Image Resolution And Print Quality

In this Photoshop tutorial, we're going to look at how image resolution affects print quality.

Have you ever downloaded an image from the internet and then printed it, only to get results that were, well, less than you expected? The image looked great on your computer screen, but when you printed it, it either printed at the size of a postage stamp or it printed at a decent size but looked blurry or "blocky"? The culprit is image resolution.

Actually, that's not really fair to say. Image resolution didn't purposely set out to make your life miserable when you printed your internet photo. The problem was simply that most photos on the internet have very small pixel dimensions, usually in the neighborhood of 640 pixels wide by 480 pixels high, or even smaller, and that's because images don't need to be very large in order to appear at a decent size and good quality on your computer screen, and also because smaller images download much faster on websites than larger images do (which is a whole other topic that we don't need to get into here).

So what can you do to make photos you download off the internet appear just as high quality when printed as photos you took yourself with your digital camera? The answer - absolutely nothing. There simply are not enough pixels in most internet images to allow them to print at high quality, at least not without printing them at the size of a postage stamp, that is. Let's find out why.

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First of all, let's get off the topic of downloading images from the internet, since we really shouldn't be doing that anyway without permission from the copyright owner, and look at image resolution in general. I cover it in much more detail in the Image Resolution, Pixel Dimensions and Document Size tutorial, but let's do a short recap.

The term "image resolution" means how many of your image's pixels will fit inside each inch of paper when printed. Obviously, since your photo has a fixed number of pixels, the more of them you squeeze inside each inch of paper, the smaller the image will appear on the paper. Likewise, the fewer pixels you print per inch, the larger the image will appear on paper. The number of pixels that will be printed per inch is known as the resolution of the image, or "image resolution". Image resolution has everything to do with printing your image. It has nothing to do with how your image appears on your computer screen, which is why images you download off the internet usually appear much larger and higher quality on your screen than they do when you print them.

Let's use a photo as an example:

An unflattering photo of a horse.

I can't help but laugh every time I see this photo of a horse I took while driving around the countryside one day. Normally this horse stands proud, powerful, full of grace and dignity, yet I seem to have caught him in a rather unflattering moment. He's standing on a bit of a strange angle, he has a piece of straw dangling from his hair, and he seems to be in the middle of chewing his food. Either that, or he's desperately trying to crack a smile for me. In either case, since this guy is already embarrassed, as am I for having taken this wonderful photo, let's use this image as an example.

First, let's look at what Photoshop can tell us about the current size of this photo. I'll go up to the Image menu at the top of the screen and choose Image Size, which brings up the appropriately-named Image Size dialog box:

The Image Size dialog box shows us the current size of the photo.

The Image Size dialog box is divided into two main sections, Pixel Dimensions at the top and Document Size directly below it. The Pixel Dimensions section tells us how many pixels are in our image. The Document Size section tells us how large the image will appear on paper if we print it. If we look at the Pixel Dimensions section, we can see that this photo has a width of 1200 pixels and a height of 800 pixels. That may sound like a lot of pixels (1200 x 800 = 960,000 pixels!), and it certainly would be if we were displaying this image on a computer screen. In fact, at 1200 x 800, it may be too large to fit entirely on your screen! But just because it looks nice and big on the screen doesn't necessarily mean it will print nice and big, at least not with any degree of quality. Let's take a closer look at what the Document Size section is telling us:

The Document Size sections tells us how large or small the photo will print based on a specific resolution.

The Document Size section of the Image Size dialog box tells us two things - what the current resolution of our image is, and how large or small the image will appear if we print it based on that resolution. Currently, our resolution value is set to 72 pixels/inch, which means that out of the 1200 pixels that make up our photo from left to right (the width), 72 of them will print inside each inch of paper, and out of the 800 pixels that make up the image from top to bottom (the height), 72 of them will print inside each inch of paper. The value in the Resolution box is for both width and height, not the total number of pixels that will print. In other words, for every square inch of paper, 72 pixels from our image will be printed from left to right and 72 pixels will be printed from top to bottom. The total number of pixels printed in every square inch of paper would then be, in this case anyway, 72 x 72 (72 pixels for the width times 72 pixels for the height), which gives us 5184 pixels!

