Image Resolution, Pixels and Size
“Why doesn’t my image fill the computer screen?”
“Why does my image look so pixelated when I zoom in on the details?”
“Why is my image too large to send by email?”
“Someone asked me for a high resolution image. What do they mean?”
These questions can be answered by looking at three related photo details:
- Pixel Dimensions
- File Size
Understanding the relationship between resolution, pixel dimensions and file size can help you unravel many challenges you face when sharing digital images online and printing photos.
The pixels dimensions are the width and length of a photo measured in pixels. The number of pixels in a photo depends on the sensor size in your camera. The sensor records image information in pixels, the smallest element in a digital photo. The more pixels in your photo, the more image information is stored in the digital file.
For example, a camera with a 20 MP (megapixel) sensor, like my Canon PowerShot SX420 IS, can produce a photo that contains approximately 20,000,000 pixels. One megapixel is equal to one million pixels.
At its highest quality, this camera produces an image that is 5152 pixels wide by 3864 pixels high in a 4 by 3 ratio.
5152 x 3864 = 19,907,328 pixels (rounded to 20 MP)
Pixel Dimensions and Resolution
In most cases, the terms “pixel dimensions” and “resolution” are used interchangeably. For printed images, there is an extra factor.
For cameras, pixel dimensions and resolution are synonymous. A full resolution image is the unedited photo that your camera produces, or an edited photo that contains the same number of pixels as the unedited original (not cropped or resized). When someone asks you for a high resolution image, they want a high quality photo that contains the most pixels possible.
The more pixels in your image, the higher the quality. You can also be more versatile when cropping and editing the photo in a photo editing program like Photoshop.
For digital photos that are displayed on your mobile device, monitor or TV, the pixel dimensions and resolution are synonymous.
For example, if your computer monitor has a display resolution of 1920 x 1080 pixels (a common resolution), a photo that is 1920 x 1080 pixels or more will fill the screen without distortion.
Suppose you are on Zoom and want an image that you are sharing to fill the screen. That image would need to be at least 1080 pixels high. If your image were 200 x 200 pixels, it would display in a tiny portion of the screen. Furthermore, if you zoom in on it to fill the screen, it would look pixelated and distorted.
For photos that you want to print, there is a third detail to consider: dpi (dots per inch) or ppi (pixels per inch). The size of your printed photo depends on both the pixel dimensions and how many dots, or pixels, the printer produces per inch.
For example, if you want to print a 6” by 4” photo at 300 dpi, you would need to have a photo that is at least 1800 x 1200 pixels or more. 300 dpi is a common requirement for high quality prints.
The file size of your photo depends on the number of pixels in the photo. More pixels equals a larger file size, measured in kilobytes (KB) or megabytes (MB).
You may not be able to share some very large photos in email and may need to reduce the file size.
Reducing the file size also will reduce the pixels dimensions of your photo.
Social Media Image Requirements
Most social media sites have pixel dimension requirements for the images you share or display on their sites. Facebook, Instagram, and Youtube, for example, have requirements for images displayed in posts and for your profile picture or header image.