The use of screen projectors previously was plagued with the absolute need to have the projector centered vertically and horizontal on the projection screen to avoid having otherwise great looking rectangular images and graphs turn into unattractive trapezoids.  Lens shift was developed to eliminate this problem, add flexibility in presentation projector placement, and let presenters focus more on the content of the presentations rather than figuring out how to make the equipment cooperate.

What is Lens Shift?

Lens shift is exactly what it implies: the physical shifting of the projector lens within the projector.  Although this does give presenters increased flexibility in where they position their presentation projector, they should keep in mind the degree of lens shift.  Lens shift was designed to correct placement problems in most situations, but it does have its limitations and boundaries.  Some presentation projectors will correct (or shift) the image more so than others, so a little research might be required if you think you might be faced with more extreme presentation environments.

However, there is a limit to the degree of lens shift that is available with any given projector, and more importantly, even where sufficient lens shift range exists to address severe placement problems, excessive use of lens shift can introduce visible image artifacts.

Lens Shift Types

Lens shift can be horizontal (left to right), vertical (up and down), and many screen projectors are beginning to offer variable lens shift on their newer models that would allow you to even shift on a diagonal. Although vertical lens shift is the most common type of lens shift, there is an increasing demand for horizontal and variable lens shift as well.

Vertical lens shift allows the projector to optically move — or shift — the image up and down so the projector will always be properly placed, even in varying installation scenarios. Horizontal lens shift works in exactly the same way except it will allow the projector to move the image from side to side so that the projector doesn't have to be placed directly in front of where the image should be.

Lens Shift and Keystone Correction, or both?

Lens shift is typically preferred to keystone correction.  This is because keystone correction is a result of the projector digitally correcting the image within the presentation projector and then displaying the image rather than manipulating the lens.  Because of this, image quality is lost when using keystone correction.  Although it is a great feature, it is not recommended to rely on it too heavily if you plan to use your projector to show video or use it in a home theater set up because it can create noticeable artifacts in the corrected image.  For displaying text and static images (such as PowerPoint slides), using keystone correction in your projector is a perfectly acceptable way to correct distorted projections. You can read more about this our article about keystone correction.

If the projector does not have lens shift, one alternative is to tilt the projector such that the image fills the screen from the position you want to place the projector. However, this will result in a trapezoidal shaped image. You can square it up using keystone correction, but this is something you should avoid if possible since it can soften the image a bit.

While a projector lens shift may not be necessary at all if your projector is installed to the correct specifications, you should still consider it a beneficial feature when purchasing a projector. It will provide you with flexibility now and in the future and simply the installation process for you. Example: On a 9x12 screen, a projector with an 8:1 (T:B) lens shift offset ratio will be positioned 1 ft above the bottom edge of the image in normal orientation, or 1 ft. below the top edge of the image in inverted orientation.