Let's do some simple math ourselves to make sure that the width and height being shown to us in the Document Size section is correct. We know from the Pixel Dimensions section that we have 1200 pixels from left to right in our image and 800 pixels from top to bottom. Our print resolution is currently set to 72 pixels/inch, so to figure out how large our image will be when printed, all we need to do is divide the number of pixels from left to right by 72, which will give us our print width, and the number of pixels from top to bottom by 72, which will give us our print height. Let's do that:

1200 pixels wide divided by 72 pixels per inch = 16.667 inches
800 pixels high divided by 72 pixels per inch = 11.111 inches

Based on our own simple calculations, at a resolution of 72 pixels/inch (ppi for short), our image would be 16.667 inches wide by 11.111 inches high when printed. And if we look at the Document Size section once again:

Confirming the print size shown in the Document Size section.

That's exactly what it says! Wow, a 1200 x 800 pixel photo is large enough for an 11 x 14 inch print, with a little extra to spare! That's great!

Sadly, no. If only life were that simple.

The fact is, 72 pixels/inch is not enough to give us sharp, good quality, professional looking images when printed. It's not even close. To give you an idea of what I mean, here's a rough approximation of how the photo would look on paper if we tried to print it at a resolution of 72 pixels/inch. You'll have to use your imagination a bit here and try to imagine this at 11 x 16 inches:

The photo as it would appear on paper when printed at only 72 pixels/inch.

Doesn't exactly look good, does it? The problem is that at 72 pixels/inch, the image information is being spread out too far on the paper for the photo to appear sharp and detailed, sort of like spreading too little peanut butter over too much toast. The photo now appears soft, dull and generally unappealing. We don't see this problem on a computer screen because computer monitors are generally referred to as low resolution devices. Even a photo with relatively small pixel dimensions, like 640 x 480, will look great on a computer screen. Printers, however, are high resolution devices, and if you want your photos to appear sharp and detailed when printed, you'll need a resolution much higher than 72 pixels/inch.

So how high of a resolution value do you need for professional quality printing? The generally accepted value is 300 pixels/inch. Printing an image at a resolution of 300 pixels/inch squeezes the pixels in close enough together to keep everything looking sharp. In fact, 300 is usually a bit more than you need. You can often get by with a resolution of 240 pixels/inch without noticing any loss of image quality. The professional standard, though, is 300 pixels/inch.

Let's take our same image then at 1200 pixels wide by 800 pixels high, change our resolution from 72 pixels/inch to 300 pixels/inch, and see what we get. Here's the Image Size dialog box again showing the new resolution of 300 pixels/inch. Notice in the Pixel Dimensions section at the top that we still have 1200 pixels for the width and 800 pixels for the height. The only thing that's changed is our resolution, from 72 to 300:

The print resolution has been changed to 300 pixels/inch.

With our resolution now increased from 72 to 300 pixels/inch, this means that out of the 1200 pixels that make up our image from left to right, 300 of them will now print inside every inch of paper, and out of the 800 pixels contained in our image from top to bottom, 300 of them will now print inside every inch of paper. Naturally, with so many more pixels squeezing into each inch of paper, we'd expect the photo to print much smaller, and sure enough, the Document Size section is now showing that our photo will print at a size of only 4 inches wide by 2.667 inches high:

The photo will now print at a much smaller size than before.

Where did those new width and height values come from? Again, some simple math is all we need:

1200 pixels wide divided by 300 pixels per inch = 4 inches
800 pixels high divided by 300 pixels per inch = 2.667 inches

The photo will now print much smaller than it would at a resolution of 72 pixels/inch, but what we lose in physical size, we more than make up for in image quality. At 300 pixels/inch (or even 240 pixels/inch), we'd enjoy sharp, detailed, professional quality print results:

Higher print resolutions result in smaller photos but much better image quality.

Of course, most people don't print their photos at weird sizes like 4 x 2.667, so how do we make sure we're going to get professional quality print results with more standard print sizes like 4 x 6? An excellent question, and the answer comes to us once again through some boring yet simple math.

