Photoshop | Supplement


 

 

 

 

 

 

 

 

 

 

 

 

 

Check the Image Size dialog at the beginning a project. Consider your target linear resolution (ppi) and the printed or screen dimensions you're aiming for.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Scaling an image up or down can result in a loss of image quality, if you resample the image.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Photoshop artists generally try to avoid sacrifices in image quality through down or upsampling.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Tweaking canvas size, which allows you to add or subtract pixels from the edges of your image, is one way to adjust the dimensions of your image without reducing image quality.

File Input and Output Essentials

To publish images successfully, every designer must understand certain technical fundamentals of linear resolution and image dimensions. I cannot overemphasize the importance of these concepts. I travel the country giving seminars, and I encounter designers all the time who are masters of Photoshop effects but cannot output an image at the correct dimensions for printing!

With that in mind, let's look at some technical essentials of file input and output, and pointers for using the right file formats. We'll encounter this topic (and expand on it) as we move through the course.

Image resolution is one of the more confusing topics to new Photoshop users. And the term resolution is unfortunately often misused and therefore often misunderstood. Here are a few basic tips to help you understand resolution:

 
 
  • Use resolution terminology that matches the building blocks of your image. In Photoshop we are always working with pixels, so we measure resolution in pixels per inch (ppi), rather than dots per inch (dpi) which is used for discussing printed images where pixels are converted into dots.

  • Distinguish between linear and dimensional resolution. Linear resolution refers to the number of building blocks, in this case pixels, per linear unit (usually inches), for instance 300 ppi. Dimensional resolution is used to describe the dimensions or width and height of an image using pixels—for instance, 1024 pixels horizontally x 768 pixels vertically.

  • Adjust your dimensional and linear resolutions to match your output device. When you prepare your image for output, you will want to adjust the linear resolution of your image to match the requirements of your output device. For print images, linear resolution is typically 300 ppi (the linear resolution required by high quality printers). For Web images, linear resolution is typically 72 ppi (the linear resolution of many monitors). Printing requires more pixels than Web displays.

 
 

Each image you create will have a specific combination of dimensions and linear resolution. For example, a print image may have dimensions of 5" x 7" at 300 dpi, resulting in a 1500 pixels x 2100 pixels file. A Web image may have dimensions of 2" x 3" at 72 ppi, or 144 pixels x 216 pixels.

Typical print image: 5" x 7" (1500 pixels x 2100 pixels) @ 300 ppi
Typical Web image: 2" x 3" (144 pixels x 216 pixels) @ 72 ppi

And as you will see below, linear and dimension resolutions are interrelated and controlled in Photoshop through the Image Size dialog.

Linear Resolution

The bitmap images that we edit in Photoshop are made out of pixels. Those little colored squares are the key to everything in Photoshop. The more pixels packed into an image at specific output dimensions, the more detail it has—and the more detail it has (generally speaking), the better it pleases the eye.

The level of detail in an image is controlled by its linear resolution. This is measured in pixels per inch or ppi, though print designers will often refer to dots per inch (dpi) and lines per inch (lpi) when working with printers.

We refer to the resolution when capturing or creating an image as input resolution. For example, many digital cameras capture images at a linear resolution of 72 ppi (pixels per inch). Remember, we can use 72 ppi images for Web displays but not for quality printing.

For many print jobs, the standard image linear resolution is 300 ppi—the higher the ppi, the more pixels printed, and the higher the quality, which is essential for printed designs. For high-end projects or larger size jobs like posters or billboards, the output resolution is typically 300 ppi or above.

An image at 100 ppi (pixels per inch) will literally display 100 pixels x 100 pixels per square inch. You can count them if you don't believe me.

If you are creating images to be used over the Web, you will focus primarily on the dimensions of your image at a low linear resolution such as 72 ppi. Screens typically have a low resolution of 72 ppi, so this is the standard for Web design.

Controlling Image Dimensions

Understanding how linear resolution (ppi) works is particularly important because it affects image dimensions: the width and height of an image upon printout, as well as how much of your screen is taken up by the image.

If you are a print-oriented designer, this is your bread and butter, because you need to provide enough pixels per inch in order to print your images at high quality.

Let's open up an image in Photoshop to explore how it works. Open up the Arches park image we discussed in Lecture One (Arches_RGB_300.jpg from your Lecture One downloads) and go to Image > Image Size. This is the Image Size dialog, a thing of great power and beauty:

The Image Size dialog tells you all about your image's dimension and resolution and lets you make changes to it.

In your work, you'll visit the Image Size dialog often (generally at the beginning or end of a project) to make changes to the dimensions or resolution of an image.

The Pixel Dimensions area of the dialog displays the dimensions of your image in pixels and its file size in kilobytes (K) or megabytes (M). The Document Size area displays the output document dimensions (by default in inches) and the linear resolution in pixels per inch.

Let's review what the dialog tells us about the Arches image:

 
 
  • File size: The file size of the image is 3.09 MB.

  • Width and height (dimensional resolution): This image is 1200 pixels wide and 900 pixels high.

  • Linear resolution: The resolution of the image is currently 300 pixels—perfect for print.

  • Document size (output print dimensions in inches): The image will print out at 4 inches wide and 3 inches high.

