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Ask the Expert: Five things you need to know about lens selection

Have you ever bought tires for a new car? You (or the mechanic) first take into account the size of the tire in relation to your car, and then discuss your day-to-day driving conditions.  Will you be driving a lot in the snow?  Off road?  Or only on beautiful sunny days?  No matter if it’s a high-end sports car or an old truck, if you drive on a poorly matched set of tires, you won’t get the performance you paid for — especially in bad weather conditions.  Think of your lens the same way.


November 23, 2010
By Bob Moore

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There are many very helpful online lens calculators available at different sites, but there are still some thoughts to consider prior to using the calculator.  Since PTZ and dome cameras already have set lenses included, here are five things you should think of when selecting a lens for a fixed camera:

1.Will the camera be used in day and night conditions?

Just like buying tires, you should first consider the camera’s intended environment.  If you require a camera that works in both day and night time conditions, or if you plan to utilize infra red (IR) illumination as part of your solution, you will also need a lens that is meant for both light and dark conditions known as an IR compensated lens.  An IR compensated lens complements the auto IR cut filter found in a true day/night camera.

During daytime, the light coming into the image sensor goes through the lens first and then through the IR cut filter.  With a daylight-only lens, you’ll be able to focus the camera in the day, but in low light conditions or when using IR illumination, the IR cut filter will automatically remove itself from the image sensor and your image will be distorted because of the additional colors in the IR spectrum that are introduced.  IR compensated lenses are more expensive than daytime-only lenses due to the added complexity of focusing a broader color spectrum onto the sensor.

Unless you have a camera that has automatic back focus, you will need an IR compensated lens to ensure your focus remains in both day and night time conditions.

2.What will be my ‘true resolution’?

Consumers probably don’t know it, but when buying a 10 megapixel (MP) digital camera from their favorite electronics store, they are not going to get true 10 MP resolution. Chances are that a 10 MP digital camera will have a 5 or 8 MP lens at best.  This means is that the clarity of each pixel will not have optimal sharpness and that you will lose true resolution.  So why don’t they offer 10 MP lenses for the 10 MP camera?  It’s because the casual end-user won’t notice enough of a difference between the 10 and the 5 MP lens to justify such an expensive additional cost.  It’s really a marketing tactic.

But since there is much more on the line for the security professional, it’s crucial that you match your image sensors resolution to the lens itself so that you get the amount of usable pixels desired.  If there is a disparity between the lens and sensor, it will be very apparent when doing post forensic analysis of your image: You will get pixilation earlier than expected.
 
The bottom line: If you buy a 3 MP camera but only use a standard lens supporting 4CIF/VGA resolution, you have basically bought a VGA-rated camera.  There are no standards on the level of sharpness required for a pixel, so be sure to test out some different lenses during installation.
 


  
3. What’s the desired field of view and scene landscape?

The main criterion for field of view is to get the exact scene you envisioned before installation. For instance, if you are looking down a hallway, you will want to have a telephoto lens.  If you are looking to grab a wide viewing angle, then you will need a lower focal numbered lens. There are also lenses that can see 180 degrees (some call them 360 degree, but that is a misnomer).

Be very careful, however, when using wider angled lenses to ensure that you have a higher resolution sensor.  The wider the angle, the more diluted your pixels on target become — therefore, the sensor will need to do more work.

Varifocal lenses will provide the installer the greatest flexibility as they will cover a broader area.  Note that these are more expensive than fixed lenses.

4. Does my lens match the camera’s chip size?

Make sure your lens is larger than your chip size. For example, if your lens is rated for 1/4” and you have a 1/3” image sensor, you will get an image that will be missing all four corners. Chip size ratings can be found in datasheets and documentation for the camera, and lens rating in both the selector tools and printed on the lens itself.  It’s best to match exactly your lens size to the image sensor size, but if that is not possible, always get a bigger lens size than your image sensor.  You will get 100 per cent of the usable data for that chip sensor.
   
5. What type of iris is best?

There are many iris types to choose from when picking your lens and camera. Auto-Iris lenses help to get the most out of the lens/camera relationship. These lenses will automatically open or close the aperture to let in more or less light according to the environment. 

For optimal resolution with higher megapixel cameras, some manufacturers offer P-Iris lenses — sometimes referred to as “precise-iris.” These lenses work together with specialized software inside the camera to steer the motor inside the P-Iris lens to enable automatic and precise control of the iris opening.  This creates an optimized aperture and allows for pixel clarity across the entire image sensor to maximize contrast, clarity, resolution and depth of field. 
Auto-Iris lenses have less clarity than P-Iris because you don’t have the same precise control over the position of the iris opening; however, both options provide a better image in varying light conditions than a fixed iris.

Stay Focused

Selecting the right lens is definitely more science than art.  There are concrete steps you can take to make lens selection precise instead of educated guesswork. Take advantage of the fact that the web offers many great lens selectors from all of the major manufacturers to help make the right match.  Keep these five questions top-of-mind when using these calculators and you’ll always find the best focal length to keep your pixels on target.

Robert Moore is Canadian Country Manager, Axis Communications. Robert.Moore@axis.com


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