Let's say you've taken some photos of your recent family vacation using your digital camera and you want to print out some 4 x 6's on your printer. We know now that in order to achieve professional quality prints, we need set the resolution of our images to a minimum of 240 pixels/inch, although 300 pixels per inch is the official standard. Let's look at both of these resolution values though to see how large of an image, in pixels, we'll need out of the camera in order to print 4 x 6's with good image quality. First, let's look at 240 pixels per inch:

To figure out how large, in pixels, our images need to be in order to print 4 x 6's at professional quality, all we need to do is multiply 240 x 4 for the width, and then 240 x 6 for the height (or vice versa depending on if your photo is in landscape or portrait mode). Let's do that:

240 pixels per inch x 4 inches wide = 960 pixels
240 pixels per inch x 6 inches high = 1440 pixels

Based on our math, we can see that in order to print a digital photo as a 4 x 6 at 240 pixels/inch resolution, which should give us excellent quality, our photo's pixel dimensions need to beat least 960 x 1440. We can see exactly how many pixels that is by multiplying 960 by 1440, which gives us 1,382,400 pixels. Let's round that up to 1.4 million pixels. That may sound like a lot of pixels but it really isn't, not when you consider that 1.4 million is the minimum number of pixels you'd need to print good quality 4 x 6 photos using the minimum resolution we can use to achieve good quality, which is 240 pixels/inch. The good news at least is that these days, most digital cameras on the market are 5MP ("mega pixels", or "millions of pixels") and higher, so they'd have no trouble printing good quality 4 x 6's even using 300 pixels/inch for the resolution.

Of course, we haven't actually looked at how many pixels we'd need to print professional quality 4 x 6's at 300 pixels/inch, so let's do that now. We'll use the same simple formula as above, where we'll multiply 300 by 4 and then 300 by 6 to give us the pixel dimensions we'll need:

300 pixels per inch x 4 inches wide = 1200 pixels
300 pixels per inch x 6 inches high = 1800 pixels

Let's do one more quick calculation to see how many pixels we need in total:

1200 pixels wide times 1800 pixels high = 2,160,000

So, in order to print a photo as a 4 x 6 using the professional standard of 300 pixels/inch for resolution, our photo needs to be 1200 pixels wide by 1800 pixels high (or vice versa), which means we'll need a total of 2,160,000 pixels, which again should be no problem for most digital cameras on the market today which are 5MP and higher.

What if you have a photo you absolutely love and feel it deserves an 8 x 10 print rather than a 4 x 6? How large of an image in pixels do we need to print a good quality 8 x 10? The answer is as easy as when we needed to find out how large of an image we'd need for a 4 x 6. All we need to do is multiply the resolution value in pixels by the width in inches and do the same thing for the height. Let's first use 240 pixels per inch as our resolution:

240 pixels per inch x 8 inches wide = 1920 pixels
240 pixels per inch x 10 inches wide = 2400 pixels
Total number of pixels = 1920 pixels wide x 2400 pixels high = 4,608,000 pixels

From our little bit of math, we can see that in order to print a photo at good quality as an 8 x 10, our photo needs to be 1920 pixels wide by 2400 pixels high (or vice versa), for a total of approximately 4.6 million pixels. Now we're starting to push the limits of lower end digital cameras. A 4MP digital camera wouldn't capture quite enough pixels to be able to print an image at 8 x 10 at 240 pixels/inch resolution. It would fall about 600,000 pixels short. You could still print an 8 x 10 image of course, but you most likely wouldn't get professional looking results.

Let's do the same calculations for an 8 x 10 at 300 pixels/inch resolution:

300 pixels per inch x 8 inches wide = 2400 pixels
300 pixels per inch x 10 inches wide = 3000 pixels
Total number of pixels = 2400 pixels wide x 3000 pixels high = 7,200,000 pixels

Now we're really pushing the limits as far as digital cameras currently on the market. In order to be able to print a photo as an 8 x 10 using the 300 pixels/inch resolution standard, our photo needs to be 2400 pixels wide by 3000 pixels high (or vice versa), for a total of 7.2 million pixels! Now that's a lot of pixels! This means you need at least a 7.2MP digital camera in order to be able to print your photos as 8 x 10's and still get true, professional quality prints. Of course, keep in mind that most photos require at least a little cropping, which means you'll need to start with even more pixels. If you know you're going to be printing a lot of photos as 8 x 10's, investing in a good quality 8 MP or higher camera is highly recommended.

And there we have it!

