LESSON 2: Knowing Your Lens

Lens Basics


Did you know? The word lens comes from the Latin word of the same name meaning 'lentil', because the basic convex (curved) lens resembles a lentil. 

As discussed in LESSON 1: Knowing Your Camera, a lens performs two jobs: a) to gather a large amount of light from the scene, and b) to focus the light from a plane of the scene to create a visibly sharp image. In fact, there is a third function that almost all lenses have. They take what arrives as a curved image from the scene and transform it into the flat image that is recorded by the camera. Lenses that do this are called rectilinear (rectus 'straight' + linea 'line') lenses. Here is an example of an image of how an image looks both with and without this rectilinear transformation:


A lens brings the light from a scene into focus onto the camera sensor perfectly, but only from the plane of focus. This is an imaginary geometric plane at a distance in front of the camera, perpendicular to the axis of the lens. The further away from this plane that we find elements in the scene, the more out of focus they become in the final image. This is particularly evident when the aperture is completely open when capturing an image. You can see that the term "fo•cal length" in the image below falls on the plane of focus, but the lines above and below this term become increasingly out of focus:


When one reduces the size of the aperture, the range of the image that we see as being in focus increases. This range is termed depth of field. Here is the same image shot with a very small aperture: 


With this in mind, one can decide to use a larger aperture when one desire to have less of the scene in focus (for example, a portrait of a single person), or use a smaller aperture when one desires to have more of the scene in focus (for example, a large group of people).

Depth of field is also affected by the distance from the lens to the subject in focus. As one moves the lens closer to the subject, the depth of field decreases. As one moves further away from the subject, the depth of field increases. 

The plane of focus (what you want to be in focus in the image) can be adjusted manually by rotating a ring on the lens called the focus ring. Cameras also have a technology called autofocus, a feature which automatically focuses the lens on a point in the scene. Autofocus is especially useful in capturing split-second moment where the photographer does not have much time to manually focus on the subject.

Focal Length

When the parallel light rays from a scene hit the lens, they meet at a point. The distance from the lens to this point is called the focal length and is measured in millimeters.


The focal length of a lens is important because determines two characteristics of a lens: a) the angle of view from which it can gather light, and b) the magnification of objects in the scene. The shorter the focal length, the wider the angle of view and the less magnification of objects. The longer the focal length, the narrower the angle of view and the greater the magnification of objects. 


The Normal Lens


When the focal length of a lens is equal to the diagonal length of the camera's sensor, the image that is produced approximates that of human vision. If we take a picture of two objects which are at different distances from the lens, then we switch places with the camera and look at those same objects, their relative sizes will appear the same to us as they do in the picture.

Short Focal Length Lens


A short focal length lens captures light from the scene in a wider angle than a normal lens does. This is why it is commonly called a wide angle lens. I'm addition to getting a wider view of the scene, such a lens also has a lower magnification than a normal lens. The parts of the scene seen through a short focal length lens will be smaller than when seen through a normal lens. The relative sizes between two objects in a scene will be exaggerated: objects closer to the lens will appear larger in comparison to objects further away. 

These characteristics are useful when you need to photograph a large group of people or when you want to emphasize an object in a scene by making it appear larger than it really is (for example, a bride's bouquet). 

Long Focal Length Lens


A lens with a long focal length has a narrower angle of view than that of a normal lens. It also provides greater magnification of objects in a scene. These characteristics are useful when you want to get a large image of a subject that is small or far away, such as a bird on a tree branch. 

Also the narrow angle of view allows the photographer to capture the subject in the foreground and eliminate much of the unwanted background. The red outline in the image below indicates the angle of view of a lens with a very long focal length:


Zoom Lenses


A zoom lens is simply a lens that lets you change its focal length. Some zoom lenses have focal lengths ranging from short to very short, while others have focal lengths ranging from long to very long. And there are some with focal lengths ranging from wide to long. 

A zoom lens allows one to change the magnification of a subject with a twist of the zoom ring without having to move closer to that subject. This feature is quite useful when moving around might cause a commotion, such as when photographing during a wedding ceremony. 

