Why do lines appear in the picture when we photograph a computer screen?

All graphics shown on our computer screens are made of pixels. Pixel is a short term for picture element that refers to a single point in a picture. In a computer screen, pixels are the square-like element that forms an image. Monitors display pictures by dividing the screen into thousands or even millions of pixels arranged horizontally and vertically. The tiny pixels are so close together that they appear connected.

Now, computer screens do not generate an entire image at once. Instead, a tightly spaced horizontal lines separately switch on or off these pixels. Each row of pixels is rendered by these horizontal lines one at a time until the entire image is complete. Imagine the screen as filling in color by numbers from left to the right one line at a time.

A whole image, technically pertained as “frame,” is entirely produced at about 0.016 seconds. Another frame will then be generated right after the first one. These transitions are so fast that the human brain perceives it together as a constant movement. No matter how hard you try, naked human eyes will never see the completion of a single frame.

The number of consecutive frames per second is called the frame rate or frame frequency measured in hertz. One hertz equals one frame per second (fps). A 59.99-hertz screen generates an entire structure nearly 60 times per second. If your screen has 4000 pixels, it does one pixel every .015 seconds.

Computer screen and Digital cameras

Similarly, the sensors of digital cameras are made up of a pixel-grid. Unlike computer screens that generate pixels for image output, cameras do the other way around. They instead create pixels for image input. Digital cameras use what is known as a rolling shutter. They do not simply capture the whole image at once but instead generate the pixels one line at a time.

This is similar to how scanners work. When you connect a scanner to a computer and then scan an image, you will primarily notice that the picture is not complete. At first, only the upper part of the image will be generated to the computer. The scanned image will only be done when all of the pixels were sent to the machine. Digital cameras do this but at a seemingly faster rate.

For example, you have a Macbook that has 5,184,000 pixels arranged in 1,800 rows and 2,880 columns. You also have a camera that has 22,118,400 pixels in 1280 by 1024 pixel-grid. Note that since the Macbook has bigger pixels, it is faster in generating an entire frame than the Macbook.

When you look at the Macbook screen through your camera, you might observe that there are lines, curves, and circles in the image. No, neither your Macbook nor your camera is defective! The pixels were only playing an optical illusion on you.

Abstract pixel patterns forming shapes, lines and different colors

The Moire Pattern

Your Macbook completes the frame in a shorter time than your camera does. Generally, computer monitors refresh at about 60-120 hertz, but cameras work at about 50 fps. As a result, you can see a disjoint picture where there are either overdrawn (a different pixel is on top of another) or blank (no pixels at all). This is called a partial frame.

The strange pattern of lines happens when two different pixel grids interact. Since the two pixel-grids are out of sync, the camera will record an inaccurate representation of the screen. When taking a photo, the camera will make its own decisions of what color and how bright these colors will be in every pixel. When taking a video, the camera will record fewer frames per second, causing the video to flicker.

Unless the Macbook and the camera will have the same pixel value and will have their pixel grids lines up correctly, you will see an optical effect called moire. A moire results from interference between two patterns that were not wholly identical and were either displaced, rotated, or have a slightly different pitch.

TIP: To avoid having moire patterns when preferring a photo in your computer screen, adjust your camera’s aperture or shutter speed. Adjust your camera to a rate of 1/30 or 1/15 to fully capture the rolled-out frame of pixels. If you do not have the options to change the shutter speed, try looking for ‘sports’ in the automatic modes.