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So we need to learn the difference between resizing our images and resampling our images. To do this, we're going to look at the Image Size dialog box. So under Image > Image Size. At the top of the Image Size dialog box is the size of the image in megapixels. And right underneath that are the Dimensions. You can see that right here I've got it set to pixels, but of course we can use the drop-down menu here and change this to other values if we want to. Right now, we'll just go ahead and leave it at the pixel values.
Underneath that, we have a width, height, and resolution area, so we can see that I can print this image at 14.05 inches by 9.367 at 300 pixels per inch. Now as we start preparing our image for the desired output device, we're going to be changing the width, height, and resolution. We're also going to choose whether or not we're going to resample the image. By default, the resample option is checked on, which means that if I want to make this image. Larger or smaller, Photoshop is actually going to either make up or discard information in order to create the file size that I tell it to, so let's take a look at our three different scenarios.
For the first one, let's imagine that I want to print this. At four by six to the printing press. Now I need to know my line screen for the printing press so lets say its 175. Then I would go ahead and change my width to six inches and you'll notice that the height automatically changed to four. And that's because of this little icon right here. This is locking the aspect ratio so whenever I change the width, the height will change accordingly. Then I need to double the line screen so we said 175 so we multiply that by 2. And we need a resolution of 350 pixels per inch. We can see at the top here that the image size is actually been decreased so what was initially a 33 meg file has now been or will be sampled down to an 8.41 meg file.
And we can also look and see that the actual pixel count. Has been lowered as well. Now, if I change my dimensions from pixels to percent, we can see that we're almost cutting the file size or the number of pixels in the width and height in half. So, when we're lowering the file size, Photoshop is going to discard almost half of the pixels in the original file. And as long as I'm making the image smaller or resampling down the image I really don't have to worry about losing image quality. As long as I have the resample checked on and I have it set to automatic Photoshop will automatically select the best resampling algorithm or the best math in order to create the highest quality image so.
Just so that you know, it's going to automatically select, by cubic a sharper when I'm reducing my file size. But I'll go ahead and leave it automatic so that I really don't have to worry about that. Now I'm going to hold down the option key on the Mac or the alt key on Windows And you'll see that the cancel button turns to reset. I'm actually just going to reset the dialog box. This time I'm going to uncheck the resample option because I want to show you what would happen if we told photoshop that it couldn't.
Increase or decrease, basically it can't add any pixels or remove any pixels from the file. Well now, when I change the width to six inches, you can see that the image size remains the same, and the dimensions remain the same. So, if I'm telling Photoshop that. It can only print six inches wide but it still has the same number of pixels to use well then its gotta use more pixels per inch and sure enough we see that under resolution. It's no longer 300 but its been boosted up to 702.5. But sending this high of a resolution to the printing press. Isn't going to magically give us any better quality. It's really just sending unneeded information to print at that line screen. And it's really only going to make the file larger. So I am better off checking the resample option and then changing my resolution back down to 350 pixels per inch.
You will also notice that on the left-hand side here, we are getting a preview of what this will look like when it's resized. And when I hover my cursor over the area here, we can either zoom out or we can zoom in. And you will notice when I position cursor over my image, I can actually click anywhere in my image file, and that area where the square is, where my cursor is, where I clicked, that is going to be the area that is represented here in the preview. Alright. So at this point, I would go ahead and click okay, and you can see that my file actually got smaller, so Photoshop has discarded the information that it doesn't need.
Let's undo that. We can either use command Z on the Mac or control Z on Windows, or we can select Edit > Undo Image Size. All right, let's take a look at the next scenario. I'm going to imagine that I want to send this image to my lab, or print it to my inkjet printer. So, I'm going to want to be looking at 300 pixels per inch. I'll choose image, and then we'll choose image size. But this time I want to print this much larger. I'm actually going to print it 24 by 16. So now we can see at the top, what was a 33.9 meg file is going to have to be resized up or interoplated up in order to create this almost 100 meg file. And we can see the actual pixel dimensions have also increased. And if we change this to percent, we can see that we are actually going to resample up a 170% which is actually quite a significant amount. But that's okay, because I've got my Resample option set to automatic. And, just in case, you want to know the details, Photoshop is actually going to use this reserve details in order to enlarge this image. So you might be wondering what the benefit would be to actually resize the image in Photoshop before you print it because I know that some people would just prefer to send the image to the printer at the size that it is and have the printer actually resize the image.
But, if you're really looking for the highest quality image, I would suggest that you go into image size, and you resize the image, whether it's larger or smaller. And then click OK so that Photoshop can calculate that. Because now, when you're retouching your image, and you're working with your image, you're actually seeing the complete file. And right before you go to print, you'll probably want to sharpen this image. And I really do want to sharpen the image. At the size that I'm going to send down to the printer, as opposed to just, maybe, guessing and sending the print at its native resolution.
And having the printer interpolate up the file. Without adding any sharpening. Alright, let's go ahead and undo that. We'll select Edit > Undo Image Size, and finally we need to take a look at this third scenario, which is to prepare this image for the screen. So again I'll select Image and then Image Size. And in this case, I'm going to change the width and height to a specific pixel dimension. So under the width, I'm going to change my units of measurement to pixels.
And let's say I'm going to display this on a projector that displays at 1024 by 768, so I'll go ahead and put this in here. 1024 and you can see that the height is actually less than 768. So the aspect ratio here isn't exactly the same aspect ratio of the projector that I'm going to project it on but as long as that height is less than the 768 then I'm still okay. If I had changed the height to 768, then you'll notice that the width is higher than the 1024 of that projector so it would probably resample the file which I might not want, so I just need to make sure that I set this. The width to 1024 and then the height will just be a little bit short, because again the original file is not at that same aspect ratio. I can also change the resolution if I wanted to, but you'll notice this time when I change it from 300 down to 72, nothing changed as far as the width and height because I had them set to pixels.
And of course, when you're displaying images on the screen, the total pixel count is really what matters. So when I click okay we can see that Photoshop has discarded a bunch of information. It has made the image smaller and in fact its made the image the exact pixel dimensions that we entered in the image size. And there you have it. Now you know how resampling and resizing affects your images in Photoshop when you're preparing them for multiple output devices.
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