File size, Image size, megapixels, and resolution are all common terms that we hear in digital photography. However few of us really understand what they mean and how they relate to each other. Let’s break it down so we can make sense of this.
Pixels and Megapixels
First off we all understand what a pixel is. A pixel is the tiniest building block of a digital photo. Think of them like the tiles that make a mosaic. When you zoom in really close on a photo you can see the pixels. Each pixel that we see in a photo is actually made up three different colored pixels; red, green and blue, stacked on top of each other to make up one color in a range of over 16 million. A digital photo is just a huge grid of many millions of pixels. In fact, a million pixels is called a Megapixel. So for a camera with a 20-megapixel sensor, it records the scene in 20 million pixels.
Image size is an old measure of photo size that Adobe Photoshop and some other pixel editors use. Image size looks at the width and height in pixels, then multiplies by the number of channels (three for an RGB photo) to show the number of bytes the photo would take as a single-layer 8-bit photo saved as a TIFF. Wow, makes sense right? I’ll give you an example. A photo that is 3000 pixels by 2000 pixels, is 6 million pixels or (6 megapixels) times three channels for RGB, 18 million bytes or 18 MB. Many years ago, the file saved out would be about 18 MB as a TIFF. However today this is overly simplistic. A photo often contains multiple layers, masks, previews and thumbnails that can make the size of disk, or the file size, much larger.
File size is much easier to understand, at least on the surface. It’s the amount of storage space on disk that a photo takes up. As I mentioned above, in the old days image size and file size meant roughly the same thing. However today they are often every different. File size can be much smaller than image size if the photo is compressed, like in the case of a JPEG. JPEG compression can make a photo much smaller on disk. This is good for storage space and transmission. RAW photos are also similarly smaller than their file size. RAW photos have only a single color channel of data, so they are about a third the size on disk compared to their image size. RAW photos are generally about the same as the megapixel rating for the camera. For example, a 20-megapixel camera creates RAW photos that are about 20 MB in size.
There are other cases where the file size can be much larger than the image size. Remember that image size is calculated for a single 8-bit layer. However, when you create a 16-bit photo, it will double in size. For each layer that you add, it doubles in size too. The same goes for layer masks and transparency. Plus modern files also have embedded previews, thumbnails, color profiles and metadata that increase their size as well. I’ll give you an example to show just how quickly the file size can grow. Let’s start with a 20-megapixel RAW file. As I mentioned above, the file size will be about 20 MB. As soon as you open this photo as a 16-bit PSD the file will blow up to 320 MB! That’s 20 MB for each channel (RGB, plus transparency), times two for being 16-bit, times two again for the file-sized, maximum compatibility, embedded preview. Some applications, like Adobe Photoshop, use compression and other optimizations to try to decrease layered file sizes in these cases. We are looking adding compression and other optimizations to saved PSD files as well for the future.
Last but not least is resolution. This is the most misused term of the bunch. Some people talk about their camera having 20-megapixel resolution, which is not the correct way to use the term. Resolution is a measure of density, how tightly packed the pixels are in a photo. Resolution is measured in pixels per inch (or pixels per cm). Some printer manufacturers also use the term dots per inch, but that is a bit different but also often confusing. Resolution really only comes into play when you are going to print your photos. The size of a print is determined by the number of pixels on a side, divided by the resolution. For example, a photo that is 3000 x 2000 pixels, at a resolution of 300 ppi would yield a print of 10” x 6.67”. The same 3000 x 2000 pixels on a television or normal computer display (72 or 96 ppi) would fill a display 31.25” x 20.83”. That’s why when you zoom-in on a photo to 100%, you need to pan around to see all of it. Most digital cameras define their resolution at 72 ppi, which creates a huge “canvas” if you were to print it, but it would not be sharp at that size. That’s why you use the Resize module or the Print dialog to properly rescale the resolution to what your printer needs.