There are several considerations when choosing cameras for your installation:  Viewable size, picture clarity, lighting, mounting, and application are the most common factors.  The following is a general guideline to follow when choosing cameras, and a great way to start the process of narrowing down your choices.

Clarity:  The clarity of pictures being recorded starts at the camera.  The measurement used by camera manufactures is Television Lines (or TVL).  This measures the cameras ability to create a image that is true to life and lot digital like or pixel like.  This is not to be confused with resolution.  Resolution is the measurement of the size of the recording.  Both are important to creating quality pictures.  If you need to be able to see crystal clarity full screen we recommend you set your DVR to the highest resolution possible (D1 or 4CIF on the best units available) since this reduces the amount of distortion created by "image stretching" (IE the image begins larger and therefore does not need to be enlarged to fit the screen).  However, if you intend to do this often and need the images to be crystal clear we also recommend you use cameras of at least 500 TVL since they provide excellent clarity at large resolution. 

Lighting: Different cameras perform differently in different lighting.  Terms like Day/Night, True Day/Night, Lowlight, WDR, Lux and Infrared are very confusing. 

Lux: This is a the scientific measurement for an amount of light.  Unfortunately not every company uses the same method to measure the "Lux Rating" of their cameras and these numbers can't always be used to compare between different manufacturers.

IR or Infrared Light: If you are concerned about seeing in zero light, you will require a camera with Infrared capabilities.  These cams have floodlights attached to them that emit light invisible to the naked eye, but give the camera the ability to see in complete darkness.  It's important to have a strong enough light to cover the distance you need to cover, however too much light can hurt as well.  Know the distance you need to cover and pick a camera that is appropriate for your situation.  Some cameras are sensitive to IR light but do not emit it.  Always keep in mind that IR light from a camera or Illuminator can always be seen by multiple cameras.  A good design could essentially light an entire area up in invisible infrared lighting.  Two downfalls of IR light is that it is highly reflective and insects can see it.  This means that bugs may swarm to an IR light much like any other light.  Small objects like bugs, smoke or steam particles, and dust then create high reflection and can obscure the view of a camera or in the least create false motion detection by your DVR and consume your hard drive.

DN and TDN or Day/Night and True Day/Night: Cameras with these indicators are usually equipped with excellent lowlight capabilities.  Essentially they give you the ability to see perfectly in very dim situations.  The difference between the two is the True Day/Night cameras usually have even better lowlight than their Day/Night counterparts.  When you combine D/N with IR light you gain the ability to see anytime, anywhere.  The lowlight portions will see until it becomes too dark for them, and then the IR light will take over.  This is just like any other IR cam, except that the light threshold is much lower giving you true color pictures much longer than a standard IR camera would.

WDR or Wide Dynamic Range: This newer technology is perfect when there are multiple zones of light or changing lighting situations.  The WDR chipset is designed to compensate for glare and provide good lowlight images simultaneously.  This application is ideal for camera views that face windows, open doors, bars and nightclubs, movie theaters, and anywhere else where you may have a problem with light blinding a camera or causing it to adjust to the light and lose viewing in the darker areas. Rules of thumb include if you mount in a darker area and are recording a lighter one or vice versa you are much more likely to require or benefit from a WDR camera.

Viewable size: The viewable size is the width of the image at a given distance (How wide your camera will cover at a given distance).  This is determined by the lens and CCD chip width.  If your chip is wider, it allows a wider array of light coming through the lens to come in contact with the light sensing CCD chip.  Draw an X.  The X represents the lens.  On one side of the lens is your CCD chip, and on the other are your objects you are trying to view.  If you widen the angle of the X on the CCD side, you widen the angle you can see through the lens on the object side.  Quarter inch and third inch CCDs are the most common CCDs in the market.  Essentially, if you want to see a wider area, you should consider a camera with a 1/3" CCD and smaller lens width (3.6mm and 4 mm are very common).  You can often use a lens calculator to assist in making your choice. Verifocal lenses give you the ability to tune your camera to exactly what you need.

Mounting: One reason to choose a bullet, dome, box, or other camera is what you are up against in your installation.  The most easily mounted cameras are bullets.  They have a bracket that is fastened to a surface and the camera is affixed to a radial arm from it and then pointed to the area needed.  This gives you fast flexibility to quickly change the camera's direction and great flexibility in where you mount the camera as well.  The downfall is that they are also the most easily moved camera as well.  Essentially anyone with a broomstick can move the camera.  Box cameras are also great for these reasons and present a nice theft deterrent.  Domes are flush mounted and only slightly more difficult to install.  They are shorter and have to have a clear field of view about the mounting area to see.  However are superior in a few ways.  They are more difficult to modify once installed, making them fairly tamper proof and are offered in vandal resistant models that hold up to abuse rather well. 

Application: Inevitably application will factor into your decision.  How is your lighting?  Is your installation location ideal for a certain camera design?  Is your object you are trying to monitor close by or far away?   Do you have to be concerned with tampering or vandalism?  These and many other factors will without a doubt affect your decision and selection of the right cameras for you.