Lens Selection RecommendationsBy John Honovich, Published Jan 30, 2013, 07:00pm EST
Selecting the right lens can be confusing and tough, especially because lenses are often integrated into cameras, severely restricting flexibility to mix and match the right lens feature sets. In this tutorial, we examine 7 key criteria in selecting the appropriate lens. They are:
- Lens Length / FoV Width
- Varifocal or Fixed Focal
- Iris Type
- Shutter Speed
- Color vs D/N Lenses
- Lens 'Resolution'
Lens Length / FoV Width
You need to know what lens length you need, because this determines how wide (or narrow) a Field of View (FoV) you can see. Do you want a 3mm, 8mm, 12mm, 50mm lens? The longer the lens, the narrower the FoV. See our lens length and FoV tutorials for details.
Typically in surveillance, 10mm or less is used because wide FoVs are most common. But there are many applications where one needs to see far away, requiring longer lenses. To determine exactly the lens length needed, use a lens calculator. For an example of what you need to see miles away, see this super long lens discussion.
Varifocal or Fixed Focal
Lens length is either fixed or adjustable. For instance, the lens may be 3mm with no other options (that's fixed) or adjustable between 3mm - 10mm (that's varifocal). Overwhelmingly, surveillance users prefer varifocal (see our survey results on varifocal vs fixed for more). However, fixed focal lenses are less expensive and typically found on lower cost cameras, so are often used to reduce costs.
If you use varifocal, you have more flexibility adjusting the FoV. If you used fixed, the only choice to do the same is move the camera closer or farther away from the subject.
F-Stop is a strong indicator and determiner of low light performance. The higher the F number, the less light the lens will pass and the more likely the camera will have problems in low light. For instance, an f/1.4 lens takes in a lot more light than a f/2.8 one (actually 4x). See our F-stop tutorial.
Always check the lens F-stop, especially if comparing different form factors or wide price ranges, as the differences can be extreme and have a huge practical impact
Iris Control Type
The iris is the opening that allows light to pass through the lens and to the camera. The four common choices are fixed, manual, auto and P iris. While higher end cameras typically use auto or P iris, our testing does not show much benefits for iris choice. For more on the differences between these controls, see our Iris tutorial.
The shutter is NOT part of the lens but is related because it significantly impacts the sensor's exposure to light. Many cameras use both iris and shutter speed control to regulate light input (see our shutter speed vs iris tutorial).
The shutter determines how long the sensor is exposed to light and is typically expressed in fractions of a second (1/1000s, 1/100s, 1/30s, 1/3s, etc.). See our shutter speed tutorial for more.
One big issue to beware is slow shutter speeds. Many manufacturers market this as a miracle technologically because, in really dark scenes, it can make an image bright but it creates blurring and ghosting of moving objects. This is why we say, this feature called Sens Up, is for suckers.
While cameras are typically marketed by their resolution, i.e. pixel count, like 1MP, 2MP, 3MP, etc., lenses are not. First, pixel count has no direct meaning because there are no pixels in lenses. Pixels are a part of image sensors, not lenses.
However, to make it simpler for users, lens manufacturers sometimes rate their lenses for resolution, such as 1.3MP or full HD, etc. Technically, the more accurate term is line pairs / millimeter (lp/mm) or how many lines a lens can display in a given area, with the more the better. The problem is that lens manufacturer rarely reveal this metric, making it useless as a judging criteria.
The best bet is to use the lens that the camera manufacturer recommends. As long as your manufacturer is reputable, they will recommend or require you to use a certain tested and verified lens. Using a cheap lens will not save much money but can cause problems. The exception to this rule of thumb is if you have done your own comparative tests with the alternatives you have considered. For instance, see our HD lens shootout.
Megapixel and Super Megapixel
Two rules of thumbs here:
- Never use an SD lens on a megapixel camera. You will almost certainly significantly degrade image quality.
- For 3MP or less resolution, many quality lenses exist. Just make sure they are rated for MP and the camera manufacturer approves. Above 3MP, it is much more difficult. First, varifocal lenses are much rarer. Secondly, the lenses tend to be significantly more expensive.
Color vs Day/Night
If you are using a day/night camera with a mechanical cut filter, use an IR corrected (sometimes marketed Day/Night) lens. Otherwise, you may have a focus / blurriness issue at night. The good news is IR corrected lenses are common and do not cost much more.
Even if you have the right lens, you still need to make sure it is in focus. This is especially important since most use varifocal lenses that require fine focusing after adjusting for the right lens length for one's scene. Auto back focusing / auto focusing are increasingly common and help greatly with this. IP camera installation tools should be considered but it's tricky finding the right one as there's no great, general solution (as there was with analog cameras). For more, see our focusing tutorial.
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