All About Lenses
Many first-time SLR owners don’t really understand lens technology and don’t know the meaning of terms such as “maximum aperture”, “fast lens”, “focal length”, “zoom lens”, “prime lens” and wide angle/telephoto distinctions. I hope this article will enable you to understand the specifications of your current lens lineup and help you to choose lenses for your camera in the future.
What is Focal Length?
Focal length is the most important specification of your lens. What does "Focal Length" actually measure? The technical answer is that the focal length is the distance from the front element of the lens to the point at which light passing through the lens is focused, i.e. the sensor, measured in millimeters.
All that you really need to know is that the focal length tells you the length and magnifying power of the lens. Focal length can be fixed such as 50mm or if you have a zoom lens, it would be variable; for example 18 – 55mm or 70–300mm.
A short focal length of up to 35mm is considered wide angle; focal lengths between 35mm and 70mm are considered normal, because this range approximates what the human eye sees; and anything beyond 80mm gets into the telephoto range because it's a magnified view. What Is the Difference Between a Prime Lens and a Zoom Lens?
A prime lens has a single, fixed focal length such as 50mm, 100 mm or 300mm whereas a zoom lens has range of focal lengths, like 18-55mm, 70-300mm, or 18-200mm.
Zoom lenses are obviously more convenient to use, but there are engineering trade-offs involved in a lens that can move through a wide range of focal lengths. Prime lenses perform better--and many are less expensive than zoom lenses. Serious photographers might have a few prime lenses in common focal lengths such as 35mm, 50mm and 85mm, 300mm, but most folks get by with a couple of zoom lenses that cover wide-angle and telephoto ranges with very satisfactory results.
What about Aperture?
Aperture is the 2nd important lens specification. All lens apertures can be made smaller or "stopped down" to f/22. The reverse is not true - every lens on the market CANNOT be opened up to f/1.4. The widest aperture a lens can be opened to is called the maximum aperture.
The smaller the f-number, the more light can be let in to enable faster shutter speeds for low-light photography.
Prime lenses, having a single focal length, have a set maximum aperture. Zoom lenses can have a constant maximum aperture at all focal lengths or, as is the case with most zoom lenses, they will have a variable maximum aperture setting at different focal lengths; such as f/4 – 5.6. The smaller f-number (f/4) is the aperture at the wide-angle end and the larger number (f/5.6) is the maximum aperture when zoomed to the longest focal lengths.
The kit lens that came with your digital camera might be an 18-55mm f/3.5-5.6 zoom:
* At 18mm the maximum aperture is f/3.5 * At 55mm the maximum aperture narrows to f/5.6
In the middle of the zoom range it’s f/4-4.5.
Fast Lenses
Most lenses which have constant aperture at a low f-number such as f/1.4, 1.8, 2 and 2.8 are usually more expensive because they are larger diameter lenses with more glass. They are considered “fast lenses” because the larger aperture allows in more light resulting in faster shutter speeds. Of course, the lens that offers a bigger aperture (the smallest f-number) is usually the better choice--and it will usually be more expensive.
Most lens manufacturers have several zoom lenses with constant maximum apertures. On these lenses, the maximum aperture is the same, regardless of how much the lens is zoomed.
How can you tell which lenses have constant maximum apertures? If you see a zoom lens with only ONE aperture number then it has a constant maximum aperture.
Example: 17-55mm f/2.8 (2.8 is the max aperture at both 17mm and 55mm)
What are the benefits of lenses with wide maximum apertures?
1. Natural light photos, which are always preferred to flash.
If you are shooting indoors and your camera tells you that with your aperture set to f/4, your shutter speed will have to be 1/30 of a second you know that's not going to be fast enough to take a portrait shot of a moving child in the house. You will have to use flash or make sure the child sits very still and you should also use a tripod.
If you had a lens with a maximum aperture of f/2.8 (which lets in double the amount of light of f/4) then you can use a shutter speed of 1/90 second, which means that you can get sharp photos without the flash.
2. Fast Shutter Speeds to prevent motion blur.
If you have taken photos on an overcast day of a flower in the backyard with a breeze blowing you know this is a challenge. When you set the aperture on your zoon lens as wide as it can go, probably f/5.6, the camera shows a shutter speed of 1/125 of a second. That's not fast enough to get a sharp photo of the flower. You're stuck unless you go to a higher ISO which increases noise.
If you had a lens with a maximum aperture of f/2.8, you could get the shot because your shutter speed could be 1/500 of a second.
