It’s a good idea to orient yourself to an image file before making any changes to it. There are a few key areas in the Photoshop file window to make note of:
- Image window – this is where you will find a sub-menu that contains the Image Size option.
- View size – in the sample below, the file is being viewed at 50% of its actual size.
- Document size – the number on the left side is the approximate size of the saved, flattened file. The number on the right side is the approximate size of the layered file size, including channels.
When you click on the Document size area on the bottom, you get a quick snapshot of the file’s physical size in width and height, channels (color space and bits per channel), and resolution. When you click on the arrow next to the Document size area, a menu is revealed that shows more information, notably Document Size, Document Profile, and Document Dimensions. High resolution images (required for print) are 300 pixels/inch (ppi), whereas low resolution images (required for web and screen viewing) are 72 pixels/inch (ppi).
To resize the image, go to the Image menu and select Image Size. The Image Size window will pop up. There are 2 sections in this window: Pixel Dimensions and Document Size. Pixels are the small picture elements that comprise digital images. Pixel Dimensions are the number of pixels that reside in the physical dimensions defined in Document Size. For the flower image, the Document Size is 6.947″ wide x 4.93 ” high, with a 300 ppi resolution. If you divide the pixel width of 2084 by 300 (pixels/inch), you’ll get the 6.947″ inch width.
Document Size Scaling
One of the most common ways to scale an image is in the Document Size section. To scale the image down in size, simply change the Document Size width and height, keeping all 3 options of Scale Styles, Constrain Proportions, and Resample Image selected. Scaling down in this manner does not change the resolution or the image quality. Notice that the number of pixels in the Pixel Dimensions section decreases as you scale down, but the resolution stays the same.
When you scale an image up in size, you need to be more careful. First of all, you should NOT resample the image when scaling up. Photoshop adds pixels to an image when resampling upward using the interpolation method you select. Printers have always warned me against using resampling when upscaling beyond a 20% increase. Although the image may look ok on your computer monitor, it has actually lost detail and sharpness, and won’t look good when printed. Downsampling (resampling when scaling downward) is not problematic; even though you’re deleting pixels, the pixels per inch remains the same.
In the sample below, we’ve scaled the image up 200% without resampling. The Pixel Dimensions stayed the same, but the resolution decreased from 300 ppi to 150 ppi. That’s because those same pixels are now being spread out over a larger area.
With Resampling – Instead of changing the Document Size width and height, you could decrease an image by decreasing its Resolution. With Scale Styles, Constrain Proportions, and Resample Image selected, change the 300 ppi to 72 for a low-resolution image. This will affect only the Pixel Dimensions: pixels are deleted. The physical dimensions in Document Size remain unchanged.
Without Resampling – With Resample Image NOT selected, change the 300 ppi to 72 for a low-resolution image. This will affect only the Document Size width and height. You’re spreading the same number of pixels over a larger area.
When to Change Pixel Dimensions
For web images that require a specific width or height, I often have to use a 2-step process for downsizing an image. First, I downsize without resampling as we just did above. That will change my resolution to 72 without altering the Pixel Dimensions. Click OK. Open the Image Size window again, and this time, select resampling to change the Pixel Dimensions to the desired width and/or height. In the sample below, I’ve change the width to 1600 pixels. The Document Size dimensions reduced automatically to retain the 72 ppi.
Now that you’ve got the general idea behind the Image Size window, play around with a high resolution image in Photoshop. Make note of what happens when you change something. Compare the differences between sampling and resampling, altering the pixel dimensions vs. the document size dimensions. Ask me any questions you might have. And feel free to let me know what techniques you use that work well for you!