Pixels and print quality

Generating a good print from a digital photo requires that you feed the printer a certain number of pixels per linear inch, or ppi. So the pixel count of a photo determines how large you can print the image without noticing a loss of picture quality.

Figures 4-6 through 4-8 illustrate this issue. The first image has resolution of 300 ppi; the second, 150 ppi; and the third, 75 ppi. Why does the 75-ppi image look so much worse than its higher-resolution counterparts? Because at 75 ppi, the pixels are bigger, and the bigger the pixel, the more easily your eye can figure out that it’s really just looking at a bunch of squares. Areas that contain diagonal and curved lines, such as the edges of the coins and the handwritten lettering in the figure, take on a stair-stepped appearance.

If you look closely at the black borders that surround Figures 4-6 through 4-8, you can get a clearer idea of how resolution affects pixel size. Each image sports a 2-pixel border. But the border in Figure 4-8 is twice as thick as the one in Figure 4-7 because a pixel at 75 ppi is twice as large as a pixel at 150 ppi. Similarly, the border around the 150-ppi image is twice as wide as the border around the 300-ppi image.

How many pixels are enough to guarantee great prints? Well, it depends in part on how close people will be when viewing the pictures. Consider a photo on a billboard, for example. If you could climb up for a close inspection, you would see that the picture doesn’t look very good because billboard photos are typically very low-resolution images, with few pixels per inch. But when you view them from far away, as most of us do, they look okay because our eyes blend all those big pixels together. Unless you’re doing billboard photography, however, people will be viewing your images at a much closer range, so a higher resolution is required.

The resolution you need to produce the best prints also varies depending on the printer, but I usually shoot for 300 ppi. For quick reference, Table 4-1 shows you the approximate pixel count you need in order to produce traditional print sizes at that resolution. The first set of pixel values represents the pixel dimensions (horizontal pixels by vertical pixels); the value in parentheses represents the total pixel count, in megapixels (MP). Your mileage may vary, however, so don't panic if an image you want to print has fewer than 300 ppi. You may be satisfied with prints made with a lower resolution — just don't go too low, for the reasons made obvious by Figures 4-7 and 4-8.

Chapter 9 provides an in-depth look at printing, talking more about the whole pixel-count issue and exploring other factors that affect print quality. In the meantime, keep these pixel pointers in mind:

• Select the resolution that matches your print needs before you shoot. Yes, some photo programs enable you to change the number of pixels in an existing image, a process called resampling. But adding pixels — upsampling — isn’t a good idea. When you take this step, the photo-editing software simply makes its best guess as to what color and brightness to make the new pixels. And even high-end photo-editing programs don’t do a good job of pulling pixels out of thin air, as illustrated by Figure 4-9.

  To create this figure, I started with the 75-ppi image shown in Figure 4-8 and resampled the image to 300 ppi in Adobe Photoshop, one of the best photo-editing programs available. Compare this new image with the 300-ppi version in Figure 4-6, and you can see just how poorly the computer adds pixels.

  With some images, you can get away with minimal upsampling — say, 10 to 15 percent — but with other images, you notice a quality loss with even slight pixel infusions. Images with large, flat areas of color tend to survive upsampling better than pictures with lots of intricate details.

• Use the highest resolution setting for pictures you may want to crop later. The numbers in Table 4-1 assume that you’re printing the entire photo. If you crop the photo before printing, you need more original pixels to generate a given print size because you’re getting rid of a portion of the image. For example, the left image in Figure 4-10 shows the tightest framing I could achieve with my camera, given the distance between the bird and my lens. Because I captured the image at a resolution of 24 mp, I could crop to the better composition shown on the right and still have plenty of pixels to produce a good print. In fact, the cropped file contains about 5.3 mp, so I can output a much larger print than fits on this page.

Pixels and screen images

Although resolution has a dramatic effect on the quality of printed photos, it’s irrelevant to the quality of pictures viewed on a monitor, a television, or another screen device. The number of pixels controls only the size at which the picture appears.

A bit of background to help you understand this issue: Like digital cameras, computer monitors (and other display devices) create everything you see on the screen out of pixels. When you display a digital photo, the monitor simply uses one screen pixel to display one image pixel.

For example, Figure 4-11 shows a screen capture taken from a 24-inch monitor that I use for a Windows computer. Windows enables you to adjust the resolution of the monitor (as does the Mac operating system), with each setting resulting in a different number of screen pixels. Usually, you get the best display when using the monitor's native (default) resolution, which on this monitor, is 1920 x 1200 pixels.

