Photoshop Top 40
Illustration by John Hersey

3. Image Size


Photoshop Top 40

with Deke McClelland

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Video: 3. Image Size

(Music playing) Deke's Photoshop? Deke's Photoshop? Top 40! #3! It's hard to imagine a more mission critical, misunderstood feature than Image Size. This one dialog box controls the two most fundamental attributes of an image, its pixel dimensions and its resolution. Image size is so frequently misused and misrepresented that I might characterize it as dangerous but employed properly there is no more essential command in all of Photoshop.
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Watch the Online Video Course Photoshop Top 40
7h 13m Intermediate Dec 21, 2009

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There's nothing people love more than lists, and Photoshop Top 40 offers a great one, highlighting the best features in Photoshop. Deke McClelland counts down to #1, detailing one great feature after another in this popular digital imaging application. The videos cover tools, commands, and concepts, emphasizing what's really important in Photoshop.

Topics include:
  • Assembling multiple pieces of artwork with layer comps
  • Creating a black-and-white image from a color photograph
  • Merging multiple channels to create an alpha channel with calculations
  • Selecting images with the Pen tool
  • Masking images using the Brush tool
Design Photography
Deke McClelland

3. Image Size

(Music playing) Deke's Photoshop? Deke's Photoshop? Top 40! #3! It's hard to imagine a more mission critical, misunderstood feature than Image Size. This one dialog box controls the two most fundamental attributes of an image, its pixel dimensions and its resolution. Image size is so frequently misused and misrepresented that I might characterize it as dangerous but employed properly there is no more essential command in all of Photoshop.

Feature #3 is the Image Size command, which allows you to change the number of pixels inside of an image to make it either smaller or larger, or you can change your resolution value in order to specify how large the image prints. Now the reason that image size is so extremely important inside of Photoshop is that it allows you to monkey with the pixels, which are equivalent to the molecular detail inside of a digital photograph. I'm looking at a photo of the Moab Utah skyline from photographer Miroslav and I'm going to go up to the Image menu and choose the Image Size command or you press Ctrl+Alt+I or Command+Option+I on the Mac.

Now that brings up a pretty unfriendly dialog box but the good news is that it's very well organized. So right up here at the top of the dialog box, we see the pixel dimensions of the image, how wide it is, how tall it is in pixels? You can change those values if you like, just bear in mind that if you change those values at all you're going to rewrite every single pixel inside of the image. That is something you want to do though frequently and I'll tell you how it works. Next we have document size which determines how large the image prints and that's all.

The width and height value in and inches and resolution value are all about printing and nothing more. Then we drop-down to these checkboxes here, which determine if you decide to change the number of pixels inside of an image. You don't have to, but if you do then these guys determine how those pixels will get rewritten and I want you to know a word. See this one right there Resample. To resample an image is to change the number of pixels, you can downsample to reduce the number of pixels inside of an image or upsample to increase the size of the image and we'll come back to that in a moment but imagine that we just want to print this image.

So we start off inspecting it to see how big it is. Its 9999 pixels wide which is an enormous image by the way, by 4000 pixels tall and if we do the math right there, 9999 times 4031 which seems like a difficult equation unless you consider that the Width is almost 10,000 pixels, so just 10,000 times 4000 that gives you 40 million pixels inside this image. This is a 40 megapixel image, which is larger than the overwhelming majority of digital cameras can capture.

So it might be stitched together. Who knows what's going on with it? It looks beautiful though so that's great. Now what that means is even at a resolution value of 300 pixels which is a lot, by the way that's the high-end industry standard and bear in mind that value is measured in a linear hand so that's 300 pixels wide by 300 pixels tall, so the total of 300 times 300 equals 90,000 pixels in a square inch. We still have a Width of 33 inches and Height of 13 inches, so this is a very large image but let's say I wanted to print even larger than this.

I wanted to print 45 inches wide. Well, if I just entering the values here, if I say yeah I want this to be 45 inches wide now, the size of the image is going to jump up. Now the width goes to 13,500 pixels and where the image used to be 115 MB, that's huge by the way for a flat image 115 MB, it's going to jump up to 200 MB. But that'll keep it 300 pixels per inch, but here's the deal. There're not 300 good pixels. Photoshop is going in and rewriting every single pixel inside the image and what do you think it's going to do when it comes to a little bush like this.

