- Using the main application features
- Using the panels
- Tools on the Tools panel
- Changing the image size
- Choosing a bits per channel mode
- Cropping and rotating images
- Using the Layers panel
- Creating adjustment layers
- Editing adjustment layers
- Limiting the effect of an adjustment layer
- Saving adjustment presets
- Merging and deleting adjustment layers
- Working with layer groups
- Applying content-aware scaling
- Choosing a mode for the History panel
- Making snapshots of history states
- Working with nonlinear histories
- Using presets
- Streamlining your workflow
Changing the image size
There are three ways to choose a file resolution:
- When opening a raw digital photo, set the output resolution in the Workflow Options dialog (see page 44).
- For a JPEG photo or other image that you need to change the resolution for, use the Image Size dialog after opening the file in Photoshop.
- When scanning, set the input resolution to control how many pixels the device is to capture.
Changing the image size without resampling
Via Image > Image Size (Ctrl-Alt-I/ Cmd-Option-I), you can change the resolution of your file and/or its width and height. If you do so with the Resample Image option unchecked A (to prevent resampling), the image quality will remain at its current level. This method is recommended for digital photos and scanned images.
By default, JPEG photos from a digital camera have a low resolution (72–180 ppi) and very large width and height dimensions, with a sufficient number of pixels for high-quality output (prints as large as 8" × 10")—provided you increase the resolution to the proper value. When you increase the resolution to suit your output device, the pixel dimensions remain constant and the print dimensions are reduced.
Unlike photos from a digital camera, scanned images usually have small document size dimensions, yet contain a high resolution and sufficient pixel dimensions to produce large, high-quality prints. For a scanned image, change the Width or Height (under Document Size) with Resample Image unchecked; the Resolution and Pixel Dimensions values will remain constant.
Changing the image size with resampling
If your file contains too few pixels to meet the resolution requirement of your target output device, you’ll have to increase the resolution with Resample Image checked.B Pixels will be added to the file (its pixel dimensions will increase) and its storage size will increase. Increase the resolution only to a level that is sufficient for achieving the desired output quality.
When you downsample a file (decrease its resolution with Resample Image checked), pixels are discarded permanently. For a Web graphic, downsampling isn’t an issue, as users will view it on a computer display, which is a low-resolution device. For print output, on the other hand, try to avoid resampling, because it reduces the image sharpness (although you can use a sharpening filter to remedy blurring from a minor degree of resampling).