How important is attention to focus with CCTV lenses, given many are wide angle lenses with very small hyperfocal distances? Shouldn’t everything in the field of view of such a lens be in focus?
WITH a wide lens, everything past the hyperfocal distance of a wide lens will be more or less in focus but that doesn’t mean focus will be as good as it could be for your operational requirements. There are plenty of compact cameras with fixed focal lengths and non-adjustable lenses out there – fixed lenses of around 4mm are becoming a trend. Most do an ok job but you only have to look at performance in the real world to see that they still have a focal sweet spot around 8-10m that gives maximum sharpness in the centre.
With a wide varifocal lens at the widest setting, the same general rule applies – there will be a sweet spot in the centre at a discernible distance, and the rest of the image will be generally in focus. But you can improve focus in your application by tweaking at commissioning and during maintenance to ensure best operational performance, especially if you want to place your point of focus deeper into a scene.
Careful focusing becomes more important at longer focal lengths. We’ve mucked around with many varifocal lenses and focus really does count at the long end. If you focus on some junk on a shelf in the workshop 5 metres from the lens, don’t expect to get best performance at 12 or 20m on the street. You need to commission for the application with the camera installed.
Focus for face recognition is important, too. We’d recommend longer focal lengths for faces – the longer the less distortion, the less aberrations, the less irrelevant pixel-eating image components – walls, windows, ceilings, trees, sky, opposite sides of streets, or details at a distance or at the edges that are too soft to be useful. ♦