Is it going to invent new leaves? Is it going to invent new needles? Is it going to invent new rocks? No, because it does know anything about this landscape. Photoshop is completely ignorant of this subject of the photograph in the first place, all it's doing is smoothing transitions between pixels and what that typically means when you're upsampling in an image is you're making it fuzzier so you're not doing yourself any good whatsoever. So what I'm going to tell you is never ever upsample, which may seem like radical advice, but if upsampling doesn't do you any good then, what are you left with.

So, if I decide I want to take my existing pixels and I want to print them 45 inches wide, so I don't have to rewrite any of the pixels inside the image, then I turnoff Resample Image. I'll go in and turn off that checkbox and noticed that one checkbox controls half of this dialog box. As soon as turnoff Resample Image, I'd turnoff all the checkboxes, I turnoff the interpolation algorithm down here and I also turnoff my access to the pixel dimensions and I'm left just with the Document Size, just the print size values. Then I'd change the Width value to 45 inches, Height and Resolution are both going to change in kind, Height is going to go up along with Width because I'm increasing the size of the image and it's remaining proportional, Resolution is going to go down because I'm packing fewer pixels into an inch and in this case, I'm only packing 220 pixels into a given inch.

Now that's about as low as you want to go if your viewer is going to be seeing the image from a foot or less distance because any closer than that, the halftone dots that are produced by the commercial printing process are going to resolve your pixels and your viewer stands a chance of being able to see those pixel transitions which really ruins the appearance of a continuous-tone photograph. However, you can go even lower than that. If I decide to take this up the 5 feet wide, 60 inches wide and the Resolution drops down to 166, well presumably that image is going to be a farther away from me than a foot.

It's going to be mounted high on a wall. It could even be on a billboard, if it was even bigger than this, that kind of thing, and then you can let the Resolution value drop because your viewer is going to be sufficiently far away that they're not going to see the pixels. All right, so that's one way to work if I decide to do that. 60 inches right, click OK, no changes happen to my image on screen, why is that? Because all I did was change the image in print. It will print at a different size now but haven't changed a single pixel inside this image. All right! Now let's say what we want to do is we want to send a proof of this image out to a client, we want to email it to him or I want to email the image to a friend or a family member something like that, then I need to reduce the size of this image and when I say reduce the size of this image.

I don't mean the size at which it prints. Why would I care about that? I'm going to email it. I'm caring about how big the image is actually going to be because it doesn't do anybody any good, notice the zoom ratio 17. 5% right now, which means I'm way zoomed out. I could zoom in and get all kinds of more detail here and that's going to be wasted on somebody who is not even going to look at the image inside of Photoshop. They presumably are just going to look at the image inside of their Email client or their Web browsers or something along those lines and then they are not going to zoom in the image, and then they're going to be zoomed out.

They'll have too many pixels so it's not doing them any good and it takes too long to send the image on the first place and its too ginormous. So we need to downsample the image. So upsampling, I don't recommend typically bad, doesn't do you any good. Downsampling, very, very useful for screen work. So I'm going to go up to the Image menu, choose Image Size, Ctrl+Alt+I, Command+Option+I on the Mac and this time, I would turn Resample Image on. I would also make sure that Constrain Proportions is on so that my Height and Width are changed by the same amount and then Scale Styles, good idea, turn it on.

That way Photoshop will try to, it does not very succeed but will try to scale your Drop Shadows and your other layer effects so that they look right inside of the new image. All right, then I'm going to change the size of my image and I'm going to reduce the size to 1750 pixels. Now where am I coming up with that? That happens to fit on screen right now, so that I can show you the image inside the video that happens to work out quite nicely and then I would just let everything else fall where may.

I don't care about the Resolution by the way at this point. Resolution is only for print. If you hear somebody tell you, well for your web graphics, you want to have your Resolution value set to about 72 pixels per inch then slap him, because that is totally wrong. There is no Resolution associated with a screen. You do have a screen resolution but the web browser and your Email client and all that stuff doesn't want to know anything about it, doesn't care. All right, so we drop down at this point in then, having turned on all the checkboxes, having determined the Width for my image and so on, what is the interpolation method going to be? Now, if you don't want to worry about this, you don't have to.

Just leave it set to Bicubic (best for smooth gradients). It doesn't even make sense nor is it true. There are no gradients inside of this image for that and it doesn't really have anything to do with gradients in the first place. It's just Bicubic Interpolation, which happens to be a great method. But if you're curious about the other one, so I will run through them, very quickly, Nearest Neighbor just throws pixels away, just keep some pixels, throws some of the pixels away. You end up with jagged transitions. Useful for screen shots under very specific conditions otherwise do not touch it. Bilinear don't need it.

