Choosing between varifocal and fixed focal lens is one of the most fundamental decisions in video surveillance.
With a fixed focal lens, the Field of View (FoV) of the camera can not be changed unless the lens is changed. By contrast, varifocal lenses allow for immediate adjustment of the FoV simply by adjusting a control of the lens / camera.
But which one is used more and why?
Inside, our exclusive integrator survey results answer these questions.
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Fixed focal lenses are not lower quality than varifocal lenses...
Is it a myth that varifocal lenses have higher f-stops generally than their fixed lens counterparts, and that the difference is exacerbated as you move the varifocal towards the high range of its the focal length?
By definition, any time the focal width remains constant but the length increases, F stop will increase. That is typically the case with varifocal lens, i.e., adjust to a longer focal length while width remains constant.
Perhaps one could find a fixed focal length lens for a higher value (e.g., 9mm) with a lower f stop than, e.g., a 3-9mm. I've never heard, though, of someone pursuing this. Maybe though...
In my early days of doing surveillance video, when fixed focal length lenses were much more the norm and prepared installers usually kept a wide selection of lenses with them, I really wanted to get away from that and go with vari-focal as often as possible. I discovered Tamron, which at the time was marketing as having the only varifocal lenses on the market that maintained a constant F-stop throughout the full range of FOV adjustment. I was very pleased with the optics and mechanics of their lenses, and I did seem to get more consistent light handling throughout the FOV range than with most other brands, so they became my go-to lenses for a long time. I must confess, though, I could use an explanation on how they would have achieved this... a widening aperture as the focal length increases? Was it marketing mumbo-jumbo and the more consistent light gathering was simply because of the quality build? If it was truly an accurate claim, what other manufacturers are doing it now, too?
...varifocal lenses on the market that maintained a constant F-stop throughout the full range of FOV adjustment.
What's the model, do they still make it? It's hard to imagine that it could be solved by quality. Look thru a paper towel roll vs. a toilet paper roll, Charmin or not the toilet paper roll gonna win, no?
If they were widening and narrowing the aperture, and purposely restricting the best possible performance of the lens, by limiting the light, just to make their marketing claims true, that would be wrong hysterical.
In the world of photography, fixed focal length lenses (often called prime lenses) enjoy a significant quality margin over vari-focal (zoom) lenses, and generally have a smaller numerical "F" number meaning more light gathering.
Because the sensors in security cameras are so small, lower F number are relatively easier to acheive than with photographic camera lenses. Also, lower F numbers mean shallower depth of field (DOF). Shallow DOF is often prized in photography as it isolates the main subject from the background. This is almost always a real negative in security.
Also, while low F numbers are good for low-light performance, most security cameras use other means to improve low-light performance.
So, interestingly, the ideal security lens in any given situration is almost the reverse of the design of a photographic lens.
Paul, thanks for the feedback. I want to expand on one point you made:
"Also, while low F numbers are good for low-light performance, most security cameras use other means to improve low-light performance."
I agree with this. However, as a practical matter I have never seen a surveillance camera, for example, with an f/2.0 lens be good at low light. Generally, if a manufacturer wants a camera to be good at low light, they will easily spend the small delta to get an f/1.0, f/1.2, or f/1.4 lens, along with what they are spending on image processing, etc.
The one exception is integrated IR budget cameras, which generally have high f stops but include their own light source.
So, interestingly, the ideal security lens in any given situration is almost the reverse of the design of a photographic lens. (EA)
Interesting, yes, indubitable, no.
...most security cameras use other means to improve low-light performance.
First, although there may be 'other means' available to surveillence cameras, that's because low-light performance is so key to surveillence that other more artistic concerns, e.g. color space, low noise/distortion, are sacrificed if need be to that end. These are things that no photographic camera would dare do. But that, as John counters, doesn't mean lower F numbers are worse or even immaterial, they still comprise the front line assault against the forces of darkness, and all other things being equal, lower is better, and a lot lower is a lot better.
Also, lower F numbers mean shallower depth of field (DOF).
This is true. However true DOF issues are rarely seen in security cameras. Why? Two technical reasons:
1) Sensor size smaller sensor = deeper DOF. Witness the now ubiquitous availability of full-frame 35mm sensors in Pro DSLRs after old school photographers had enough of the 4/3 system with its crap crop factor of 2. Main reason shallower DOF afforded by larger sensors.
2) The nominal distance to subject is usually large enough to render it imperceptible. We're not talking close-up portrait with blurry daisies in the background. Also, if the distance is small enough to show the effect, often it doesn't matter if the background is out. Like in an LPC system.
And regardless, it seems unlikely that the industry is choosing higher f-upped lenses because they fear shallower FOV's.
So I would say although there are differents emphasises and constraints on each discipline, the lens design concerns generally follow the same vector.
Well noted Paul. Coming from a photographic background, and now learning surveillance technology, it is often hard to grasp concepts that seem to be turned upside down. I try to look at it as a completely different concept, or I get into trouble, trying to apply what I thought I previously knew about photography to surveillance cameras/lenses. I do want to eventually try large sensor (35mm) surveillance cameras.
Seems that varifocal lens is more popular and flexible.
Good points all. I've often found that Optics knowledge is missing or downplayed amongst IT professionals moving into the surveillance realm. Had one instructor claim that any adjustment necessary could be made with the cameras setup software. As Charlie Pierce used to say 'We're gathering light first and foremost'.
Charlie Pierce is a legend! We did an interview with him a few years ago. See: Charlie Pierce: How Video Surveillance / CCTV Has Changed
Also, Charlie likes to give hugs. (He is a genuinely nice person)
During previous installation, indeed many customers prefers to use vari-focal lenses due to the reasons mentioned above...nice sharing.
Simple yet easy to missed.
