How can one Photoshop image grow and shrink dramatically during production, even without compression? It’s a question that comes up regularly. And the answer isn’t just academic, because understanding why could save you a lot of storage space on your hard drive or in the cloud.
What’s the Document Size in the Image Size Dialog Box?
The first place you might see a document size in Photoshop is at the top of the Image > Image Size dialog box. This number tells you how much disk space the document would use it if it was saved as a single layer. If the Resample option is on, you can watch the Image Size number change as you adjust the Width, Height, or Resolution. (If Resample is off, Image Size doesn’t change because the number of pixels remains constant no matter how you change the numbers.)
The 20.6 megabytes shown above is based on the document using 8 bits per channel (bpc).
Why Does a Photoshop Document Show Two Sizes?
If you configure the Status bar or the Info panel to display the document size, you’ll notice two sizes listed for the same document, separated by a slash. The first number is the same as in Image Size: the document size if saved as a single layer. The second number accounts for layers, channels, and masks, so for layered Photoshop documents the second size would be both larger and more representative of the size on disk.
Why Are Document Sizes in Photoshop Different than on the Desktop?
Having seen the various document sizes reported in Photoshop, you might wonder why those sizes don’t match what your Mac or Windows desktop tells you. For example, the document we’ve looked at so far is a basic 8×10-inch photo at 300 pixels per inch (ppi). But even that same photo can take up very different amounts of disk space depending on the file format and file compression you chose, as in the figure below. The first two examples, Photoshop format and TIFF uncompressed, are close to the document size reported inside Photoshop. The rest are smaller. However, while ZIP compressed TIFF may be the smallest, it can take much longer to save changes and may be compatible with fewer applications.
If you’re handling hundreds or thousands of images for a book or a web site, file size differences can add up quickly, not just affecting how much space you need to store your images but also how much time it takes to upload or download them from project servers, or to back them up.
Images can be compressed further if they use a lossy compression method such as JPEG. Lossy compression doesn’t preserve all of the original quality of the image, which is why in professional workflows JPEG is used only for output where small file size is much more important than perfect quality, such as web publishing.
Remember that this works in reverse, too. The 0.3 MB compressed JPEG file will expand to occupy 21.8 MB of RAM when Photoshop decompresses it for editing. Also, the File > Save for Web command creates smaller files than File > Save As because in the interest of conserving Internet bandwidth Save for Web doesn’t include extras like a preview thumbnail image and various types of metadata (you can choose to preserve some metadata).
Why Are Document Sizes Different Even Without Compression?
If you try comparing file size differences yourself, you may see variations that can’t be explained by the factors seen so far. There are more variables that can affect the size of a basic Photoshop image, including but not limited to the number of:
- Channels (grayscale is one channel, RGB is 3 channels, and CMYK is 4 channels)
- Layer masks
- Embedded Smart Objects
The exact amount that each feature adds to the document size depends on the content in them. You can see examples of document size variations in the figure below.
Other factors affecting file size include embedding a color profile, and turning on the Maximize Compatibility option that may appear when you save (leave it on to see previews of Photoshop files in other applications.)
Why Do Raw Photo Files Get Bigger in Photoshop?
If you’ve ever wondered why a raw file seems to get a lot bigger after it’s opened and saved in Photoshop, there are some clues in the previous figure but there’s also a key piece of information to understand: Camera raw files are typically one gray channel full of numbers, because the raw sensor data hasn’t yet been interpreted into pixels with RGB values. When you open a raw file from Adobe Camera Raw or Lightroom into Photoshop, what was a one-channel grayscale file becomes a three-channel RGB file. Since we know that the number of channels affects file size, naturally a three-channel RGB file will be larger than a one-channel raw file.
Another factor is bit depth. Many digital cameras capture 12 to 14 bits per channel into that raw file. You can set Camera Raw or Lightroom to open that raw file in Photoshop at 8 bpc or 16 bpc, and that too will affect the document size of the resulting Photoshop file. If you convert a 12 bpc raw file to a 16 bpc RGB Photoshop document, you should expect a large increase in file size. You can then expect a large decrease in file size when you save a final copy of the document with much smaller pixel dimensions, as a single flattened layer, in the lossy JPEG format, for posting on your web site. The next figure shows the possible variation in file size for an image that started as a 5184 × 3456 pixel raw file.
The document size of an image is always a compromise between
quality, flexibility, speed of opening and saving, and compressing to save disk space or Internet bandwidth. Knowing the factors that affect the size of a document helps you understand why an image’s size on disk rises or falls.