Testing Wide Angle Megapixel Lenses - Theia SY125MAuthor: John Honovich, Published on May 26, 2009
More pixels provide the opportunity for greater field of views and less cameras. However, stock lenses provided with cameras only offer moderately wide fields (60-70 degrees).
A few companies are now developing wider angle lenses to help solve this problem. In this group, Theia is receiving the most marketing attention (see some image examples of their lenses in use and a report on their technology).
This report shares finding from a test of the Theia's SY125M lens, a 1.3mm manual iris lens that provides 125 degree FOV for 1/3" imagers. The MSRP of SY125M is $440.
Here are our key findings from the test of Theia's SY125M:
- The SY125M provides a far greater Field of View (about 200-300% greater area).
- The greater FoV comes at the expense of significantly reduced pixel density / pixels per foot
- The lens provided the greatest advantage for large areas but provided little advantage and some disadvantages for small areas
- The lens benefits from modest installation modifications to maximize the viewing area of the lens
Overview of the Tests
The concept of the test is to compare megapixel cameras with stock lenses at the widest Field of View (at 4mm) to the use of those cameras with the SY125M.
For this test, we used the SY125M with (2) 2MP cameras: the Vivotek 7161 and theIQinVision 752 (the Vivotek is 1/3" imager and the IQinVision is a 1/2" imager). As you will see, the size of the imager makes a difference as the Theia lens (at 1/2.5") is slightly larger than the Vivotek imager but smaller than the IQinVision.
Increasingly customers and integrators are deciding to use megapixel and want to see the best ways to deploy megapixel cameras. Stock lenses are generally included (and therefore essentially free) while the SY125M has an MSRP of $440.
From a practical standpoint, the question is: What scenarios does this lens provide $400 or more worth of value?
To examine this, we tested 5 different locations - 1 indoor, 1 outdoor covered and 3 outdoor locations. The tests were done during the day or at night with street lighting. This lens is not day/night so it can be used at night but only with street lighting [Theia has their 110 series that is day/night]
Pixel Density vs FOV Tradeoffs
The most important finding from the test is the tradeoff between pixel density and the size of the Field of View. By this I mean, an object with the Theia lens has significantly less pixels than an object captured using a 4mm lens.
While this should not be surprising, this issue is not generally reflected in Theia's marketing material. On the one hand, the tradeoff between pixel density and Field of View is physically necessary. There are no magic pixels and if you cover a wider area with the small number of pixels, the density will decrease. However, between the general CSI effect and Theia's marketing material (see their expectation page), people may not appreciate the tradeoff.
This is important for surveillance video as pixel density impacts the type of surveillance that can be accomplished in different manners. If you are trying to determine the facial features or numbers of a license plate, you need a high number of pixels. Below a certain level, identification becomes impossible. However, for general activity surveillance (like seeing if a person or car is present), you require dramatically less pixels and can be successful over a broad range.
As a practical manner, in small areas (under 10M /30 feet), there was a practical tradeoff. Switching from stock to Theia lenses eliminated the ability to conduct personal identification, allowing only action identification. This can be a significant tradeoff if you expect personal identification.
However, in larger area (over 20M / 60 feet), there was not much of a practical tradeoff. At this FoV, even with a stock lens, you cannot achieve personal identification. With the Theia lens you were still able to perform the same action identification but with 200-300% more coverage area.
Download Video Clips
Download all the video clips from the 5 testing scenarios [Note: 83MB zip file]. In the 2 screencasts below, we explain how we setup the cameras and comment on the video clips.
Physical Setup Overview
In this 5 minute clip, we show the physical features of the camera and how to setup the lens with an IQinVision camera.
Overview of Scenarios Tested
In this screencast, we walk through the videos from each scenario, providing recommendations and commentary on the advantages and challenges of each one.
An important point that should be noted is the physical positioning on the camera with Theia lenses. Because of the far wider field of view the lens provides, modifying the camera positioning (relative to the use of stock lenses) can be quite useful:
- The camera should be positioned at a greater downtilt than that of cameras with stock lenses. Whereas a traditional security camera may be downtilt 10-20 degrees, the Theia lens benefits from a 30-45 degrees downtilt. This is because Theia's FoV is so wide it captures much more area higher than traditional lenses. As a practical issue, without the added downtilt, this results in capturing the sky (which is generally wasteful).
- The camera should be mounted farther out from the building or wall the camera is mounted on. Again, because of the wider field of view, the lens captures areas down and to the side that traditional security lens do not. Typically, cameras are mounted with short mounts (3 to 6 inches) with no issue of capturing the wall or objects immediately next to the camera. When using the SY125M, you may consider using an extension mount. This will eliminate this problem and also capture more area that is valuable to monitors.
As demonstrated in the videos, there are certainly benefits from the use of the SY125M lens but users should be careful about understanding its limitations.
- If you are interested in action identification over wide areas, the SY125M can eliminate the need for a second camera, more than justifying the cost of the lens. In this scenario, the reduction in pixel density is not likely to be a practical problem.
- However, if you are monitoring a small location or need to get license plates or facial details, you should be careful about the use of Theia's lenses as they are likely not to meet this requirement.
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