Lenses
Megapixel cameras are constrained by the optical limits of lenses. True megapixel lenses are rare, expensive and therefore have not kept pace with image sensor increases. “One thing people use megapixel cameras for is the wider view requiring fewer cameras, but it's a challenge to do that with more background motion,” Moody said. “Lens adjustment in external areas of a large warehouse or retail situation may pick up too much background motion interference, with no relevance for the site's consideration. It is important that the scene does not overspill onto a surrounding road, where there may be excessive traffic flow.”
There are 5- or 10-megapixel lenses for machine vision, but they can retail from US$700 to $1,400 each — about the same price or even more than the camera itself. “There's not a standard lens that can provide the resolution for a 5-megapixel image sensor,” Schreiber said. “The pixel size is so small at 2 microns that it cannot be held with a standard lens. I can give you an example of a 5-megapixel camera using a bad lens resulting in a worse image than a 1.3-megapixel camera with a good lens.”
A good megapixel lens should have the same resolution at the edges as well as the center, Khanna said. Image performance should be maintained throughout lighting changes, particular for outdoor scenes that require IR correction.
Fisheye lenses are catching on in megapixel surveillance, thanks to increased sensor sizes. A good hemispheric camera should select a fisheye lens that is responsive and matches its sensor, Pineau said.
Looking at the spec sheets is good reference, but nothing beats field tests. “We do quite a lot of large projects, where the client requests 100 to 200 cameras, so this type of application warrants looking at megapixel or HD cameras to reduce the camera quantities,” Moody said. “We go out with footage, show them how it was tested in various conditions and show them the results for cameras 1, 2, 3 and 4. We will show them the one we would recommend; it is important that the client understands the choice and why we have recommended a certain type of camera over another.”
On paper, there are relatively few differences between cameras. However, software and programming will affect performance; two cameras using the same hardware can produce completely different results. In our next section, we explore best practices in megapixel camera installation, particularly for illumination, as well as real-life project specifications.