LensFocus: Understanding lens lexicon
4 min read
Lenses. Without them your projector isn’t worth much. No lens, no projection. The lens of your projector is the tool that processes and forms the image into a visual result. And if you have the right lens, it can even improve the quality of your projection. But before choosing your lens, do you know your lens terminology? Let’s start with these three must-knows: throw ratio, lens shift and high-contrast specs.
Lens (noun): a piece of transparent material (such as glass) that has two opposite regular surfaces either both curved or one curved and the other plane and that is used either singly or combined in an optical instrument for forming an image by focusing rays of light. [Merriam-Webster dictionary]
What is throw ratio?
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The throw ratio is a lens specification used to describe the relation between the projection distance from lens to image and the image/screen width. It determines what image size can be projected from a certain distance away. This easy calculation and its derivatives are essential when choosing the right solution for your project. If you know the size of your canvas and you have an idea of where the projection unit will be placed in your infrastructure, you can calculate the required throw ratio and select the right lens and projector for your set-up.
For example, a projector with a lens throw ratio of 2.0 : 1 would need to be positioned at a distance that is twice the width of the screen. The smaller the throw ratio, the closer you can position your projector to cover the image surface. In a set-up where the projector needs to be positioned very far from the screen you’d need a "long throw lens" with a throw ratio like 7.5 : 1.
Changing the lens on your projector could help achieving the required throw ratio.
However, the same lens can have different throw ratios for different projector types. How is that possible? Well, it’s related to the DMD (Digital Micromirror Device) type of your projector. These chips gather the image pixels before its projected. A projection lens has a certain magnification range, and so the throw ratio depends on the width of the DMD that is magnified. We’ll make it a bit more tangible with a comparison of two Barco projectors.
Projector | DMD | Resolution | DMD width | Throw ratio with |
| F80-Q9 | 0.67" | 2716 x 1600 | 14.666 mm | 1.06 - 1.60 |
| F70-W8 | 0.96" | 1920 x 1200 | 20.736 mm | 0.75 – 1.13 |
What is lens shift?
Lens shift allows the lens to move within the projector housing repositioning the projected image without having to physically tilt or swivel the projector. The lens shift comes in both horizontal and vertical versions, motioning the lens and image respectively from left to right or up and down in relation to the optical axis.
Why is this important to know? A large shift capability offers greater flexibility and easier integration in your venue. You can mount or place your device wherever you have the space. The alternative is tilting the projector and then use keystone adjustments to make the image again rectangular, but digital keystone corrections convert and compress the image with a loss in resolution and causing noticeable artifacts on the edges.
The Barco lenses have an amazing lens shift, unprecedented by other projector manufacturers, going up to 130% +/- vertical lens shift and up to 50% +/- horizontal lens shift – all depending on the type of projector and lens.
P.S. When comparing the lens shift specs with other manufacturers, please keep in mind the different calculations. Barco uses the optical axis as its 0% point of reference, where some of our competitors use the bottom of the screen as the 0% axis. There’s no wrong or right way, it’s just a matter of knowing.
What are high-contrast lenses?
The HC (or high-contrast) lenses are lenses built to enable higher contrast projections. (Oh, thank you, Captain Obvious.) Perhaps a better question is: ‘How does it work?’ and ‘When should I use these HC lenses?’
Each lens has an opening, also called the aperture, through which the projected light travels. A HC lens is built with a smaller opening which allows a smaller bundle of rays to go through the optical system. You could compare it with the diaphragm of a camera. As it blocks only the rays with the biggest angles, the lens allows for better contrast.
This type of lenses is typically used in applications where the need for contrast exceeds the brightness requirements. (Home) cinema, for instance, are usually designed to keep out the ambient light and consequently don’t require high lumen specs. Using the HC lenses improves the contrast for a premium movie or gaming experience with truer colors and blacker blacks. In domes and night-vision simulators too, the contrast of HC lenses has its advantages.
Conclusion
Spec sheets are great tools to help you compare solutions and make your decision. However, it’s important to understand what these specs are telling. And even more so, to know how the specs are calculated. Don’t get fooled by marketing specs, but push for true specs.
Still not sure on how to read your lens spec sheet? Reach out and we’ll gladly answer any lens-related questions and support you to make sure you get the best solution for your project.