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mage Quality: Image Resolution, Pixel Dimensions and Document Size

Pixel Dimensions

Here's a photo I took one day while strolling through a park. I saw this little guy (or girl, who knows) posing for me on the flowers and happened to have my camera handy. My camera, by the way, is an 8MP camera, and the reason why I'm telling you this will be explained shortly.

Obviously, the photo you're seeing above is a much smaller version of the photo, since the actual-size version would be too large to fit on the screen. Let's pretend though for the sake of this lesson that we're working with the full size version of the photo. In order to see exactly how large the photo is, once we have it open inside Photoshop, we can simply go up to the Image menu at the top of the screen and choose Image Size from the list of options, which will bring up Photoshop's Image Size dialog box, as shown below.

The Image Size dialog box can seem a bit frightening and confusing, but it's not meant to be and really, it's quite simple. It's divided into two sections, Pixel Dimensions and Document Size. For the moment, let's ignore the Document Size part and focus only on Pixel Dimensions.

The term "pixel dimensions" here, to me, is confusing because it sounds like we're talking about the dimensions of each individual pixel, and that's not the case. What Photoshop is really telling us is the width and height of our image in pixels. In other words, how many pixels are in our image from left to right, and how many pixels are in our image from top to bottom. It's also telling us one other important piece of information which is the file size of our image. The dimensions and file size shown here are of the full size version of the photo above (the insect on the flower) before I resized it to something more suitable for a web page. So here, Photoshop is telling me that my photo has a width of 3456 pixels and a height of 2304 pixels. In other words, it contains 3456 pixels from left to right, and 2304 pixels from top to bottom. To find out exactly how many pixels I have in my photo then, I can simply multiply the width times the height, which in this case is 3456 x 2304, which gives me a grand total of 7,962,624 pixels. That's a whole lot of pixels.

Remember earlier when I mentioned that the camera I used to take this photo was an 8MP camera? Well, the "MP" stands for "mega pixel", and "mega" means "million", so "8MP" means 8 million pixels. This means that when I take a photo with my digital camera, the photo will be made up of 8 million pixels (approximately, anyway). If you have a 5MP camera, your photos will be made up of 5 million pixels. 4MP cameras give you photos made up of 4 million pixels, and so on. So if we take a look again at what the Pixel Dimensions section of the Image Size dialog box is telling us about my photo above, it's saying that my photo has dimensions of 3456 pixels wide by 2304 pixels high, for a total of 7,962,624 pixels, which is pretty darn close to 8 million, and that's why my camera can be sold as an 8MP camera.

So that's what the first part of the Image Size dialog box is telling us - the width and height of our image in pixels. So far so good. Let's take a look now at the second part of the dialog box, "Document Size" which is where we really start to make sense of image resolution.

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RGB and Color Channels in Photoshop Explained

Did you know that Photoshop is color blind?

When I say "color blind", I don't mean it has a little trouble distinguishing between certain shades of green and purple. I mean it's completely and totally blind when it comes to color. All Photoshop sees is black and white. Well, black, white, and a lot of shades of gray in between, but that's it. The world's most powerful image editor, an industry standard among photographers, designers, and virtually all creative professionals, capable of producing millions, even billions of colors has no idea what color is.

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You may be looking at a photo you took on your last vacation of the crystal blue waters on the ocean, but all Photoshop sees is a gray ocean. Did you manage to snap a picture of a rainbow arching across the sky after a summer evening storm? Photoshop sees it as a beautiful assortment of shades of gray. And that famous pot of gold at the end of it? To Photoshop, it's a big ol' pot of gray.

Don't feel sorry for Photoshop though. It's perfectly happy in its colorless world. In fact, the only reason it shows us our images in color at all is because we as human beings expect to see them in color. We wouldn't know what to think if everything was appearing in black and white. But not Photoshop. To it, life just couldn't be sweeter than in black, white and gray.

Alright, so if Photoshop doesn't have a clue what color is, and all it knows and sees is black, white and gray, how does it manage to show our images in color? I mean, here's an image I have open in Photoshop:

An photo open in a document window in Photoshop.

Obviously, this little guy (or girl) is in color. In fact, I don't think birds come much more colorful than this. But it's not just the bird. The leaves in the background are in color. The piece of wood the bird is standing on is in color. The whole thing is in color! And this image is open in Photoshop, so how can it be that Photoshop doesn't see color? And if it really doesn't see color, how is Photoshop doing such a great job of showing us something it doesn't see?

