60. Change Image Size or Resolution
Before You Begin
59 About Size and Resolution
6 Import a Scanned Image
62 Increase the Area Around an Image
66 Print an Image
As you learned in 59 About Size and Resolution, an image's size is tied directly to the number of pixels in the image as well as the relative size of the pixels. When you create images with a digital camera or scan printed images with a scanner, you choose the resolution you want to usefor instance, 300 pixels per inch. The resolution you choose also determines the resulting print size. For example, an image that's 2048 pixels wide by 1536 pixels tall (the typical dimensions of an image taken with a 3 megapixel camera), whose resolution is 300 pixels per inch, will print at 6.827" by 5.120".
So what do you do if you want to print your image at a different sizelarger or smallerwhile maintaining or even increasing its resolution to, say, 300 DPI? Answer: You use resampling. When you use resampling to increase an image's print size and/or its resolution, new pixels are inserted between existing ones. The Editor determines the colors for these new pixels by sampling the color value of each surrounding pixel, calculating a value within the sample range, and assigning that value to that new pixel. Conversely, when you reduce an image's print size, resampling removes pixels from the image then adjusts the colors of the pixels remaining in the image by approximating the blended color values of the pixels that were removed.
To calculate the print dimensions of an image yourself, take the image size in pixels and divide it by the number of pixels per inch. To display an image in its print size, click the Zoom tool on the Toolbox and then click the Print Size button on the Options bar. To view an image's size in pixels, resolution, and print size, display the Image Size dialog box by choosing Edit, Resize, Image Size.
Because resampling is based on best-guess estimation, using it to change an image's size or resolution by more than 20% often produces poor results. You can resize or change an image's resolution without resampling by telling the Editor that you want to turn resampling off, and therefore maintain the relationship between the size and the resolution. In this manner, you can double an image's print resolution by cutting its print size in half (the image will contain as many pixels as it did before, but the pixels will be smaller, and there will be more of them per inch). Onscreen, you won't see any apparent change at all.
Resampling The mathematical process applied during image resizing that evaluates the content of the pixels in the image in order to calculate the value of new pixels (when enlarging) or neighboring pixels (when reducing), and which re-interprets the result to minimize loss of detail.
Choose Image, Resize, Image Size
In the Editor, open the image you want to resize or whose resolution you want to change, and save it in Photoshop (*.psd) format. Then choose Image, Resize, Image Size from the menu bar. The Image Size dialog box appears.
To resize a group of images in one step, see 61 .
One fast way to remove moiré patterns, fuzziness, and spots created when you scan an image is to scan at 600 DPI and then reduce its resolution to 300 DPI while maintaining its print size. See 128 Restore Quality to a Scanned Photograph.
If you want to make sure that the image is not distorted during the resizing process, enable the Constrain Proportions check box.
If you've applied a layer style to the image and want the pattern of that style to be resized as the image is resized, enable the Scale Styles option as well. Note that the Scale Styles option does not affect the size of patterns formed by effects, so you might want to apply such embellishments after resizing the image.
Turn On Resampling
To have the Editor mathematically re-evaluate and re-render the content of the image when you change its print size or resolution, enable the Resample Image option and select a sampling formula from the list. Here's a brief description of the formulas:
If you want to print an image in some size other than its normal print size, you can "rescale" the image on the fly when you print it. If you print an image in a larger size than normal, however, the resolution is decreased proportionately to compensate (pixels are not added). If the resulting resolution falls below acceptable levels of quality, you'll see a warning, so that you can choose a different print size. Regardless, with this method, the original resolution and print size of the image are left unchanged. If you get the warning, it's best to abandon printing and then resize and resample the image to the print size you want, by following the steps in this task.
Why would you ever choose not to resample an image? When you make your image larger or smaller, the rescaling process can introduce artifacts or patterns that resampling can eliminate. However, in smoothing out any possible artifacts or unwanted patterns, resampling after you resize can result in loss of detail, especially in the background or in small areas. So limit the number of times you resample an image to once; if you have detail in the background you don't want to risk losing, do not resample.
- Bicubic. Estimates each new pixel's color value based on the values of the 16 pixels nearest to the new pixel's location relative to the original image, in a 4 x 4 array. This method is best used when enlarging an image.
- Bicubic Smoother. Similar to the Bicubic formula, except that the tendency of Bicubic resampling to create halos around highly contrasting edges is reduced. Best used when enlarging an image.
- Bicubic Sharper. Similar to the Bicubic formula, except the edges are sharper with even higher contrast. Best used when reducing the size of an image.
- Bilinear. Estimates each new pixel's color value based on the values of the four pixels nearest to the new pixel's location relative to the original image. This method is best used when reducing an image.
- Nearest Neighbor. Estimates each new pixel's color value based on the values of all the pixels that fall within a fixed proximity of the new pixel's location relative to the original image. Here, the pixel residing in the same proportionate location in the original image as that of the new pixel in the resized image is given the extra "weight" when estimating the new color value. This method is best used when reducing the size of an image, but only for those images with edges that have not been anti-aliased.
Select a New Size and/or Resolution
If you know what size you want the final image to be, type a value in the Document Size Width box; the Height value changes proportionately (or vice versa).
You can also change an image's size by adding or removing pixels. When you add pixels while maintaining the same resolution, you make the image bigger. For example, if you want the image to be twice as big, in the frame marked Pixel Dimensions, for either Width or Height, type 200 in the text box, and from the adjacent drop-down list, choose Percent. This increases the number of pixels without affecting their size (assuming that Resample Image is on).
To change the resolution, type a value in the Resolution box. Altering resolution in this manner does not change the image's print size unless you entered new values for Document Size earlier. Click OK. Because the Resample Image option is selected, new pixels are created as needed to meet your print size and resolution requirements.
To avoid resampling an image, disable the Resample Image option and change either the Height/Width or Resolution values in the Document Size area of the Image Resize dialog box. Just keep in mind that if you increase the Resolution without resampling, the image will be resized smaller.
View the Result
After you're satisfied with the result of the resizing process, make any other changes you want and then save the final image in JPEG or TIFF format, leaving your PSD image with its layers (if any) intact so that you can return at a later time and make different adjustments if you want.
Because resampling often leaves an image a bit fuzzy, it's best to follow up by treating your resampled image to an Unsharp Mask. See 149 Sharpen an Image.
Even though I increased the size of this photo by quite a lot (from 5" x 7" to 11" x 7.857"), the quality (resolution) was maintained because I selected Bicubic Smoother resampling.