Image Size and Resolution

Pixels And Bits

Adobe Photoshop only raster images, also called bitmap images. Bitmap Images are made up of individual pixels (picture elements) of information placed on a grid, which a computer understands. Photoshop can import other types of files, such as portable Document Format (.pdf) and path-based vector files, produced by Adobe Illustrator and other drawing programs. In Photoshop,other types are rasterized(translated into pixels) on export.

Image Resolution

Image resolution is not to be confused with monitor resolution or printer resolution. Image resolution is the pixel density of an image, measured in pixels per inch (ppi) of an image. Image resolution is closely related to image file size, measured in kilobytes (KB) or megabytes (MB). The more pixels per inch, the larger the image file size. Note: Print output needs a higher image resolution than screen output.

Print Output

Print output is intended for desktop printers or commercial printers. Printer resolution is measured in ink dots per inch (dpi) on the page, monitor units are pixels per inch (ppi). To make clear, crisp images, printers require much more image data, and therefore higher image resolutions, than monitors. Standard image resolutions for print output are: 240 ppi for desktop printers300 ppi for commercial printers

Resampling

Resampling is Photoshop's process of determining which pixels can be deleted or added, with least effect on the image, in a process called interpolation. Resampling down to a lower image resolution deletes pixels but usually retains image quality. Caution: Resampling up to a higher image resolution adds pixels and always results in a lower-quality image. When choosing images for print especially, make sure the image resolution and document size are close to your output needs.

Screen Output

Screen output is intended for websites, email, video, or small-screen presentations. Because the output is another computer screen, the image resolution can be close to the monitor resolution. Monitor resolution is the number of pixels a monitor can display per inch (ppi). The base is 72 ppi for Macs and 96 ppi for PCs.Display settings can increase monitor ppi and make the same image appear smaller. 72 ppi is the standard image resolution for screen output.

Document Size

There is one more piece to the imaging puzzle. Document size is the dimensions of the output image, where image resolution is measured in pixels and document size is measured in inches. Monitors can only display at the monitor resolution, regardless of the image resolution. For screen output, actual pixels (100% view) will be close to the document size on the monitor. For high-resolution print output, actual pixels (100% view) will appear much larger on the monitor than the document size.

Vector Graphics

Unlike bitmap images, vector images are not made up of pixels. Instead, they are made up of points, curves, and paths defined by mathematical objects called vectors. Also unlike bitmap images, objects that make up a vector image can be scaled up or down with no loss in image quality. For this reason, they are often used for type and logos. For a good explanation of the differences, see Adobe Photoshop CC Classroom in a Book, page 206. Photoshop CC has a basic set of vector tools, including the Pen tool and custom shapes. You can also import a vector drawing as a Smart Object, which remains editable in the program that created it.

Photoshop's Image Size Dialog Box

Use the Image Size dialog box to alter the dimensions or resolution of a file. When the Resample image option is not selected: The Width, Height, and Resolution fields are linked (note the chain icon).If you change one value, such as resolution, the other two values change automatically.The Dimensions values stay the same; no pixels are added to or deleted from the file.

Resampling in Photoshop

When the Resample image option is selected in the Image Size dialog box, the Dimensions fields and the Width and Height fields are linked: Increasing a value in any field will add pixels to the file.Decreasing a value in any field will delete pixels from the file.

Resampling While Cropping

You can change the resolution while cropping an image: 1.Click the Crop tool.2.In the Options bar, click on the second pop-up menu, and choose W x H x Resolution.3.Experiment with different image dimensions, preset crop options, and resolutions by typing values on the Options bar.4.Adjust the crop handles to define the crop boundaries.5.Press Return/Enter to crop. Note: This tool uses resampling, so any increase in resolution will degrade the image.

Resolution and Output

You've probably experienced one or both of these situations: Some images look great onscreen but are broken up and "pixelated" when printed.Most email and web images open quickly, while some take forever to download and open. If you suspect a connection, you are right: The connection is called image resolution, and it's different for different output types. Images optimized for onscreen viewing don't print well, while images optimized for printing create very large files.