To answer that question, we need to look at a couple of things. One is color modes and the other is color channels. They're both related to each other in a big way, so once you understand the first one, color modes, the second one, color channels, makes a lot more sense.

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Benefits Of Working With 16-Bit Images In Photoshop

What Does The Term "8-Bit" Mean?

You may have heard the terms 8-bit and 16-bit before, but what do they mean? Whenever you take a picture with a digital camera and save it in the JPEG format, you're creating a standard "8-bit" image. The JPEG format has been around for a long time and as digital photography and even Photoshop itself continue to advance, the limitations of the JPEG format are becoming more and more apparent. For one thing, there's no way to save a JPEG file as 16-bit because the format doesn't support 16-bit. If it's a JPEG image (with the extension ".jpg"), it's an 8-bit image. But what does that mean, "8-bit"?

If you read our tutorial RGB and Color Channels Explained, you know that every color in a digital image is made up of some combination of the three primary colors of light - red, green and blue:

It doesn't matter what color you're looking at on your screen. It's being made up of some combination of those three colors. You may be thinking, "That's impossible! There's millions of colors in my image. How can you create millions of colors out of just red, green and blue?"

Good question. The answer is, by using multiple shades of red, green and blue! The more shades of each color you have to work with and mix together, the more colors you can create. If all you had was pure red, pure green, and pure blue, the most you could create would be seven different colors, including white if you mixed all three together:

You could also include an eigth color in there as well, black, which you would get if you completely removed red, green, and blue.

But what if you had, say, 256 shades of red, 256 shades of green, and 256 shades of blue? If you do the the math, 256 times 256 times 256 equals roughly 16.8 million. That's 16.8 million colors you can now create! And that's exactly what you get with an 8-bit image - 256 shades of red, 256 shades of green, and 256 shades of blue, giving you the millions of possible colors you usually see in a digital photo:

Where does the number 256 come from? Well, 1-bit equals 2. When you move beyond 1-bit, you find its value using the expression "2 to the exponent (however many bits there are)". So, for example, to find the value of 2-bits, you would calculate "2 to the exponent 2", or "2 x 2", which equals 4. So 2-bits equals 4.

A 4-bit image would be "2 to the exponent 4", or "2 x 2 x 2 x 2", which gives us 16. So 4-bits equals 16.

We do the same thing for an 8-bit image, which would be "2 to the exponent 8", or "2 x 2 x 2 x 2 x 2 x 2 x 2 x 2", which gives us 256. That's where the number 256 comes from.

Don't worry if you found that confusing, or even worse, boring. It all has to do with how computers work. Just remember that when you save an image as a JPEG, you're saving it as an 8-bit image, which gives you 256 shades each of red, green, and blue, for a total of 16.8 million possible colors.

Now, 16.8 million colors may seem like a lot. But as they say, nothing is big or small except by comparison, and when you compare it with how many possible colors we can have in a 16-bit image, well, as they also sometimes say, you ain't seen nothin' yet.

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Seeing The Difference With JPEG Compression

It's a fact of life when it comes to working with digital photos and images. Sometimes, we need to compress them to make our file sizes smaller, especially if we're emailing them to a client or to family members, or posting them on a website. Since the jpeg file format is still the format of choice for digital photos, even though it's been around for more than 15 years, compressing images usually means using jpeg compression, which does a great job of reducing the file size. Unfortunately, it also reduces image quality, although it's not always easy to see what sort of negative impact the compression is having on the image, especially when viewing it on a computer monitor. But thanks to Photoshop and one of its rarely-used blending modes, the horrors of jpeg compression become strikingly clear.

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To see exactly what's happening to our images, we'll use Photoshop's Difference layer blend mode. The Difference blend mode gets its name from the fact that it looks for differences between two layers. Any areas in both layers that are exactly the same appear as pure black, while areas that are different in some way appear as strange colors. The Difference blend mode isn't used very often outside of the special effects world, but it does a great job of showing us how much garbage (yes, I said garbage) we're adding to our jpeg images when we compress them. Now, this isn't to say that we all need to stop compressing our photos. In a perfect world, sure, but that's just not realistic. It does help, though, for us to see and understand what's happening to our images, especially for those of us who, up until now, have been convinced there's no difference in image quality between a compressed and uncompressed digital photo.

