Resolving image resolution

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Image resolution is nothing more than an instruction to a printing device about how large to reproduce each pixel. Onscreen, when working in Photoshop, your image has no resolution at all. An image that's 3000 pixels wide and 2000 pixels tall looks and acts exactly the same in Photoshop whether you've got the image resolution at 300 ppi or 72 ppi. Same number of pixels, right?

You can always check — or change — a picture's resolution by using the Photoshop ImageOImage Size command. The Image Size dialog box (which you can see in Figure 2-6) has two separate but related sets of information about your image. At the top, you see information about the actual image itself, in the Pixel Dimensions area. Below, in the Document Size area, you see instructions for a printing device — that "size" pertains only to printing and has no impact on what you do in Photoshop.

You'll find it very handy to change the pixel dimensions and the print size at the same time in the Image Size dialog box. And, much to the delight of the math-challenged among us, Image Size does most of the calculations for you.

©2001 PhotoSpin, PhotoSpin image #0520012

Figure 2-6: At the top, real information about your image. Below, simply printing instructions.

©2001 PhotoSpin, PhotoSpin image #0520012

Figure 2-6: At the top, real information about your image. Below, simply printing instructions.

Changing the size of your artwork with Image Size

You have a number of ways to change the size of your photos and other art. In Chapter 4, I introduce you to cropping (chopping off part of the artwork to make it fit a certain size or to improve its overall appearance and impact). You can use Photoshop's Image Size command to change the image dimensions (or printing instructions) without altering the composition (the visual arrangement of the image or artwork). All the content of the original image is there, just at a different size. Of course, as you can see in Figure 2-7, if you reduce the size of an image too much, some of that original content can become virtually unrecognizable.

Figure 2-7: As the smaller image shows, reducing an image too much isn't good.

If you know the specific pixel dimensions that you need for the final image — say for a Web page — you can simply type a new number in one of the upper fields in the Image Size dialog box and click OK. In most cases, you select all three check boxes at the bottom of the dialog box, enter your desired print width or height (letting Photoshop calculate the other dimension), enter your desired print resolution, and click OK. Of course, you probably want a little more control over the process, don't you? Figure 2-8 gives you a closer look at the Image Size dialog box.

Figure 2-8: Your choices can have a substantial impact on the appearance of your final image.

In the lower part of the Image Size dialog box, you have three decisions to make. The first is rather easy: If you're resizing an image that uses layer styles (see Chapter 12), you want to mark the Scale Styles check box to preserve the image's appearance as it shrinks or grows. In a nutshell, layer styles (such as shadows, glows, and bevels) are applied to a layer at a specific size. You can scale the image without changing those sizes or you can scale the image and change the style sizes proportionally. Not scaling layer styles can dramatically alter the appearance of a resized image, as you can see in Figure 2-9. A slight bevel combined with a small drop shadow produces a subtle 3-D effect in the original (upper). Below, when the image is scaled down to one-fourth the original size without scaling the effects, your chips change to chumps, and the artwork is ruined.

Figure 2-9: Scaling an image without scaling its layer styles can ruin your image.

The middle check box, Constrain Proportions, should almost always remain checked. There might be some exceptions, but you normally want to preserve an image's aspect ratio (the relationship between height and width) when resizing to prevent distorting the image. Figure 2-10 shows you what can happen when you scale one dimension without constraining the image's proportions.

The third check box, Resample Image, is the one that most often requires the attention of that gray matter within your skull. Not only do you need to decide whether you want to resample the image (change its pixel dimensions), but you also need to decide how you want to resample. Refer to Figure 2-8 to see that you have five different ways to calculate the change (called resampling algorithms).

Figure 2-10: Resizing an image without constraining proportions. Interesting, yes, but useful?

Before I talk about those choices, let me explain what happens when the Resample Image box is left unchecked. The top portion of the Image Size dialog box becomes unavailable — you can no longer make changes in the Pixel Dimension fields (as you see in Figure 2-11). The information is there, but it won't change. When you clear the Resample Image check box, you protect the original pixels, preventing any change to the image itself.

Figure 2-11: Clear the Resample Image check box to change print size, not pixel dimensions.

So what's left? When using Image Size without resampling, you're simply changing the instructions recorded in the image for your printing device. When you enter one dimension, either width or height, Photoshop does the math and fills in both the other dimension and the new resolution. Or, if you'd like, you could change the Resolution field and have Photoshop recalculate the print dimensions although it generally makes more sense to specify a desired print size.

Take a look at Figure 2-12. I cleared the Resample Image check box and entered 10 and inches for my new print width in order to print this image to a letter-size (8.5" x 11") sheet of paper. Photoshop fills in the new height (6.667 inches) and a new resolution (307.2 ppi). But what if I want an 8 x 10 print? If I enter 8 and inches for the height, Photoshop recalculates the width to 12 inches (and enters a resolution of 256 ppi). If I want a true 8 x 10, I have to crop some of the image because a digital photo and an 8 x 10 have different aspect ratios. (You can read more about that in Chapter 4.)