Don't ever worry about that. Just pass it by. All it does is factor in fewer pixels at a time, when Photoshop is rewriting new pixels. Bicubic does a better job than Bilinear, there's no dispute about that whatsoever and then we come to these other two, Bicubic Smoother (best for enlargement), not true and Bicubic Sharper (best for reduction), sort of true. So here's the thing Bicubic Smoother what it does is it tries to smooth out noise, which is random pixel variations where random pixel variation should not occur.

For example you probably see some noise in the sky, if you look closely at it and if you wanted to smooth that noise away to a certain extent, you are not going to get rid of it entirely, but if you want to try to smooth some of it away, you would choose Bicubic Smoother. So Smoother for high a noise images, Bicubic under just general conditions and if you're going to the web and you're going to be creating a very small image and you want crisp details then Bicubic Sharper can be great, assuming you've started with a low noise image in the first place.

It will do you no good by the way if you're reducing for print. If you're downsampling for print, don't expect Bicubic Sharper to do you any good whatsoever. Its sharpening algorithm is too fine. So for our purposes, we might want to look nice and crisp on screen, so we might go with Bicubic Sharper or just stick with Bicubic. I'm going to stick with Bicubic for a moment here and then I'll show you the difference between the three. But I'm going to choose that and then click OK and that is going to change the image, now the image actually changes on screen, because I just the changed Width and Height of the image in pixels, so I'd just rewrote every single pixel in the image that quickly, amazing.

Now I'm going to press Ctrl+1, Command+1 on the Mac to zoom in to 100% and it looks sweet. It looks really, really good. All right, so I have some comparative documents open here that I've created in advance of the different algorithms. Those different interpolation methods, we've got Bicubic, which we just did, so it looks exactly the same. You're going to look to very close in the video here and then I will run it again at 200% but this is Bicubic Smoother. It's just slightly softer. You're just going to see slightly smoother details.

Really it's almost imperceivable. It's very, very similar algorithm. And in Bicubic Sharper, you should see things get a little tackier here on screen, a little crisper. Now there are some folks out there that will claim the great thing about Bicubic Sharper is that unlike something like Unsharp Mask or one of the other sharpening functions that I showed you back in a feature #5 video, you do not get halos with the Bicubic sharper. So that you don't get those slight transitions around the edges. That's not true. You do. They're just so tiny that you don't see them and if they're that tiny, they're not really contributing much to the photograph in the first place.

So this is just for a little bit of tactile sharpening on screen, nothing more. At here, I'll go back to the Bicubic default here. If you want more than that, what I'd recommend you do is you reduce the size of the image. Downsample the image using Bicubic, the one that I talked about the smooth gradients, the default setting and then after you get done downsampling the image, you'd go to Sharpen and then Smart Sharpen for an image like this. And then what I would do is I'd say something like 100%. Take that Radius value down like crazy to something like 0.5 pixels.

You could even go lower than that if you want to Lens Blur and this is one of those times where you might go ahead and turn More Accurate on, see what the final image looks like, if it ends up looking good, great! If it looks like you're over sharpening in places then take some of the values down. In my case, I might go ahead and take this guy down to 0.3 pixels with More Accurate turned on, Removes set to Lens Blur, Amount at 100%, click OK and that is a discernibly sharper image. Compare that to Bicubic Sharper and you'll see that we have a Sharper Image still with Bicubic combined with Smart Sharpen and we have controlled over the process rather than just relying on Photoshop to deliver whatever the heck it's going to give me and I'm not seeing any halos.

This is a nicely sharpened image for screen work. So there you have it, feature #3, Image Size here inside Photoshop.

Find answers to the most frequently asked questions about Photoshop Top 40 .

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Q: Is there a way to batch convert an entire folder of photos from the RBG color mode to the CMYK color mode without having to open and convert each individual image?
A: In the Actions panel in Photoshop, create an action that converts an image from RGB to CMYK. Then link to that action from File > Automate > Batch inside Photoshop.
Next, in the Bridge, select a folder of images. Choose Tools > Photoshop > Batch. Select the action inside the ensuing dialog box.
Or, in Photoshop, select File > Automate > Batch, and select the action and the folder inside the dialog box.
See also: Photoshop CS2 Actions & Automation, Chapter 2 “Action Essentials.”
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