Here, I have two document windows open in Photoshop, each one containing what appears to be the same image:

JPEG Compression and Image Quality: Two document windows open in Photoshop, each displaying a copy of the same photo.

But are they really the same image? Appearances can be deceiving, especially on a computer screen. The truth is, they're not the same. The one on the right is the original, uncompressed photo, as if it was downloaded directly from a digital camera. The one on the left is a copy of the photo saved in Photoshop at 60% image quality, which is a fairly typical setting. This means that the image on the left has been compressed.

I realize it's a little difficult to tell in the screenshot above, but if you were to look at both of these images on my screen in Photoshop, you'd have trouble telling which one is compressed and which one isn't. If we were to print them, the differences would become clear, but on a computer screen, which has a much lower resolution than a printed image would have, the differences are not so easy to spot. At least, not without using Photoshop's Difference blend mode to help us out.

I said a moment ago that the Difference blend mode looks for differences between two layers, and that any areas between the two layers that are exactly the same appear as pure black. Let's put this to the test. I'm going to select my original, uncompressed photo, and I'm going to duplicate its Background layer in the Layers palette by pressing Ctrl+J (Win) / Command+J (Mac). I now have two layers in the Layers palette - the original Background layer on the bottom and a copy of the Background layer, which Photoshop has named "Layer 1", above it:

JPEG Compression and Image Quality: The Layers palette for the original, uncompressed photo showing the Background layer and the copy of the Background layer above it.

Since "Layer 1" is a copy of the Background layer, both layers should be identical. We can use the Difference blend mode to prove that they are. I'm going to go up to the blend mode option at the top of the Layers palette and change the blend mode for "Layer 1" from "Normal" to Difference:

JPEG Compression and Image Quality: Changing the blend mode of "Layer 1" to "Difference".

With "Layer 1" set to the Difference blend mode, if I look at the image in the document window, I see that the entire image is now filled with solid black, which is the Difference blend mode's way of telling me that both layers are in fact identical:

JPEG Compression and Image Quality: The image is now completely filled with solid black, indicating that both "Layer 1" and the Background layer are identical in every way.

Let's take things even further to be absolutely certain that all we're seeing now in the image is nothing but pure black. To do that, we'll use a Levels adjustment layer. I'm going to click on the New Adjustment Layer icon at the bottom of the Layers palette:

JPEG Compression and Image Quality: Clicking the "New Adjustment Layer" icon at the bottom of the Layers palette in Photoshop.

Then I'll choose Levels from the list of adjustment layers that appears:

JPEG Compression and Image Quality: Clicking the "New Adjustment Layer" icon at the bottom of the Layers palette in Photoshop.

This brings up the Levels dialog box. I can use the Histogram in the center of the dialog box to see exactly what tonal information is being displayed in my image. If every single pixel in the image is displaying pure black, which should be the case if both of my layers are identical, then all I should see in my Histogram is a single vertical bar on the far left, which just happens to be exactly what I'm seeing:

JPEG Compression and Image Quality: The Histogram in the center of the Levels dialog box is displaying a single vertical bar on the far left, telling me that every pixel in my image is pure black.

The Histogram confirms that there are currently no other colors being displayed in my image except pure black, which means that "Layer 1" and the Background layer are 100% identical. So far, so good. We've now proven what most of us already knew, that when we make a copy of a layer, the copy is identical in every way to the original. Exciting stuff, right? Let's move on.

I'm going to click the Cancel button in the top right corner of the Levels dialog box to exit out of it for now, and I'm going to delete "Layer 1" from my Layers palette by clicking on it and dragging it down on to the trash bin icon:

JPEG Compression and Image Quality: Dragging "Layer 1" down on to the trash bin icon at the bottom of the Layers palette to delete it.

I'm now left with just my original Background layer once again:

JPEG Compression and Image Quality: The Layers palette showing the original Background layer.

Now let's see what happens when we use the exact same method to compare the uncompressed version of the photo with the compressed version of it. As I mentioned, it's not easy to see any differences between them simply by looking at them on the computer screen, but let's find out what the Difference blend mode has to say about it. With each version of my photo open in its own document window, I'm going to click inside the compressed version's document window (the one on the left) with my Move Tool and drag the image into the uncompressed version's document window (the one on the right):

JPEG Compression and Image Quality: Using the Move Tool and dragging the compressed version of the image on the left into the uncompressed version's document window on the right.

