What Lens to use at what Distance

Warning: This article was published many years ago (greater than two) Mar 26, 2011. Some information may be outdated.

In the past few months, we have been working together trying to break into city surveillance and such related project. While providing demo (proof of concept) its was quite shocking to see that our colleagues in the Industries and ourself, unable to address the issue & concern from the Police Dept and the users.

To make easy the understanding of all the parameter’s which influence the working of the Face or No Plate Recognition System, i have attached an xls file, using which you can work out the solution.. 

from the example provided in the xls greater clarity on the use of Megapixel camera is available. further I shall try to address some of the misconception “Megapixel means you can Virtual Zoom and Read No plates”

This misconception is because the of the over usage of the PTZ camera.. and confusion between the vPTZ(Digital) and PTZ (optical) Zoom Capability. from the calc in the xls file the following data is available 

Example 1: Using a 25mm Lens with 3Mega Camera and 25mm Lens with VGA Camera for Number Plate Detection requirements (ie read by Humans and not ALPR)

Example1: Will Insert the image soon..

Example 2: Using a 8mm Lens with 3Mega Camera and 8mm Lens with VGA Camera for Number Plate Detection requirements (ie read by Humans and not ALPR)

Example2: Will Insert the image soon..


From the Example 1 above we notice that 3Mega Camera can read from a distance or 27Mtr while the VGA can read only from 15mtr but for this reason alone we do not use the megapixel camera, we use the megapixel camera because the 3mega Camera provides a view of 7mtr wide x 5mtr high when compared to 2Mtr wide x 1.6Mtr High from VGA Camera. 

Hence when we have a large image (almost 4 time big) from the 3mega camera hence we capture many more vehicle at 27 mtr and when we combine this with the 8x vPTZ we can move within the Image to enlarge any section up to 27mtr and similarly from example 2 the vPTZ is limited to 9mtr from the camera. 

The confusion of the Police Dept is because the camera have infinity focus, ie The images are providing as far as possible view (limited by the light conditions), very very much beyond the limitation of 15pixel for Detection of an 1″ Font Size on the number plate and hence when the police does a vPTZ beyond the limit, they do not get to read the number plate and then they are frustrated.

Warning: While at the Demo, the PTZ camera shows clearly the images of the Number plate but pls point it out, that the Number Plate shown is from a Parked Vehicle and not moving. and when they try to zoom into a no plate when the traffic is moving ( at 10Km and above) and take a snapshot, the images do not provide the details.. any guess? why this is so? Mostly this is because the camera are using the interlaced scan and not progressive and by the time the zoom has auto focused  the vehicle has moved out of the FoV. If the Others are providing HD PTZ Camera using H.264, pls ensure that you ask for Number Plate when many vehicle

Pls feel free to call or email me for any further details..

Srikanth Kamath

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