Some folks will recommend that you use a "view finder" to pick your lens. I, however, will not. The view finder is an excellent tool for field of view verification and customer "hands-on" understanding, but it is not a tool that was designed to actually pick a lens for an application. Lastly, there are several different electronic calculators that are available on the internet (from various manufacturer sites). Regardless of the method that you choose, your final lens will still be based upon what you are trying to see, how much detail, what distance and with what format size camera.
In most cases, with a fixed camera I will recommend that a vari-focal lens be used. This is a lens that is considered to be a limited zoom lens. The vari-focal lens leaves the designer and the installer a bit of lateral motion with the actual final view. If you find that you need a bit more magnification, you can zoom it in; if you need a bit more width, just zoom it out.
Since these vari-focial lenses are a limited zoom, the ratio of zoom usually ranges in the 4:1 to 8:1 area. That is, a 4:1 ratio lens would be a 8mm to 32mm lens. The 8mm measurement refers to the wide portion, and then multiplied by four equals 32mm as the telephoto maximum. To choose a proper vari-focal lens, you should choose a range that would place your fixed lens choice as close to the center as possible.
Going back to the actual calculations, one of the most common errors made by designers is that they don't understand a simple philosophy of distance. If you measure your scene to be 30 feet away and you mount your camera 40 feet in the air, your distance from the camera is not actually 30 feet. It is actually 50 feet. This was originally discovered by a dude named Pythagorus a couple of thousand years ago. His self-named theorem says that A-squared plus B-squared equals C-squared, where A is the distance from the scene as measured on the ground, B is the height of the camera and C is the missing leg.
Let's try it quickly. Thirty feet squared is 900. Forty feet squared is 1600. The square root of these two combined (900 + 1,600 = 2,500) is 50 feet. Yes this works with metric measurements as well and is a very important calculation to remember with all cameras that are mounted above eight feet or so, at least if you want to be accurate or close with your lens choice.
Lastly in our lens calculations is the choice between standard analog or IP cameras of different resolutions and mega-pixel cameras. As you increase the resolution of a camera, you can (exponentially) increase the width of the scene and still capture the quality of image that you require. The advantage is that you may be able to do the work of two, three or even four cameras with one. The attached view (see second photo, above) was provided by Paul Bodell of IQinVision and says it all in a single picture. The advantage of the mega-pixel camera is that you can capture several times the amount of area and still have good resolution at all points within the image. The concern with using higher resolution images in our IP solution is, of course bandwidth for transmission or recording. But, it is still a very viable solution to review and become familiar with ... especially in those large open areas of your design, such as parking lots or production floors.
If you wish to have in-depth, plain-language explanations on lenses, choices, compatibility points and much more, then you should buy one of my books. How's that for a plug?
Meanwhile, in my next column, I will discuss the various formats and types of video transmission for the cross-over analog/IP world. It's the next natural step and your options are growing daily.
A PERSONAL REQUEST FOR HELP TO THE INDUSTRY
For the past 31 years, I have been bragging to friends and family about the security industry and it gifts to the general public. So I am now addressing that same industry, personally with a request for help.
In about six weeks, a small Catholic School on the lower west end of Davenport Iowa will be permanently closing its doors. Sadly, this is happening after 100 years of dedicated service to the education of children of all faiths. This is a school where my wife and her brothers, my three sons and a countless number of other folks that are close to me, have attended. This is a school where the teachers and staff have worked for below average wages and given from their hearts for as many as 25 years. Because we are a small parish and an equally small community, our staff was never afforded compensation or retirement benefits. They will be saying goodbye with nothing more in their pockets other than the fond memories and broken hearts that years educating others will create.