Since both images have exactly the same pixel dimensions (width and height), I'm going to hold down my Shift key and then release my mouse button, which will align the two images perfectly inside the document window:

JPEG Compression and Image Quality: Both images are now aligned perfectly one above the other in the same document window.

If I look in my Layers palette, I can see that I now have two layers once again. The uncompressed version of the image is on the Background layer, and the version that was saved at 60% image quality is now above it on "Layer 1":

JPEG Compression and Image Quality: The Layers palette showing the uncompressed version on the Background layer and the compressed version on "Layer 1".

I'm going to once again change the blend mode of "Layer 1" from "Normal" to "Difference":

JPEG Compression and Image Quality: Changing the blend mode of "Layer 1" to "Difference".

And now, with the blend mode of "Layer 1" set to "Difference", if there truly is no difference between the compressed and uncompressed versions of the photo, I should be seeing nothing but pure black when I look at my image in the document window:

JPEG Compression and Image Quality: The document window after changing the blend mode of "Layer 1" (the compressed version of the photo) to "Difference".

Hmm. Can you see all that faint noise in the image above? It will depend on how you have your monitor set up. You may just be seeing black, but I can see on my screen that it's definitely not pure black like it was before when we were comparing the Background layer with an identical copy of it. There's something else there, and that "something else" is telling us that the compressed and uncompressed versions of the photo are not the same. But just how different are they? Does jpeg compression really make that much of a difference?

Let's use a Levels adjustment layer once again and let the Histogram answer that question for us. I'll click on the New Adjustment Layer icon at the bottom of the Layers palette and choose Levels from the list:

JPEG Compression and Image Quality: Selecting a "Levels" adjustment layer in the Layers palette.

This again opens the Levels dialog box. Recall from last time that the Histogram displayed a single vertical bar on the far left, which told us that there was absolutely nothing else in our image except pure black. This time, we're seeing something a bit different:

JPEG Compression and Image Quality: The Histogram is no longer just a single vertical bar.

There seems to be a lot more going on this time on the far left of the Histogram, which confirms that my eyes weren't playing tricks on me. There's definitely something else there in the image. The single vertical bar has been replaced by a larger area of black, which means the image itself now contains more than just pure black, and that means there's areas in the two versions of the photo that are no longer identical.

So what's different about them? Simple - garbage. By compressing one version of the image, we've taken the pure, untouched image information and added a whole lot of garbage to it. Noise, junk, call it what you like. The bottom line is, we've damaged the photo. How much garbage did the jpeg compression add? It may not look like much just yet, and it's still hard to see in the image itself, so I'm going to click on the small, white slider on the bottom right of the Histogram and drag it all the way over to the left until it's under the spot where the black slope begins:

JPEG Compression and Image Quality: Dragging the white slider from the bottom right of the Histogram over to the left to the point where the black slope begins.

Without getting into a lengthy discussion of how the Levels dialog box works, what I've just done is taken all that faint noise in the image and made it much brighter so we can see it more easily. Remember, any strange colors you see represent areas that are different between the original, uncompressed version of the photo and the version that was saved with jpeg compression:

JPEG Compression and Image Quality: The noise is now much more visible in the image.

Not a pretty sight, is it? Now that we can see things more easily, all of those weird colors represent all the damage we've done to the image by compressing it. The image is now filled with what are commonly referred to as "compression artifacts", which is just a fancy way of saying "we took your perfectly good image information and messed it all up". Compressing a jpeg image can greatly reduce your file size, but as the Difference blend mode is showing us, it can also greatly reduce image quality. Again, it's not always easy to see how much damage your image has suffered by looking at it on your computer screen, but you'll definitely notice the difference when printing in high resolution.

So now that we've seen how much damage we can do to a digital photo by compressing it, what can we do about it? Unfortunately, not much. The jpeg format is still your best bet for saving digital photos, and when file size is an issue, we really have no choice but to compress them. If you're working on a project for the web, you can usually get away with quite a bit of compression before image quality becomes an issue, but if your project is going to print, you'll want to use the original, uncompressed images whenever possible.

And there we have it!

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