After setting my monitor to that resolution, I used the Windows display-customization options to place a 1536 x 960-pixel photo of my office assistant and life coach in the center of the display. The picture consumes just that number of screen pixels, with the remaining screen area occupied by the various Windows desktop icons and the plain blue background I chose as the desktop background.

Of course, for most onscreen uses, you don't want the picture to take up the entire display area or even as much screen space as shown in my example. When you share pictures on Facebook, for example, post them in an online gallery, or include them in a presentation, you need to leave ample screen space for text and other page elements.

The upshot is that you need far fewer pixels for images destined for screen display purposes than you do to produce large prints. Which begs the question: What do you do if you want to be able to print your pictures and share them online or use them for other screen purposes? Always set the camera resolution to match the print size you have in mind. As illustrated in the preceding section, you can’t add pixels later successfully to achieve a good print. You can, however, dump pixels from a high-resolution original to create a copy that’s appropriately sized for the screen. Your camera may offer a built-in tool that creates a low-resolution copy; if it doesn't, you can get the job done in any photo editing program.

Chapter 9 has more details about preparing a picture for online use.

Pixels and file size

Many factors affect the size of the data file needed to store a digital picture, including the complexity of the scene (the level of detail, the number of colors, and so on). The file format in which the image is stored, usually either JPEG or Raw, explained later in this chapter, also affects file size. But all other things being equal, an image with lots of pixels has a larger file size than a low-resolution image.

Although more pixels translates to better prints, as outlined earlier in this chapter, the large files needed to contain those pixels create several problems:

• Large files require more storage space. When you’re shooting huge files, it doesn’t take too long to fill up a camera memory card, computer hard drive, or a cloud (online) storage account. And if you archive your files on DVD, which I recommend for your most treasured images, you can wind up with a huge disc collection in no time, given that the maximum storage capacity of a DVD is about 4.7GB, or gigabytes. (One GB equals 1,000MB.)
• Large files take longer for the camera to capture. The more pixels you ask your camera to capture, the longer it needs to process and record the picture file to your memory card. That additional capture time can be a hindrance if you’re trying to capture action shots at a fast pace.
• Large files strain your computer. Large files make bigger demands on your computer’s memory (RAM) when you edit them. You also need lots of empty hard drive space for editing because the computer uses that space as temporary storage while it’s processing your images. (This temporary storage is sometimes referred to as virtual memory or scratch disk space.)
• Large files are inappropriate for online use. When placed on a web page or sent via email, photos that contain bazillions of pixels are a major annoyance. First, the larger the file, the longer it takes to download. And if you send people a high-resolution image in an email, they may not be able to view the whole image without scrolling. Remember, most computer monitors can display only a limited number of pixels. (See the preceding section for details.)

All that said, large files are a fact of life if you want to capture images at your camera’s highest resolution and quality settings. But do consider whether you always need to max out the pixel count. Are you really going to want to print the picture of your grandma’s birthday cake or your new car at 8 x 10 inches or larger? If not, dialing down resolution a notch makes sense.

JPEG

Pronounced “jay-peg,” this format is standard on every camera. JPEG stands for Joint Photographic Experts Group, the organization that developed the format.

JPEG is the leading camera format, for two important reasons:

• All web browsers, email programs, and mobile devices can display JPEG images. So you can share your pictures online seconds after you shoot them. Furthermore, every image-editing program for both the Mac and Windows supports working with JPEG photos.
• JPEG files are smaller than Raw or TIFF files. That means that you can store more pictures on your memory card as well as on your computer’s hard drive or whatever image-storage device you choose. Smaller files also take less time to transmit over the web.

The drawback to JPEG is that in order to trim file size, it applies lossy compression, a process that eliminates some original image data. (Many digital-imaging experts refer to this process as simply JPEG compression.) Heavy JPEG compression can significantly reduce image quality, especially when the image is printed or displayed at a large size.

Figure 4-12 offers an example of the bad things that can happen with excessive JPEG compression. The image takes on a parquet tile look and often exhibits random color defects — notice the bluish tinges around the eyelashes and near the jaw line. These defects are known collectively in the biz as JPEG artifacts, or simply artifacts.

Now for the good news: Most cameras enable you to specify the level of JPEG compression that you want to apply, and at the highest-quality setting, the file undergoes only a little bit of compression. The result is a file that provides the benefits of the JPEG format with little, if any, noticeable damage to picture quality, as evidenced by the example in Figure 4-13. Sure, the file size is larger — the high-quality JPEG in Figure 4-13 has a file size of 400K (kilobytes) versus the 33K size of the low-quality version in Figure 4-12 — but what good does that small file size do you if the picture is lousy? Both pictures contain the same number of pixels, by the way, so the quality differences you see are purely a result of compression amounts.