3. Reduced Depth of Field
Lenses with wide maximum apertures have shallow depth of field so that the subject is in sharp focus while the background is more blurred. Nature and portrait photographers use this all the time to reduce the impact of a busy background.
So What Maximum Aperture Do You Need?
The answer to this question depends a lot on where and when you want to take photos. If you want to take photos indoors or in dim light outdoors such as at dusk or for evening or indoor sports, then you need a lens with a wide maximum aperture.
If you only take photos in broad daylight, then a wide maximum aperture isn't as crucial.
Zoom lenses with constant maximum apertures are very useful to photographers who want to have creative control over their SLRs by using aperture priority and manual modes, but are not as useful to those who shoot in Auto mode.
The following chart should give you an idea of the maximum aperture that's best for these lighting conditions:
Lighting Max Aperture Reason
Indoors f/1.4 - f/2.8 If you want to take photos indoors without a flash, you need a lens with a very wide maximum aperture to let in plenty of light
Overcast f/2.8 - f/3.5 Overcast days are especially challenging for sports photographers who need to use fast shutter speeds to freeze motion
Nature f2.8-f4 Good for dusk and dawn photos without a tripod
Broad Daylight Any aperture In good light it really doesn't matter what the maximum aperture of the lens is because you probably won't be using it as it would let in too much light and over-expose the image.
What about Lens Diameter?
Lens diameter is not standard among lenses. If you only have 2 or 3 lenses, chances are they won’t have the same diameter. Fast lenses and long telephoto lenses have larger diameters. The main concern with diameter is to make sure you know what it is if you wish to purchase a filter for your lens. It’s usually shown on the end of your lens. Common sizes are 58mm, 67mm, 72mm and 77mm.
What Is Image Stabilization?
Some lenses come with built-in stabilization that reduces camera shake as if you were shooting with a faster shutter speed than you really are. This feature is great for telephoto lenses with their smaller apertures in low light or for shooting pictures without a tripod. If you have a longer telephoto lens that is not stabilized you will have to use a tripod unless your shutter speed is fast.
Many newer image-stabilized lenses are a good bargain compared to what the older ones were only 4 -5 years ago, so look for the designation of IS (Canon) or VR (Nikon) in the name of the lens. (Example: Canon 17–85mm f4 – 5.6 IS). Sigma and Tamron also make stabilized lenses. Sigma calls it OS and Tamron calls it VC. Do keep in mind that an image-stabilized lens won't be able to freeze the action if the subject itself is moving. In that case you would have to use a faster shutter speed. All of my lenses are stabilized but it’s not as necessary on wide-angle lenses as it is on longer telephoto lenses of 200 – 400mm.
Several new digital SLR models by Sony, Pentax and Olympus offer image-stabilization in the camera body. In-camera stabilization allows you to purchase non-stabilized lenses which are generally less expensive than stabilized lenses. This stabilization is effective although some pundits say that it’s not quite as good as stabilization built into lenses. A recent report on an Olympus model showed the stabilization to be very effective. However it doesn’t show up in the Viewfinder when framing your picture which might be a little disconcerting. It activates only when the shutter button is pressed.
Popular Lenses
Prime lenses
range from extreme wide angle i.e. 12 - 20mm to long telephoto such
as 300, 400, 500 and 600 mm. The 35mm, 50mm, 85mm, 135mm and 300 mm
are popular focal lengths with Canon users.
Pro photographers prefer lenses in the 85 to 200 mm range for portraits, either prime or zoom lenses. They let you shoot from a good working distance of 10 – 12 feet or more at 200 mm allowing you space between you and the subject and still filling the frame. They are long enough to not cause facial distortion that you may get with a wide angle lens and to give some blur to the background. Better quality zooms: Canon 15 – 85 IS, Canon 17 – 55 f2.8 IS, Canon 24 – 105 f/4 IS and Nikon 24 – 120. 18 – 105, 18 – 200 VR. Also check out Sigma and Tamron zooms. The Sigma 18 – 200 OS, Tamron 18 – 270 VC and the new Canon 18 – 200 IS are good “all-purpose” lenses with stabilization but aren’t quite as good as those without such a wide focal range.
What are all those numbers on my lens? See this article for clarification and lens information. Sites that test and review Lenses:
Information on all Canon DSLRs and lenses.
Information on all Nikon DSLRs and lenses.
All about Canon lenses. Helpful when trying to choose a lens to buy.
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