this image will be printed, what size it will be printed at when you hit the Print command? You're not actually changing the amount of information of the file at all, only how it gets output. So I said I wanted a 4x6.
So right now the width of the camera. This is a very large dimensionally file. It's 32 roughly inches by 48 inches, but has a very low resolution at 72 pixels per inch here. If I change the Width to 4, so 4 inches, Because I'm not changing the number of pixels in the file, I'm just changing the Dimensions, in this case I'm making the dimensions go down, resolution has to go up, because it's the same number of pixels. Because just to fit into a smaller rectangle the pixels have to get smaller.
So the smaller the pixel, the higher the resolution. That's kind of how this works. Now, okay, you might understand that, great! I've turned off Resample, which means it's a non-destructive action. I'm not changing the number of pixels in the file. I'm only changing how it's going to print. You click OK, and this is where people sometimes get confused, because it didn't appear that anything happened. The file doesn't look different on screen and that's exactly what's supposed to happen because you did a Resize, not a Resample. If I had gone to 100%, let's go ahead and double-click on the Zoom tool, and I'm viewing the Image at 100% view, the Actual Pixels view.
Here you're seeing every single pixel in the image represented by 1 pixel on the screen of the display. You can see there's a lot of pixels here. It's a big file. If I go back to Image, Image Size, and you can see it's a 4x6 at 584 pixels per inch. Let's take this back to a 20 inch file, so a 20x30. The Resolution went down in this case, because I made the dimensions larger, again because Resample is turned off, the number of pixels in the file is not changing, so the pixels have to get larger to fill the larger dimensions that we're changing here.
I go ahead and click OK. Again, you're might be scratching your head on, but nothing happened on screen. So the summary there is when the Resample is turned off, you're not actually changing anything about the file within Photoshop except how the file will be printed when you use the Print command. Let's go back to Image Size again, and let's take this Resolution back to where we started, 72 which forces the dimensions to go up. Let's turn Resample on this time. Now when Resample is turned on, suddenly the Upstairs is available to use again because this is potentially a lossy operation.
You're actually going to be changing the number of pixels in the file. Well either be removing pixels, in which case the geeky term for that is down-sampling, or you'll be adding pixels that don't even exist, and that's called up-sampling. If I change the Width here to 4, the Height retains its proportion by default. So it goes to 6 inches. But note that the resolution did not change anymore, because they're not connected anymore when the Resample is turned on. You can change these independently or as when Resample was turned off, changing the width or heights change the resolution correspondingly and vice-versa, by changing the resolution, the width and height changes.
Now if you take a look at the Upstairs, it's actually giving you a clue of what's going on here. The original file was 23 MB, but because we made the dimension smaller and we did not change the resolution, we are throwing away a gigantic amount of those pixels and ending up with a 364 K File. Now if I click OK, you're like oh! Something happened. It got smaller on screen. Yes, it got smaller on screen because we actually threw away a large number of pixels. We're still viewing it at the 100% view, and because there is fewer pixels in the file it takes up less screen real estate to represent the image.
I am going to undo that, Command+Z, Ctrl+Z on Windows, and go back to Image, Image Size. Now when you have Resample turned on, which again is the default, you're not just changing how this file will be printed when you hit the Print command, you're also changing the physical size of the file, the number of pixels in that document. And like I said if you change the dimension smaller and keep the resolution low, you're getting rid of pixels that were present, so that you don't feel you need anymore. If you make the dimensions larger and/or the resolution larger than what you started with, that's called upsampling.
You're asking Photoshop to invent pixels that don't exist. So usually a question I get is okay, I've got this file. It's as big as it can be as it is, but I need to make it a little bit bigger. Maybe I'm starting with an 8x10 image, 300 dpi, and I want to print it as an 11x14 let's say. So how big can you make it in Photoshop? Well typically, what most people say is if you're going up about say 25% from your original dimensions, you're probably not going to be able to spot that too badly. You probably get a good enough job there, especially it's because the larger the print, the farther away you're intended to be from, when you're looking at it.
If you are looking at a billboard let's say, you're not standing right next to a billboard, you're looking at a billboard from hundreds of feet away potentially. So you don't need a lot of resolution in that regard because you're going to be looking at it from quite a distance. So anyway, what all I'm trying to say is if you're going to up-sample an image in Photoshop, going anywhere from 10-20 % larger than what you're starting with, it's probably not the end of the world and you probably won't even notice. If you need to go much larger than that, then there actually are third-party products that do a much better job of upsampling beyond the capabilities of what Photoshop may do on its own.
If that's something you need, I check out one of the products from onOne Software. That's just onone.com or ononesoftware.com and they have a product called Genuine Fractals that you can take a look at that does a really good job of up-sampling beyond that 20% threshold that I'm recommending you stay within. Last point to make is that, if you are resampling an image whether you're making it have fewer pixels or have more pixels, you actually have a choice of how the resampling gets done. There's another geeky term called an Algorithm. What math should Photoshop use to get you the result you're looking for? The default is Bicubic, again another geeky name.
And at least now, they actually give you a description of what that means. It's best for smooth gradients. There are two other options that you might want to take a look at, one is Bicubic Smoother, so if you're making that image larger than the number of pixels you have, you would want to change the algorithm for resampling to Bicubic Smoother. It's going to give you much better results. Bicubic Sharper is typically what you want to do if you're taking a file smaller, which most of time and especially with today's digital cameras that are capturing so many pixels, you typically are down-sampling your images, especially if you post it into a webpage or doing a photo gallery or just showing up on a mobile device or something, you're certainly not going to send these huge amount of pixels to those particular outputs.
So I actually change my default Resampling Algorithm to Bicubic Sharper and I'll show you how to do that in just a moment. But typically we're going to get a lot better results by choosing Bicubic Sharper. What that does is make the file smaller, and does a sharpening pass behind the scenes to retain edge detail. Okay, so if I was doing this as a 4x6 to my inkjet printer, in the previous video we talked about what type of resolution you need based on output, it turns out that inkjet printing, a good number is somewhere between 240 and 300 dots per inch. We said we wanted this to be a 4x6, and so I'm down-sampling this image down to about 4 MB, the Dimensions are 4x6, the Resolution is 240, I've chosen Bicubic Sharper to give me the best results, go ahead and click OK, and there is my file and I'm ready to go ahead and print it.
If you were following the Algorithm discussion there about Bicubic versus Bicubic Smoother and Bicubic Sharper, then you'll find that most of the time you're going to be starting for a large file and making it smaller, then you'll want to change your Preference. Go to Photoshop, Preferences on the Mac, and we can just go to General on PC, we're going to the Edit menu, or Command or Ctrl+K will just pop-open the dialog. Right there in the General category, the Image Interpolation default, it's right now set to Bicubic. That's the out-of-the-box default. If you're finding you're constantly making your images smaller, then change it to Bicubic Sharper as the default, so that the next time you go to Image Size, that dialog, it will already be set to the interpolation choice that's going to give you the better looking result.
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