To sum up, as long as you stick with the highest-quality JPEG file your camera can produce, this format is the right choice for all but the most demanding photographers. (If you fall in the latter camp, explore the Camera Raw format, explained in the next section.) And note that even if you choose your camera’s lowest-quality JPEG setting, it’s highly unlikely that you’ll see the level of destruction shown in Figure 4-12 — the defects in the picture are exaggerated so that you can more easily see what JPEG artifacts look like.

Figuring out which camera setting delivers the top-quality JPEG file requires a look at your camera manual because the options differ between models. Typically, compression settings are given vague monikers: Good/Better/Best or High/Normal/Basic, for example.

These names refer not to the amount of compression being applied, but to the resulting image quality. If you set your camera to the Best setting, for example, the image is compressed less than if you choose the Good setting. Also, some cameras may use similar names to refer to image-size (resolution) options rather than JPEG-quality options.

You should find a chart in your manual that indicates how many images you can fit into a certain amount of memory at different compression settings. But you need to experiment to find out exactly how each setting affects picture quality. Shoot the same image at all the different settings to get an idea of how much damage you do if you opt for a higher degree of compression.

If your camera offers several resolution settings, do the compression test for each resolution setting. Remember that resolution and compression work together to determine image quality. You can usually get away with more compression at a higher resolution. Low resolution combined with heavy compression yields results even a mother couldn’t love.

When you edit images in a photo editor, you can save the altered file in the JPEG format. If you do, though, you apply another round of lossy compression. Each pass through the JPEG compression machine does further damage, and if you edit and save to JPEG repeatedly, you can wind up with the level of artifacting shown in Figure 4-12. So you should instead save works in progress in a format such as TIFF (explained later in this chapter), which does a much better job of preserving picture quality. Should you want to share your edited photo online — which requires a JPEG image — you can create a copy of your final TIFF image and save the copy in the JPEG format.

Camera Raw

When you shoot in the JPEG format, your camera takes the data collected by the image sensor and then applies certain enhancements — exposure correction, color adjustments, sharpening, and so on — before recording the final image. These changes are based on the picture characteristics that the manufacturer believes its customers prefer.

The Camera Raw format, sometimes called simply Raw, was developed for photo purists who don’t want the camera manufacturer to make those decisions. Camera Raw records data straight from the sensor, without applying any post-capture processes. After transferring the files to a computer, you then use special software known as a raw converter to translate the sensor data into the actual photograph.

Unlike JPEG, Camera Raw isn't a standardized format. Each manufacturer uses different data specifications and names for its Raw format. Nikon Raw files are called NEF or NRW files, for example, while Canon’s versions go by the name CRW or CR2, depending on the camera model.

Whatever the specific name, Raw image capture offers the following advantages:

• No risk of JPEG compression artifacts: Raw files offer higher image quality because they don't undergo the kind of serious file compression that causes the defects associated with JPEG lossy compression. Again, though, as long as you stick with your camera’s highest-quality JPEG setting, you may have difficulty detecting much difference in quality between the JPEG and Raw version of an image unless you greatly enlarge the photo.
• Greater creative control: When you process your “uncooked” picture data in a raw converter, you can specify characteristics such as brightness, color saturation, sharpness, and so on, rather than dine on whatever the JPEG version might serve up. To keep up the cooking analogy, imagine that you put together a special dish and you realize after it comes out of the oven that you accidentally used cayenne pepper when your recipe didn’t call for any. If your dish were like a Raw file, you’d be able to go back to the beginning of your cooking session and remove the pepper. With JPEG, that pepper would be a done deal. And that’s what photographers like about Raw — files captured in this format give you much more control over the final look of your images.
• Greater latitude for editing the picture: Raw capture also gives you a bit of a photographic safety net. Suppose that you don’t get the exposure settings quite right, for example, when you shoot a picture. With JPEG, the camera decides how brightly to render the shadows and highlights, which can limit you in how much you can retouch that aspect of your picture later. With Raw, you can specify what brightness value should be white, what should be black, and so forth, giving you greater ability to achieve just the image brightness and contrast you want — a benefit that’s especially great for pictures taken in tricky light.

• Higher bit depth: Bit depth is a measure of how many distinct color values an image file can contain. With JPEG, your pictures contain 8 bits each for the red, blue, and green color components, or channels, that make up a digital image, for a total of 24 bits. That translates to roughly 16.7 million possible colors.

