Tuesday, June 18, 2013

How to Select the Lens for Your Megapixel IP Camera

Lenses for Megapixel IP Cameras
What lens should I use for my IP Camera?  Determining the right lens can be complicated and assuring that you get the right one requires some care.  This is a very important part of your IP camera system.  If you get a megapixel IP camera but don’t get the right lens you are wasting your money. 

Many IP cameras come with lenses.  For example the Axis P1354 comes with a 3 mm to 8 mm lens (100° - 34° horizontal field of view).   How do you determine if this will actually work in a specific application?  First make sure you know what the angle or mm of the lens should be.  The higher the mm numbers the lower the angle and the more magnification you will get.  It’s best to use the lens angle that is provided by the manufacturer instead of the mm of the lens since the angle can vary based on the size of the sensor.  

Lens Angle 
By measuring the field of view and how far away the camera will be from the area you want to view, you can calculate the angle of the lens.  To calculate the angle of the lens required you need to use trigonometry.  Don’t get upset, I slept through this class in high school as well.  I do remember my teacher saying, “You’re going to need this someday”, and me thinking, “Yeah, right”.  There are some calculators on line that can make this job easier.  Here is a review of the basics.   

 First we want to use the horizontal field of view rather than the vertical field of view, so let’s image we are looking down on the camera.  We split the triangle in half to get a right triangle.    

Then we can use the tangent function to calculate all the dimensions and angles.

The tangent (angle) = Opposite / Adjacent
In this case the dimensions we use are ½ the field of view (FOV) and ½ the angle, so the equation is:
The tangent (½ angle of the camera lens) = ½ the FOV / Distance from camera to FOV

Now we can calculate everything we need.  For example if the distance to the field of view is 100 ft. and the FOV is 50 ft. Here’s how we get the lens angle:

½ Tan(lens°) = ½ 50 / 100.
 Tan(lens°) = 2 x(½ 50 / 100)
Now I go to my on line calculator at http://www.pagetutor.com/trigcalc/trig.html and enter 25 where it says “Side A”, and then 100 into the “side B”, I get 14.04°. 
Then 2 x 14.04° = 28.08°.  

And of course, if you know the lens angle and want to calculate the FOV based on a certain distance, you can do this as well using the same equation and calculator.  

OK, your head hurts.  I understand.  Just take two aspirin and call us and we’ll do all the calculations. 

Other Specifications

Once you know the type of lens required you also need to review all the other lens characteristics for a specific application.  We reviewed all the lens specifications in our technical article Megapixel Lens for Megapixel IP Cameras, but here is a summary.  

Lens Format or Size: the lens size should be larger than the sensor of the camera.
Lens Mount:  There are 2 types of mounting formats used in IP cameras, C and CS type.  Make sure the lens you select matches the camera mount.
Iris:  There are manual adjustable irises, auto-iris and the latest p-iris.  If the lens is used indoors, you usually can use a manual iris camera, but outdoors you need an auto-iris lens.  If there are challenging lighting conditions then the p-iris is a good choice. 

 If you are using a megapixel IP camera then you must consider some additional specifications such as lens resolution, depth of field, iris control, and distortion, aperture and IR light capability. These are all very critical to assuring that you get the video that you expect.  If you get an inexpensive lens you certainly won’t be happy with the video.  

Yes, this is all complicated so please contact us for assistance.  We will review your objectives and make sure that your complete IP camera system performs exactly as you expect.  Please call us at 914-944-3425 or just use our contact form.