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  Keystone Correction & Lens Shift


Ideally a projector should be installed perpendicular to the screen without any angle to project a perfectly squared image.

However that is not always possible: furniture arrangement, lighting equipment etc. can be an issue so the projector has to be installed to the side or tilted upwards or downwards.
These angles will cause the image to be trapezoid.

Trapezoid image

Projector tilted upwards
or downwards

Projector to the side of the screen
projecting at an angle

The solution to this problem is to use keystone correction or lens shift.

Different projectors will offer vertical and/or horizontal keystone correction or lens shift, or both. If the position of the projector may be an issue for you or you are going to use the projector in various places where you can't guarantee the ideal position, you should consider these features.

Keystone correction

Keystone correction is a feature that is available on most, if not all digital projectors.
It is a digital process using the projector image scaler to make the image rectangular again.

Keystone correction

Vertical keystone correction

If your projector is projecting upwards or downwards, you need vertical keystone correction.
Vertical correction is available on most projectors.

Horizontal keystone correction

Horizontal correction is less common but there are projectors offering this feature.
It allows you to place the projector to the side of the screen and/or project at an angle.

Keystone correction is expressed in degrees and shows the angle that can be corrected digitally. This varies between models so check the numbers out before buying.

Keystone angle

Keystone angle example


Disadvantages of keystone correction

While keystone correction can be a very handy feature, it is not ideal in permanent installations.
Because the correction is a digital process of scaling, the number of pixels used is lower and the overall quality of the image is reduced.
If possible, it is preferrable to use lens shift instead of keystone correction to maintain the best image quality possible.

Lens shift

Lens shift as the name suggests is the process of physically shifting the projector lens to allow the projector to be installed off-center without affecting the shape or quality of the image projected.

Vertical lens shift will move the image up or down, allowing you to install the projector higher or lower.
Horizontal lens shift will move the image left or right, allowing you to install the projector to the side of the screen.

Available on some projectors (usually mid to high end ones), lens shift is usually expressed as a percentage or a ratio.


When expressed as a percentage, the offset is a percentage of the projected image height (vertical shift) or width (horizontal shift). The offset is measured from the image's vertical or horizontal centre.

Lens shift in percent

Lens shift expressed as a percentage



When expressed as a ratio, the ratio is the amount of image one side of the centre line compared to the amount if image on the other side of the centre line.

In the example below, the ratio is 4:1

Lens shift ratio

Lens shift expressed as a ratio


Which is best, keystone correction or lens shift?

As mentionned above, keystone correction can cause problems with image quality, especially on the edges, so it's best to avoid it where possible.
However if you are using a projector on the road and in various venues, then it's a very handy feature that allows you to get on with your presentation without having to worry too much about where to place the projector.

On the other hand if you are looking for a permanent installation, the ideal solution is to centre the projector but if this is not possible, opt for lens shift over keystone correction as this will give you better results.