 
 

Try printing this image from Photoshop, and you will indeed find that it prints out at 4 inches by 3 inches. Voila!

To get a better understanding of image dimensions and how they affect the size of an image on your screen, let's check out the other versions of the Arches image. Open Arches_RGB_100, Arches_RGB_72, and Arches_RGB_36 (all found in the Lecture One downloads), then open the Image Size dialog for each image. You'll note that all the images have the same output document size: 4" x 3". The only difference is their linear resolution, which affects their size on your screen.

Now stack all four of these images on screen from largest to smallest: 300, 100, 72, 36. CS4 users, when you open multiple images, they appear in a single, tabbed interface—drag the tabs out of that interface to make them individual documents you can move around individually. CS3 users, your documents all open individually and can be moved around as needed by dragging the top bar of each image.

Note how their on-screen dimensions are directly proportional to their linear resolutions. For example, the 300 ppi version of the image is exactly three times larger than the 100 ppi version.

So now you can clearly see how images with the same output dimensions (printed size) can appear at different sizes on your screen. Their on-screen dimensions depend upon their linear resolution and therefore their dimensions in pixels.

All of these images will print at 4" x 3", but the linear resolution dictates how large they'll appear on your screen and how much detail they contain.

Scaling Images

What if you want to print an image like Arches_RGB_300 at any other linear resolution or any other printed dimensions? How will you make sure it prints at the right size, without sacrificing any level of detail or quality? Through quiet mastery of the Image Size dialog, that's how! In the Image Size dialog, you can scale any image (change the output dimensions of your image) or resample it (change the number of pixels in it).

To do this, you change the values in the number fields while keeping an eye on the checkboxes at the bottom of the Image Size dialog.

Make a copy of your Arches_RGB_300 image (Image > Duplicate) and go to Image > Image Size. Make sure the Constrain Proportions and Resample Image checkboxes are checked (they should be by default). Now change the Document Size width (output dimension) from 4" to 3".

The height will automatically change from 3" to 2.25." (This is the Constrain Proportions checkbox at work.) Note how the linear resolution of 300 ppi has not changed, but the number of pixels in the image (the on screen pixel dimensions) will have dropped from 1200 pixels x 900 pixels to 900 pixels x 675 pixels.

Everything changed in the dialog except the resolution, which is still 300 ppi (print quality.)

Here you have changed the print output dimensions of the image (the size it at which it prints out) without changing the linear resolution, so the number of pixels in your image have been reduced. You would perform this type of scaling if you wanted to reduce the printed size of an image but maintain the linear resolution for printing.

What's happened to your image, though? You may not notice it, but a softening effect has occurred and you have lost quality in your image.

Any time you scale down an image without changing resolution, you lose definition. Photoshop is forced to throw away pixels because it wants to fit the same number of pixels per inch. Any time you scale up an image without changing resolution, Photoshop will add pixels by guessing at adjacent pixel values (this is known as interpolation), resulting in a blurry, or in extremes cases, a pixelated effect.

See, I told you the Image Size dialog was important! It's very easy to inadvertently destroy the quality of your images if you're not careful.

Resampling Images

If you wanted to reduce the print output dimensions of an image and prevent the image softening that may occur in the scaling process, you could resample the image.

Open up the original Arches image and make another duplicate of it. Uncheck the Resample Image checkbox and reduce the Document Size dimensions to a Width of 3". This time, your pixel dimensions will remain the same, but your print output dimensions will be reduced and your linear resolution will increase to 400 ppi. Photoshop will attempt to print the exact same number of pixels in a smaller space!

If you wanted to create a Web image from a print image like the Arches_RGB_300 file, you could keep Resample Image checked, then change the linear resolution to the standard on-screen resolution of 72 ppi.

Try it with a duplicate of the original Arches_RGB_300 image. Here the image size (kB or MB) will be reduced because some of the detail (pixels) is being discarded. The pixel dimensions would be reduced to 288 pixels in width.

Now try printing the results for yourself so you can actually see the difference in dimension and resolution in a printout. Print each of the different sampled image sizes and look at them closely.

I know this whole topic of resolution is a bit confusing at first, but hang in there until you get it straight in your head. Being clear about how to correctly use the Image Size dialog will allow you to control the image dimension, print dimension, and linear resolution of your images for various output devices with a minimum loss of image quality.

Canvas Size

Canvas size is an issue that's related to image dimensions because it can affect the overall output dimensions. You can crop or add extra space around an image in the Canvas Size dialog (Image > Canvas Size).

Try it with a duplicate of the original Arches_RGB_300 image. Go to Image > Canvas Size and you'll see your current image size and output dimensions. From here, you could trim down the size of the canvas, such as lopping off a half inch on all sides, or you could add some extra space around the image to make a border.

Let's add a border. Check the Relative checkbox and enter 1 inch in the Width and Height fields. By default, the center square of the anchor area will be selected, meaning that your changes will affect each side of the image equally. Click OK to see your border.

If you'd like to pick a color for your border, you can do it in the Canvas extension color menu.

Note: If you check your Image Size dialog, you'll now find that your image is 5 inches wide by 4 inches high. Photoshop has added a frame of one inch around the image.