Especially with varifocal cameras, and even with fixed focal length units that have fairly wide angles of view, the built-in IR illuminators usually do not cover a very wide field. So you usually have to settle for poor illumination at the edges when using wide angle cameras with their built-in illuminators.
Built-in illuminators are also a serious problem with any sort of dome camera because any dust, smudge, rain, scratches, fogging, crazing, etc., of the dome gets brightly illuminated by the built-in illuminator, and shows up badly in the image.
And often, plastic domes are degraded by exposure to direct sunlight in fairly short order. And I have yet to find any camera manufacturer selling replacement domes, which they really should! Also, an optical-grade glass dome, with multicoating to reduce reflections (like a high quality camera filter) might be nice. But it would be expensive, and the coatings might be delicate. Domes are just a bad idea unless you have many thousands of dollars to spend on the dome system, and are willing to maintain and replace it as needed.
Just as we'd never recommend putting a camera inside of a window to "look" outside, especially if that camera is using a built-in illuminator, a dome type camera will suffer from reflections and dirt effects, especially when you use its built-in illuminator.
Bullet style cameras also have problems when using their built-in illuminators, because spiders build webs from the camera's overhanging "sun shade" down across the field of view. These webs are then brightly illuminated by the built-in illuminator, and show up badly, blowing in the breezes, triggering motion alerts, etc.
The only camera style I've had good luck with, when using their built-in illuminators, are turret style. They aren't "looking through" a cover that is illuminated from within by the built-in illuminator. And spiders tend not to build webs across flat surfaces, and if they do string a web across the lens cover, it is NOT lit up by the illuminators.
As attractive as some of the dome and bullet style cameras are, I almost never consider them, especially for outdoor use. And especially if they'll be mounted where you can't easily reach them for the necessary daily (or more frequent) de-webbing.
The other point to consider is that you often get far better coverage angles and ranges, and dramatically reduce the problems of spider webs and dirty domes If you disable the built-in illuminators and use separate external illuminators. You can get more power and wider angle lighting that way.
Also consider white light illumination (dawn to dusk lighting) for the areas. That often allows full time color operation for the cameras, which can be helpful, especially with better low-light cameras.
Domes and bullets using their built-in IR illuminators are often disappointing, if not downright annoying in the real world, especially outdoors.
And yet, when I see packaged camera-NVR systems, they inevitably come with bullet cams. I don't get it!
Also: Realize that focal length is just one variable in the formula to find angle of view for any camera. You must also know the sensor dimensions. The important figures are angles of view. One camera's 2.8mm lens will NOT give the same angle of view as another's.