Okay, then, back to the subject of resampling! When you resample an image (change the pixel dimensions), Photoshop takes the image and maps it to the new size, attempting to preserve the image's appearances as much as possible at the new size, using the new number of pixels. Of course, if you take an image that's more than 3000 pixels wide and resample it to 300 pixels wide, you're going to lose some of the detail. (Remember what happened to Hugo the Bulldog earlier in this chapter in Figure 2-3!)

Figure 2-11: Clear the Resample Image check box to change print size, not pixel dimensions.

Figure 2-12: Enter a value, and Photoshop recalculates the fields automatically.

Figure 2-12: Enter a value, and Photoshop recalculates the fields automatically.

To resample or to crop: That is the question

In order to better understand the difference between resampling an image and cropping an image, consider this situation:

1. A painter paints a picture. He or she paints it at whatever size he or she thinks is appropriate. (Or, perhaps, on the only piece of canvas he or she can afford on that particular day.)

2. A patron likes the artwork, but the painting is too large for the frame that works best with the dining room table. Yeah, patrons can be like that, can't they?

3. The patron asks the artist to make the painting fit the frame.

4. The artist decides between cropping and resampling. He or she can grab a pair of scissors and cut off some of the painting (cropping) or the artist can re-create the painting from scratch at a smaller size, repainting the scene. Thankfully, Photoshop does the "repainting" for us, using Image Size with its resampling algorithms.

5. The artist charges the patron for the extra work.

(Let's not forget this final, crucial step!)

Cropping cuts away part of the image to meet a target size. Resampling retains all the image but shrinks or enlarges it to meet the target size.

Choosing a resampling method

After you decide to change the pixel dimensions of your image via the Image Size command, you need to select one of the five resampling algorithms (take another peek at Figure 2-8). Table 2-1 sums up my advice on choosing the right one.

If you'll be resizing a number of images in the same way, either upsampling or downsampling, open Photoshop's PreferencesOGeneral and select the optimal resampling method in the Image Interpolation menu. That algorithm then becomes the default in Image Size, saving you the trouble of changing the algorithm each time.

Keep in mind that the resampling method that you select in the Preferences is also used for the EditOTransform commands, which you use to scale, rotate, and otherwise alter individual elements within your artwork. (You can read about the Transform commands in Chapter 10.)

Table 2-1 Selecting a Resampling Algorithm

Algorithm Name

Use It For


Nearest Neighbor

Artwork containing large areas of solid color like a pie chart or a simple logo; avoid using with photos

Maintains pixel color


Artwork having horizontal and vertical lines and blocks of color, such as Web page buttons; avoid using with photos

Maintains the sharpness of your edges


General-purpose resampling for minor changes in image size

Calculates the average of surrounding pixels when generating a new pixel

Bicubic Smoother

Increasing the size of photographic images or artwork with gradients and gradual shifts in color

Gives the most pleasing result when increasing the number of pixels (upsampling)

Bicubic Sharper

For reducing the number of pixels in a photo (down sampling)

Maintains the crispness of your image

■ ■

Figure 2-13: The close-up shows inkjet printer droplets.

Picking an image resolution

After you have resampling under your belt, how do you know what size you should be resampling to? How many pixels do you need? Here are your general guidelines:

ii Photos for your inkjet printer:

Inkjet printers are stochastic printing devices: That is, they use a series of droplets to replicate each pixel in your image, as shown in Figure 2-13. In theory, the optimal image resolution is one-third of the printer's rated resolution. For example, a printer rated at 720 ppi works best with images at 240 ppi. For a 1440 ppi printer, the formula calls for an image resolution of 480 ppi (and that goes for the 2880 x 1440 printers, too). However, most folks find that regardless of the printer's rating, they never need an image resolution higher than 300 ppi.

i Web images: Ignore resolution. Ignore the entire Document Size area of the Image Size dialog box. Consider only the image's pixel dimensions. Determine what area of the Web page the image will occupy and then resize to exactly those pixel dimensions.

i Page layout programs and commercial printing: If your image is to be placed into a page layout program's document and sent to a commercial printing facility, you need to know the line screen frequency (the resolution, so to speak) of the printing press on which the job will be run. Ask the print shop or the person handling the page layout. Your image resolution should be either exactly 1.5 times or exactly twice the line screen frequency, either one. (You shouldn't notice any difference in the final printed product with either resolution.)

i PowerPoint presentations and word processing documents: Generally speaking, 72 ppi is appropriate for images that you place into a presentation or Word document. You should resize to the exact dimensions of the area on the page or slide that the image fills.

Sometimes resampling an image to 300 ppi does more harm than good. You might get a better print at the image's original resolution than you will at the higher resolution after resampling. If your image's original resolution is 240 ppi or more, consider printing at that resolution rather than upsampling to 300 ppi. When in doubt, work on a copy of your image so that you can return to the original if necessary.

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