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Home    Lesson Plans    #4: Understanding Camera Lenses

Course Requirements

  1. Before starting this lesson, you should have a basic understanding and know how to operate your digital camera. Read the following lesson to get an idea on how a camera works.

  2. You must have a digital SLR camera and a few interchangeable lenses.

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Understanding Camera Lenses

Knowing how to use a lens is essential in learning photography. Your digital SLR camera has a unique capability to allow you to change lenses depending on your needs so undertanding when, how and what lens to use is an important aspect of photography.

So, what is the main purpose of the lens? Well, a lens takes in light and focuses it to produce a sharp image on your photograph. How much of the scene will be focused depends on the lens aperture.

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In lesson #1 of our ongoing online photography course, I explained what Aperture is and its effects on exposure. Just to refresh your memory, I will briefly describe it again here. Aperture is the opening in the lens and it controls how much light can pass through to the camera's sensor to produce a photograph. The lens opening is expresssed in f-stops and are represented by numbers like f/2.8, f/5.6, and so on. The smaller the f-stop number, the bigger the opening and the more light passes through it. The bigger opening opening also produces a shallow depth of field.

Lens Aperture Openings
FIGURE 1: Lens Opening - Aperture Setting

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Depth of field

Depth of field basically refers to how much of your scene is in sharp focus. Smaller f-stop numbers produces less area that is in sharp focus.

Depth of Field
Figure 2: Depth of field

For more in depth explanation about aperture and depth of field, please refer back to Lesson #1: Aperture & Depth of Field.

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Focal Length

Focal length is defined as the distance from the optical center of the lens to the focal point, which is located on the camera's digital sensor or film. This distance is expressed in millimeters (mm). Generally, lenses with longer focal lengths also have longer barrels as shown below.

Focal Length
Figure 3: Focal Length
Lens Barrels
Figure 4: Longer focal Length = longer barrels

The focal length of a lens determines how wide of an angle your camera can capture a scene and the how big the elements of the scene are. The shorter focal length produces a wider angle of view and the elements appear smaller in the scene. The longer the focal length produces a smaller angle of view and the elements appear larger in your scene.

Angle of view Photos
Figure 5: Angle of View Examples

The size of the elements in your scene is directly proportional to the focal length of the lens. Double the focal length of the lens produces double the size of the elements in your scene. As an example, refer to figure 5 above. The elements in the photo taken at 70mm are doubled in size compared to the elements in the photo taken at 35mm.

Angle of view
Figure 6: Angle of View by Focal Length for Full-Frame Sensors

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Focal Length on Full Frame and Cropped Digital Sensors

Digital Camera manufacturers like Nikon and Canon produces DSLR cameras that have different digital sensor sizes. A Full Frame DSLR digital sensor has a sensor size that is approximately the same size a a 35mm film. A cropped sensor refers to a digital sensor that is smaller in size compared to a Full Frame digital sensor. Because the sensor is smaller on a cropped camera sensor, the image projected by the lens would also be smaller (cropped) compare to what would be projected by a full frame camera. See example images below borrowed from the Nikon website (http://www.nikonusa.com/Learn-And-Explore/Nikon-Camera-Technology/g588ouey/1/The-DX-and-FX-Formats.html):

Full Frame vs Cropped Projection
Figure 7: Full Frame vs Cropped Projection
Full Frame vs Cropped Comparison
Figure 8: Full Frame vs Cropped Photo Comparison

As an example, Nikon produces DX DSLR cameras which are their cameras with cropped sensors. The Nikon D40 for example has a crop factor of 1.5. To determine the angle of view that the Nikon D40 produces, multiply the crop factor to the focal length of the lens. So the table below shows you an approximate field of view on Nikon D40 using the following focal lengths:

18mm x 1.5 (crop factor) = approx. 27mm
35mm x 1.5 (crop factor) = approx. 53mm
50mm x 1.5 (crop factor) = approx. 76mm
70mm x 1.5 (crop factor) = approx. 107mm
100mm x 1.5 (crop factor) = approx. 150mm

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Focal Length and Aperture

The amount of light that enters the camera is controlled by the size of the lens opening or aperture. Naturally, the shorter the distance between the lens and the sensor, the more intense the light is. So, lenses with a short focal length lets in more amount of light and more intense light reaches the sensor compared to a lens with a longer focal length with the same size opening. So how then do we make sure that the amount of light and the intensity of the light hitting the sensor or film is the same regardless of the focal length? This is where the aperture settings come into play. A system was designed that comes up with a standard f-stop number that takes into account the focal length and calculates the amount of light coming into the lens and the intensity of the light that reaches the sensor. So, f/5.6 represents a specific amount and intensity of light that passes through the camera. It takes into account the focal length of the lens so that a 35mm lens would have a lens opening that would be slighly smaller than a lens that is 100mm which produces an equivalent amount and intensity of light for both lenses. So in essense, as a photographer, you don't have to worry about how to calculate the right amount and intensity of the light that enters your camera. You just need to know that f/5.6 on a 35mm lens is exactly the same as f/5.6 on a 100mm lens.

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