Megapixels, Image Resolution, & Image File Size: Photography Fundamentals

Megapixels, image size, and image resolution are common terms in digital photography. However, few of us understand their meaning and how they relate to each other. We offer various photo editing courses and professional photo editing programs, but let’s break it down so we can make sense of this.

• Pixels and Megapixels
• Image Size
• Image File Size
• Image Resolution
• Get Beginner-Friendly Photo Editing Software

Pixels and Megapixels

We all understand what a pixel is. It’s the tiniest building block of a digital photo. Think of them as the tiles that make a mosaic. You can see the pixels when you zoom in close on a photo. Each pixel we see in a photo is made up of three different colored pixels; red, green, and blue. When stacked on top of each other, each can make up one color in a range of over 16 million. A digital photo is a huge grid of many millions of pixels. What do we call a million pixels? We call them megapixels. A camera with a 20-megapixel sensor records the scene in 20 million pixels.

Image Size

Image size is a measure of photo size Adobe® Photoshop® and other pixel editors use. Image size looks at 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. Still with me?

I’ll give you an example. A photo that is 3000 pixels by 2000 pixels is 6 million pixels or (6 megapixels). Times that by the three channels for RGB. So 18 million bytes or 18 MB. Many years ago, the file saved would be roughly 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 the disk or file much larger.

Image File Size

Image file size is much easier to understand, at least on the surface. It’s the amount of storage space on a disk a photo uses. As I mentioned above, image and file sizes in the old days were roughly the same. Today, they are often very different. Photo 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 a disk. This is good for storage space and when you want to send the photo elsewhere (social media, friends, etc.). Raw photos are also similarly smaller than their image file size. Raw photos have a single color channel of data, so they are about a third of the size on disk compared to their image size. Raw photos are generally about the same as the camera’s megapixel rating. For example, a 20-megapixel camera creates raw photos at about 20 MB.

In other cases, the photo file size can be much larger than the image size. Remember, image size is calculated for a single 8-bit layer. However, when you create a 16-bit photo, it will double in size. Each layer you add doubles in size as well. The same goes for layer masks and transparency. Modern files also have embedded previews, thumbnails, color profiles, and metadata, adding to their size.

Here’s an example to show how quickly the image file size can increase. Let’s start with a 20-megapixel raw file. As I mentioned above, the photo file size will also 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, and times two again for the image file size, maximum compatibility, and embedded preview. Some applications, like Photoshop, use compression and other optimizations to decrease layered photo file sizes in these cases. We will also be adding compression and other optimizations to saved PSD files here at ON1 for the future.

Image Resolution

Last but not least is image resolution. Probably the most misused term of the bunch. Some say that their camera has a 20-megapixel image resolution, which is an incorrect term. Image resolution is a measure of density–how tightly packed the pixels are in a photo. Image 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 and often confusing. The image resolution only comes into play when you are going to print your photos. The size of your prints is determined by the number of pixels on a side divided by the image resolution.

For example, a 3000 x 2000 pixels photo with 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”. So when you zoom in on a photo to 100%, you need to pan around to see all of it. Most digital cameras define their image resolution at 72 PPI, which creates a huge “canvas” if you were to print it, but it wouldn’t be sharp at that size. So save yourselves the headache and use the Resize module or Print options in ON1 to rescale the image resolution to what your printer needs properly.

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