Advanced cropping, resizing, resampling | Photoshop

Important: This document provides background information. For instructions on how to resize photos, see Image size and resolution in Photoshop Help.

You can resize images in several ways using Adobe Photoshop. Each way to resize an image has advantages and drawbacks. To get the best result when you resize images, it's best to understand the concepts behind and the results of each method of resizing. These concepts affect how the crop tool options operate as well. If you resize and crop images without being aware of the concepts involved, then you can see unexpected results.

The size of an image when you view it on the screen is different from the size of an image when you print it. If you understand these differences, then you can develop a better understanding of which settings to change when you resize an image.

The screen resolution of your monitor is the number of pixels that your monitor can display. For example, a monitor with a screen resolution of 640 x 480 pixels displays 640 pixels for the width and 480 pixels for the height. There are several different screen resolutions you can use, and the physical size of the monitor screen usually determines the resolutions available. For example, large monitors typically display higher resolutions than small monitors because they have more pixels.

To find out what your screen resolution is, choose Start > Control Panel > Display > Settings. Look at the screen resolution (Windows) or System Preferences > Displays and look in the Resolution list (Mac OS).

Images are of a fixed pixel size when they appear on your monitor. Your screen resolution determines how large the image appears onscreen. A monitor set to 640 x 480 pixels displays fewer pixels than a monitor displaying 1024 x 768 pixels. Therefore, each of the pixels on the 640 x 480 pixel monitor is larger than each of the pixels displayed on the 1024 x 768 pixel monitor.

A 100 x 100-pixel image uses about one-sixth of the screen at 640 x 480, but it only takes up about one-tenth of the screen at 1024 x 768. Therefore, the image looks smaller at 1024 x 768 pixels than at 640 x 480 pixels.

The other values used in resizing images, the physical size of the image when printed, and the resolution, aren't used until the image is printed. Then, the physical size of the image, the resolution, and the pixel dimensions determine the amount of data in the image and its print quality. In general, higher resolution images print at a higher quality. See the sections that follow for more information on resolution and physical size in the below sections.

When you use the Image Size dialog box to resize your images (choose Image > Image Size), four aspects of your image can change:

• the Pixel Dimensions: the width and height of the image
• the size of the image when it's open in Photoshop: this value appears next to the Pixel Dimensions area title
• the Document Size: physical size of the image when printed, including a width and height
• the resolution of the image when printed: this value appears in the Document Size area, in pixels per inch or pixels per centimeter.
  
  

Photoshop calculates the physical size, resolution, and pixel dimensions of an image as follows:

• Physical size = resolution x pixel dimensions
• Resolution = physical size / pixel dimensions
• Pixel dimensions = physical size / resolution

The Image Size dialog box allows you to resize your images in two ways. You can increase or decrease the amount of data in the image (resampling). Or, you can maintain the same amount of data in the image (resizing without resampling). When you resample, the image quality can degrade to some extent. You could have to do some extra work, such as using the Unsharp Mask filter to sharpen your image, to compensate for the resampling.

Tip: To reset the Image Size dialog box to its original state, press Alt (Windows) or Option (Mac OS). Pressing these keys changes the Cancel button to a Reset button.

When you resize and resample an image, you change the amount of data in that file. To resample your image, ensure that Resample Image is selected at the bottom of the Image Size dialog box. Resample is on by default.

Resampling changes the total number of pixels in the image, which are displayed as Width and Height in pixels in the Image Size dialog box. When you increase the number of pixels in this part of the dialog box (upsampling), the application adds data to the image. When you decrease the number of pixels (downsampling), the application removes data. Whenever data is removed from or added to the image, the image quality degrades to some extent. Removal of data from an image is typically preferable to the addition of data. That's because upsampling requires that Photoshop guess which pixels to add. This procedure is more complex than guessing which pixels to remove when you downsample. You get the best results working with images that you bring into Photoshop in the proper resolution for the output you want. You could get the results you need by resizing your image without resampling. However, if you resample your images, do so only once.

When you turn on Resample, you can change any of the values in the Image Size dialog box: pixel dimensions, physical size, or resolution. If you change one value, then you affect the others. The pixel dimensions are always affected.

• Changing the pixel dimensions affects the physical size but not the resolution.
• Changing the resolution affects the pixel dimensions but not the physical size.
• Changing the physical size affects the pixel dimensions but not the resolution.

You cannot set the file size; it changes when you change the total amount of data in the image (the pixel dimensions). Note the file size value before you change the other values in the dialog box. Then, you can use the file size information to understand how much data is removed or added to your image when you resample it. For example, if the file size changes from 250 KB to 500 KB, you add twice as much data to the image, which can degrade it. Degraded images can look blurry, jagged, or blocky.

When you resize an image and do not resample it, you change the size of the image without changing the amount of data in that image. Resizing without resampling changes the physical size of the image without changing the pixel dimensions in the image. No data is added to or removed from the image. When you deselect, or turn off, Resample, the pixel dimension fields are not available. The only two values you can change are the physical size (Width and Height in Document Size) or the resolution (pixels/inch). When you resize without resampling, you can set either the physical size, or the resolution of the image. To keep the total amount of pixels in the image the same, Photoshop compensates for the value you set by increasing or decreasing the other value. For example, if you set the physical size, Photoshop changes the resolution.

When the pixel dimensions are constant and you decrease the physical size of an image, the resolution increases correspondingly. If you decrease the physical size of an image by half, the resolution doubles. Twice as many pixels can fit into the same space. If you double the size of an image, the resolution decreases by half, because the pixels are twice as far apart to fit the physical size.

