Understanding how camera lenses work in photography
There are many kinds of lenses available for cameras today. The main way lenses vary is by the focal length. Let's take a look at some different lens types.
Zoom lens
The most popular type of lens by far. A zoom lens allows your camera to use various focal lengths...in other words, it allows you to magnify your view to get a closer look at your subjects and get a wider angle of view. A zoom lens increases flexibility when shooting subjects. A zoom lens is not a single lens, but a number of pieces of glass that align in order to give the range of zooms available. Because of the additional pieces of glass in zoom lenses, there are some sacrifices in terms of quality. Zoom lenses are generally less sharp than primes, especially around the edges.
Prime Lenses
Prime lenses are lenses with a single focal length. This type of lens will be cheaper than other lenses because they have less moving parts. Using a prime lens might seem like a disadvantage because magnify a subject you have to physically move towards them. The advantages to using prime lenses is that because they use less moving parts, they will be cheaper, sharper and can have bigger apertures than zoom lenses. This makes for an excellent lens to shoot in low light conditions and will force you to work on your framing, which is not a bad thing.
Telephoto
A telephoto is a lens with a very large focal length. Around 100mm or larger length. It let's you get close to the action by magnifying the subjects and bringing them in closer. A telephoto lens can also be a zoom lens if it has a range of focal lengths. I, for example have a 70-200mm telephoto zoom lens. Longer lenses also help capture a narrower depth of field, so it's easier to get blurry backgrounds.
Wide Angle
A Wide Angle has very short focal lengths; usually below 20mm. A wide angle lens can also be a zoom lens. For example, I have a 10-22mm Sigma Wide Angle Zoom lens. A wide angle lens will have a large depth of field, so more of an image will be in focus. Wide angle lenses are excellent for architectural shots, also shots where space is tight. To get a subject to fill the frame with a wide-angle lens, you have to get very close to the subject. A wide angle lens will also dramatically distort the image so people, especially when they are close to the edges of the frame will be distorted.
Angle of View
The Angle of view in photography refers to how much of the world a lens' perspective is able to see at one point of another. A camera with a large field of view like a wide angle lens will let you see more of a subject. A camera with a narrow angle of view like a telephoto will let you see a small part of the image.
Perspective
In addition to the angle of view, the different focal lengths have an effect on the perspective of the photographs. A wide angle lenses will tend to distort perspective, especially at the edges. A zoom lens will compress perspective.
3X, 5X, 10X
When you purchase a point and shoot camera, you may see some interesting numbers that say the camera has 5X or 10X zooms. This refers to the ratio between the lowest and the highest focal length the camera lens can achieve. It doesn't have anything to do with total magnification. When purchasing a point and shoot camera, you should find out what the real focal range is and how it translates into a 35mm equivalent.
Optical versus Digital Zoom
One thing you have to be careful about when purchasing smaller cameras is that they often boast a large digital zoom. An optical zoom is the real zoom ratio of the camera. A digital zoom enlarges what the camera records digitally to make a bigger picture. It's like making a photocopy of something small. You'll get a bigger picture, but the quality won't be as good.
Sensor Sizes
The size of the sensor that records the light going through the lens has a huge impact on the quality of your final image. Think about it this way, if you try drawing something on a very small notepad, there's only so much detail you can effectively draw on a small artpad, but if you draw on a larger canvas, you can put more detail. This is why comic books are drawn larger,then shrunk down to size. In cameras, this means that a larger sensor will allow you to record more detail. The quality of the sensor also has an impact on this, but overall, the size of the sensor makes a difference.
In the old days, cameras used to have 35mm film and today, a 35mm sensor is considered to be a full-frame camera. But even most medium to high end DSLRs won't have full-frame sensors. Because they have smaller sensors, they effective focal range of your lenses is actually different when using these cameras.
Crop Factor
Because of the different sizes of the sensors, smaller frame cameras use a smaller portion of what would be recorded in a full frame camera. This introduces something called the Crop Factor or Magnification Factor. My Canon 40D and XTI both have a crop factor of 1.6, which means that if I use a 100mm lens, the camera crops most of what would be captured by a full lens in effect "zooming into" a smaller frame. Therefore, the picture appears to be bigger and the lens behaves like a bigger lens. As a matter of fact, it behaves as a 160mm lens.
Because lens aberration is more apparent at the edges, a cropping effect can actually help because only what is in the sharpest area of the lens is stored. The other advantage is that the crop factor also makes long lenses even longer. A 50mm lens will behave like an 80mm lens, and a 200mm lens will behave like a 320mm lens. This is great except when you're trying to get a wide-angle lens for a small sensor camera. A 20mm wide-angle will now behave like a 32mm lens and that's pretty close to normal view (35mm).
Additional Features
Some higher end lenses offer additional features that will make your life easier and is the reason for their additional expense. Some of these features might be well worth it for professional shooters
Image Stabilization
Some lenses offer a feature called "Image Stabilization". Lenses with larger focal lengths are often very heavy, therefore, it's harder to keep images at great distances in focus. Some higher end lenses like Canon's will have a feature that adjusts the position of a series of stabilizing lenses within the lens casing by observing the movement of the lens in the vertical and horizontal plane with built-in gyroscopes.
Canon's Image stabilization and Nikon's vibration reduction are technologies that allow you to shoot in lower light conditions with a longer lens. You may be able to shoot handheld (without a tripod) at three of four shutter speed ratios lower than normal. So instead of shooting at 1/60th of a second you may be able to shoot reliably without a tripod at 1/30, 1/15 or 1/8 of a second. This is a huge advantage.
Image stabilization will also allow you to handhold very long lenses in focal lengths where you would normally need a tripod. There are even different image stabilization modes. Some lenses allow you to set up your stabilization so that the lens will correct only for vertical movement, allowing you to shoot panned shots where the background is blurry only horizontally, but not vertically.
Parfocal vs Varifocal
Parfocal lenses can maintain focus accross the entire range of zooms. This is convenient because it allows you to stay in focus while shooting at different focal lengths as long as your subject stays put. Most lenses other than high-end lenses are varifocal.
Front Lens Rotation
Another issue that comes into play when using circular polarizing filters is that some lenses rotate in the front whenever you zoom. Polarizers work by filtering light so that only light coming from a certain angle goes through the filter. By rotating the filter in a circle you can control the angle of the light rays coming in. If you have your angle set just right on a circular polarizer, but you use a lens which rotates as it zooms, you'll have to reset your polarizer's angle every time you shoot.
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