When projecting, the image on the screen is not always properly positioned. You don’t always get that perfect rectangular shape on the screen, rather the image appears somewhat tilted; this is called a keystone effect.
The keystone effect occurs when the projector is not perpendicular to the screen or is placed at an angle to the screen, and thus causes a kind of distortion in the projected image. This distortion can be resolved using Keystone correction.
You may not be familiar with this especially if you are new to projectors, but you should know how it works. Keystone correction could be vertical or horizontal.
The vertical keystone correction fixes the image at the top and the bottom, while the horizontal keystone correction fixes it at the sides (left and right). This can be done at different angles called the Angle of correction. It varies from ±30° to ±50° depending on the model of projector that you are using.
Types of Keystone Correction
Manual keystone correction
As the name implies, it has to do with manually adjusting the image using the keystone buttons on the projector or the remote control. Manual keystone correction allows you to adjust the lens of the projector so you can project at an angle that is higher or lower than it would be if the projector was placed on a flat surface.
How to do manual keystone correction
Turn on your projector and project an image on the screen. Locate the keystone buttons on the projector or the remote control. For most projectors, the buttons are located on the top of the device, while for others, they are on the remote control.
These buttons have a trapezoidal icon on them; this symbol will help you to locate them easily. At the sides of the icon, there are small arrows that indicate the direction for the keystone adjustment. Press the buttons to adjust the image the way you want it to be.
Although manual keystone correction is an easy way to correct the keystone effect, it significantly affects the image quality. And it can also cause some parts of the image to be out of focus.
What happens is that, after you have manually adjusted the image, refocusing it becomes difficult, and some parts may even be off the screen. See the video below to get a better understanding of this.
Another disadvantage of the manual keystone correction is that the projector needs to be placed far away from the screen for you to get the best image.
Manual keystone correction is not always the best, because of its effect on the image quality. If you can find a way to properly adjust the projector without using the manual keystone correction, that will give you better image quality.
Digital/Automatic keystone correction
Compared to the manual method, this is a faster method to correct the keystone effect in projectors. The projector uses an infra-red beam to scan the surface and automatically corrects the distortion.
This feature is common in new LCD and DLP projectors. These projectors use an image altering algorithm to alter the image before it gets to the lens. This gives a squared image that fits perfectly on the screen.
How to do Digital keystone correction
This is a very simple procedure. Turn on your projector, project an image on the screen. Locate the “screen fit” button on your projector, then press the button. This automatically corrects the distortion and fits the image on the screen.
This method of keystone corrections also has its disadvantages, one of which is that it shrinks the image after adjusting it. In a bid to fit the image perfectly on the screen, it makes the image smaller. Another disadvantage is that it reduces the image brightness, since the image is now smaller than the original resolution of the projector.
Does Keystone correction reduce resolution?
Yes, keystone correction reduces the image resolution.
This is not surprising because Keystone correction is a digital process, and like most digital processes like cropping and zooming the image resolution is always affected.
This feature reduces the pixels of the image; fewer pixels mean lower image resolution.
It has been argued that Keystone resolution destroys the visible resolution of the image by about 50%, and in that way reduces the quality of the projected image.
Keystone correction greatly reduces image quality; it should be the last resort if at all you are unable to adjust the image or you cannot manage the distortion. It is better to use only a little Keystone correction when projecting.
Is Keystone correction bad?
Keystone correction is not bad in itself, it is actually a useful feature; otherwise, why will it be incorporated in projectors? The decision to use the Keystone correction feature depends on the level of alteration that is done on the image quality.
If you can cope with the image quality after using the Keystone correction then you should try it out. The only issue here is, you cannot determine the amount of change that this feature will have on the image quality until you try it out.
So if you are willing to try it, then you should give it a shot. But if you are okay with the original distortion on the image you can as well stick to that.
How can the keystone effect be avoided?
There are a few ways to avoid a keystone effect when projecting. One way is to move the projector closer to the center of the screen. The image becomes bigger and the keyboard effect is eliminated or becomes less visible.
Another way is to tilt the screen to a certain angle. This will compensate for any distortion that may arise from the keystone effect.
Apart from the fact that Keystone correction affects image quality, it is a helpful feature. The keystone effect is a common issue when projecting, and it is usually a lot of work to correct it without using the Keystone correction feature.
You may have to change the projector’s position to get a better image. This requires some time of skillful readjustment, and for a small room, finding another position will be difficult.