For example, an image is 400 x 400 pixels, has a physical size of 4 x 4 inches, and has a resolution of 100 pixels per inch (ppi). To reduce the physical size of the image by half without resampling, you set the physical size to 2 x 2 inches. Photoshop increases the resolution to 200 ppi. Resizing the image this way keeps the total number of pixels constant (200 ppi x 2 x 2 inches = 400 x 400 pixels). If you double the physical size (to 8 x 8 inches), then the resolution decreases to 50 ppi. Adding more inches to the image size means that there can only be half as many pixels per inch. If you change the image resolution, the physical size changes as well.

Important: The pixel dimensions control the amount of data, and the resolution and the physical size are used only for printing.

Note: Pixels per inch (ppi) is the number of pixels in each inch of the image. Dots per inch (dpi) relates only to printers, and varies from printer to printer. Generally, there are 2.5 to 3 dots of ink per pixel. For example, a 600-dpi printer only requires a 150- to 300-ppi image for best quality printing.

For more information about the options in the Image Size dialog box, see About pixel dimensions and printed image resolution in Photoshop Help.

When you use the Crop tool to resize an image, the pixel dimensions and the file size change but the image isn't resampled. When using the Crop tool, the pixel dimensions and resolution incorporate more pixels per inch based on the size of the crop region. However, Photoshop isn't specifically adding or removing data from the image.

When you crop an image, you remove data from or add data to the original image size to create a different image. Because you are removing or adding data relative to the original image, the concept of resampling loses much of its meaning. That's because the number of pixels per inch can vary based on the number of pixels in the crop selection area. When the number of pixels in the crop area allows, Photoshop tries to keep the same resolution of the original image. This method is considered cropping without resampling. However, when you are not exact about the number of pixels you select, the pixel dimensions and file size changes in the new image.

The options in the Crop tool options bar change after you draw the selection area. When you first select the Crop tool, you can specify a Width, Height, and Resolution. You can measure the width and height in inches, centimeters, millimeters, points, and picas. Type the unit or the unit abbreviation after the number in the value field. For example, 100 px, 1 in, 1 inch, 10 cm, 200 mm, 100 pt, or 100 pica. If you do not specify a unit of measure in the Width and Height fields in the Crop options bar, the default unit is inches.

You can also set a value for the resolution of the cropped image in the Resolution field. Choose pixels/inch or pixels/cm from the pop-up menu.

For more information on Crop tool options, see Crop images in Photoshop Help.

When you set the physical size of the image in inches in the Crop tool options and you don't change the resolution, the pixel dimensions change. The dimensions change based on the ratio of the number of pixels you drew in the crop selection to the pixel dimensions of the original image. The resolution changes to fit the extra pixels into each inch of the image based on the image's original size.

Note: The original image used in the examples below is 4 x 4 inches, 100 ppi, 400 x 400 pixels at 468.8 KB.

In this example, Photoshop reduces the image by half of the physical size (from 4 inches square to 2 inches). Photoshop also reduces the pixel dimensions by 50%. The original resolution (100 ppi) is maintained but it increases to compensate for the extra pixels (8 pixels/inch) added to the crop rectangle.

When you set the physical size of the image in inches in the Crop tool options, and change the number of pixels per inch, the pixel dimensions change. The resulting image has more or fewer pixels in the document as a whole. You set the inches and the number of pixels in each of those inches. Photoshop removes or adds data to fit the number of pixels in each of the inches you specified.

Note: The original image used in the examples below is 4 x 4 inches, 100 ppi, 400 x 400 pixels at 468.8 KB.

In the first example, you reduced the physical size by half but balanced that by doubling the resolution. Therefore, the pixel dimensions and file size remained the same.

In the second example, you reduced the physical size by half and increased the resolution. Therefore, the pixel dimensions increased to hold the extra number of pixels per inch. The file size also increased.

In the third example, you reduced the physical size by half and reduced the resolution (the ppi). Therefore, the pixel dimensions decreased because there are fewer pixels now in the image. The file size also decreased.

When you set the pixel dimensions but you do not set the resolution, the resolution stabilizes at the same resolution as the original image. The new physical size is produced to hold the number of pixels specified in the image and per inch. The file size changes because you are changing the pixel dimensions while letting Photoshop stabilize the number of pixels per inch.

Note: The original image used in the examples below is 4 x 4 inches, 100 ppi, 400 x 400 pixels at 468.8 KB.

In these examples, the resolution are unchanged but the pixel dimensions have changed. The physical size changes to fit the number of pixels per inch you specified (pixel dimensions).

When you set the pixel dimensions and the resolution, Photoshop creates a different physical size. The image holds the number of pixels in the image and number of pixels per inch that you specified. The files size changes because you are changing the total number of pixels in the image and the number of pixels in each inch.

Note: The original image used in the examples below is 4 x 4 inches, 100 ppi, 400 x 400 pixels at 468.8 KB.

In these examples, both the pixel dimensions and resolution are changed. The physical size changes to fit the total number of pixels and the number of pixels in each inch (pixel dimensions and resolution).

When you change only the resolution in Crop tool options, the size of the image depends on the number of pixels in the crop area.

Tip: Look at the Info panel to see how many pixels are included in your crop area.

Note: The original image used in the examples below is 4 x 4 inches, 100 ppi, 400 x 400 pixels at 468.8 KB.

In these examples, Photoshop uses the size of your crop selection and the resolution you specify to resize the image. The new image's physical size and pixel dimensions fit the number of pixels in the crop selection you drew and the new resolution you set.

The Adobe Captivate demonstration below shows how to adjust image size and resolution in Photoshop CS3. The demo shows how to adjust the image without using the Resample Image option in the Image Size dialog box. For help viewing this demo, see View Adobe Captivate demos in knowledgebase documents.