  On most cameras, choosing the Raw setting delivers a higher bit count. You may be able to set the camera to collect 12 or more bits per channel, for example. However, you may not really ever notice any difference in your photos — that 8-bit palette of 16.7 million values is more than enough for superb images. Where having the extra bits can come in handy is if you really need to adjust exposure, contrast, or color after the shot in your photo editing program. In cases where you apply extreme adjustments, having the extra original bits sometimes helps avoid a problem known as banding or posterization, which creates abrupt color breaks where you should see smooth, seamless transitions. (A higher bit depth doesn’t always prevent the problem, however, so don’t expect miracles.)

Of course, like most things in life, the benefits of Raw do come at a price. First, most low-priced cameras don’t offer the format. But Raw also costs you in the following ways, which may be enough for you to stick with JPEG even if your camera is dual-natured in terms of file format:

• Raw files require some post-capture computer time. You can’t take a Raw image straight from the camera and share it online, use it in a screen presentation, retouch it in a photo editor, or, well, do much of anything with it until you process it in a Raw converter, which you can find in photo-editing programs such as Adobe Photoshop. Most camera manufacturers also provide a tool to do the job. As part of the conversion process, you specify critical image characteristics: color, sharpness, contrast, and so on. Then you save a copy of the processed Raw file in a standard file format, such as JPEG or TIFF (explained in the next section).

  Processing Raw files isn't difficult, but it does take time that you may prefer to spend behind the camera. And if you have a hate-hate relationship with computers, Raw is definitely not for you. Some cameras do have a built-in “mini converter” — that is, you can create a JPEG copy of a Raw image right in the camera. That feature makes the process faster and simpler, but it's not ideal because you have to make judgments about the picture based on a small camera monitor and you typically can specify only a handful of image characteristics. In other words, it's all the inconvenience of Raw without the creative-control benefits.

• You may not be able to view thumbnails of your images on your computer without converting them, either. You can still see the Raw filenames, but not image thumbnails.

  However — and here’s some good news — most camera companies offer their own image-browsing software for free. (Look on the CD that came with your camera or check the manufacturer's online support site for a program download link.) You should be able to view and maybe even print Raw images by using those manufacturer-created programs — just not in most third-party programs.

• Raw files are larger than JPEG files. Again, Raw files aren’t compressed, so they can be significantly larger than JPEG files, even if you capture the JPEG image at the lowest possible amount of compression. So you can store fewer Raw files on a memory card than JPEG files. (If your camera offers the Raw format, the manual should provide details on the size of its Raw files versus JPEG versions.) In addition, because you probably want to hang on to your original Raw files after you convert them, just for safety’s sake, you ultimately wind up with at least two copies of each image — the Raw original and the one that you converted to a standard format.

In short, because of the added complication of working with Raw files, you’re better off sticking with JPEG if you’re a photo-editing novice, a computer novice, or both. Frankly, the in-camera processing that occurs with JPEG is likely to produce results that are at least as good as, if not better than, what you can do in your photo editor if you’re not skilled at the task. (Just remember the earlier warning about saving files that you edit in the TIFF format instead of JPEG.)

Some cameras offer a setting that creates both a Raw file and a JPEG file so that you have a version of the image that you can use immediately and another that you can use if you’re inclined to spend hours editing it. Of course, you consume even more camera memory with this option because you’re creating two files — and later, you’ll have yet one more file after you process the Raw file on your computer. Depending on the camera, you may be able to choose a different resolution for your JPEG photos than you use for the Raw versions. You also may be able to select the amount of JPEG compression that's applied to the JPEG version.

TIFF

The third player in the image file format game is TIFF, which stands for Tagged Image File Format. Like Camera Raw, TIFF also offers higher picture quality than JPEG because TIFF files don't go through the JPEG lossy-compression mill. TIFF has long been the standard format for images destined for professional printing, where the highest quality is required.

Before the emergence of Camera Raw, some high-end cameras offered TIFF as an alternative to JPEG. Most cameras, now, though, don’t offer the option to record images as TIFF files. Instead, TIFF is a destination format: You save an edited JPEG file in TIFF format, for example, so that no more compression and data loss occur. Or if your camera offers the ability to save files in the Camera Raw format, you can save them in TIFF format after you process them so that you preserve the best image quality.

TIFF does present two problems: First, TIFF files are much larger than JPEG files. In addition, web browsers and email programs can’t display TIFF photos. If you do want to share a TIFF file online, you need to open the file in a photo editor and save a copy of it in the JPEG format.