If you decide to go with a longer lens you'll need to think about housings, too.

Would you ever install a very long lens and if so, what brand would you choose for a CCTV application? We are trying to avoid cabling with an elevated view from a location much closer to our infrastructure.

A: Being able to reach into a scene optically across air space is very appealing but you will need to be sure the results will be better than what you’d get with a local camera and economical wireless links. With the right lens and a vibration-free mounting point you can achieve good result, but they will be different results than those you’d get cabling to a location.

For a start, compression may make the image more cluttered than you want it to be – in good light this will be less of an issue than it may be in poor light. That vibration-free mounting point – it’s really important. You can mitigate some effects with faster shutter speed, but this will reduce the time light has to reach the sensor so your image streams will be darker and noisier. Image stabilisation will help.

Aperture selection will be another delicate balance. You want it smaller so your images are sharper (this will take some fiddling to get right) and so you have enhanced depth of field but the smaller the aperture, the less light, the more noise, the slower the compensating shutter speed, the greater the motion blur. If you buy a quality camera that can handle higher ISO, that’s a great start.

Something else to consider with long lenses is the impact of dust, haze, smog, pollution and heatwaves. These will reflect and refract light enough to reduce contrast and wash out colours over long ranges. You can bump up the contrast in camera settings and this may help. Some CCTV cameras have inherent low contrast and reflection or refraction from the atmosphere will make this performance worse still.

In terms of brand, we’d be looking at Fujinon.

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