Some lenses only focus light at a single focal length. These are called prime lenses. Since a zoom lens includes many focal lengths, you might wonder why someone would choose to use a prime lens over a zoom lens. While a zoom lens is versatile with regard to magnification and angle of view, prime lenses have a number of advantages over zoom lenses. They are smaller, lighter, can capture a larger amount of light and render a higher quality image than a zoom lens. 

Macro Lenses


To increase the size of an object in a scene, you can either use a lens with a longer focal length or you can move the camera closer. However, most lenses cannot focus on objects that are too close to them. Special lenses, called macro (or sometimes micro) lenses, are able to focus on objects much closer than non-macro lenses. For example, trying to get close to a bowl of popcorn with a non-macro lens would result in an image with about five or six popcorn kernels across the frame of the image. With a macro lens, one could get close enough so that a single kernel occupied the entire image frame. Because of this special characteristic of macro lenses, they are best employed when photographing small objects, such as the bride and groom's wedding rings. 

LESSON 1: Knowing Your Camera


Did you know? The human eye has the equivalent of 120 million pixels - 5 times more than a typical camera!

A camera is tool made up of many parts that work together in order to see a three-dimensional, real-life scene and record it to the camera as a two-dimensional image. These parts are similar in function to those used by the human eye and mind to accomplish the same task. Below I cover the basic components of the camera so that you are familiar with your tool and can use it to create a photograph.

The Lens


Light from a scene first travels through the lens. This component is made of glass elements housed in a cylindrical barrel, which is itself made from plastic and/or metal. Its job is to gather a large amount of light from the scene and also to focus the light from a plane of the scene to create a visibly sharp image. For more on lenses, read "Lesson 2: Knowing Your Lens".  

The Aperture


Within the lens there is the diaphragm, which is a group of blades that move to regulate the size of the opening at the centre of the lens. This opening is called the aperture. Changing the size of the aperture affects two characteristics of an image: a) the brightness, and b) how much of the image appears in focus. The larger the aperture, the brighter the image and the less that appears in focus. The smaller the aperture, the dimmer the image and the more that appears in focus.

The Viewfinder


Once light exits through the lens, it travels through the viewfinder assembly. Comprised of a mirror and a glass prism, the viewfinder reflects the incoming image up and out the back of the camera. This real-time image can be seen by the photographer and used to aim at the desired scene.

The Shutter

When one decides they want to record the scene, they press a button on the camera to activate the shutter. This is a flat, rectangular component consisting of two metal curtains. In their normal state, the curtains stop the light from advancing further into the camera. When the shutter button is pressed, the first curtain moves across the shutter plane to create an opening and allow light to pass through. The second curtain then follows behind to close this opening. How soon the second curtain follows the first is called the shutter speed. The shutter speed affects two characteristics of an image: a) the brightness, and b) the amount of motion blur of a moving object. The faster the shutter speed, the dimmer the image and the less motion blur. The shower the shutter speed, the brighter the image and the more motion blur.

The Sensor


The light that passes through the shutter next hits the sensor of the camera. Also rectangular in shape, the sensor is comprised of an array of millions of microscopic sensor elements, called pixels, that are sensitive to light. Some pixels record the color blue, some record the color red and some record the color green. Each pixel registers the cumulative light that it senses by converting it to an electric signal and recording the strength of that signal as a number. The result is an array of value pairs containing the colour that was recorded and its brightness level. These values represent the image of the real life scene captured. The resultant image is stored on a memory card inside the camera.



On the back of the camera is found an LCD screen that has for functions:

  1. Monitoring the camera settings

  2. Playback of the recorded images

  3. Accessing the camera menus

  4. Viewing a real-time image of the scene

The last function is called Live View and is toggled on and off by pressing a button on the camera. This moves the mirror up so that it no longer reflects the image through the viewfinder and allows it to travel straight through to the sensor. Here the image is read continuously and displayed on the LCD screen. During Live View, one can magnify the view of a portion of the scene in order to verify that it appears in focus.