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Focal length, usually represented in millimeters (mm), is the basic description of a photographic lens magnification. We also look at the different types of lens available.
Introduction to Photography Series
  1. Beginners Introduction to Photography
  2. How to Understand Focal Length and Lenses
  3. Get to Know your Camera and Get off Auto
  4. A Guide to Camera Shooting Modes
  5. Understanding Shutter Speeds
  6. Aperture, Depth of Field and The Relationship Between Them
  7. Exposure Triangle: Aperture, Shutter Speed and ISO
  8. 10 Top Photography Composition Rules You Need To Know
  9. White Balance Explained
  10. Flash Photography
  11. Why You Need A Tripod for Photography
  12. A Beginners Guide to Start Shooting in RAW
  13. Understanding Histograms

Types of Lens

There are two main types of lens available - zoom lens and prime lenses. A prime lens has a fixed focal length, such as 50mm or 135mm. Zoom lenses are able to change zoom and are expressed as a range, such as 70mm - 300mm.

Examples of various lenses

Examples of various lenses

There are many advantages of zoom lenses; they are much more versatile as you don't need to keep changing lens risking dust entry into the camera body and onto the sensor.

Prime lenses on the other hand usually have higher quality optics designed specifically for the set focal length. They are often cheaper as there are fewer lenses internally and fewer mechanisms, something which also makes them lighter and more compact. They also tend to have a wider maximum aperture, say f/2 or even f/1.4 which are a distinct advantage when shooting in low light conditions.

Focal Length

Focal length is not what is commonly believed, the actual length of a lens, but instead a calculation of an optical distance from the point where light rays from an object focus on the digital sensor or 35mm film surface. In short, the focal length of the lens is the measure of how "zoomed in" your lens is.. The focal length of a lens is calculated when the lens is focused at infinity.

The focal length of the lens describes the angle of view - how much of the scene will be captured, and the magnification - how large the object will be. The longer the length, the narrower the angle of view and the higher the magnification. The shorter the focal length, the wider the angle of view and the lower the magnification. This is demonstrated in the images below.

Visual example of various lens focal lengths

Visual example of various lens focal lengths

Focal length affects perceptual perspective too. As the focal length and magnification of a lens increases, the image appears more compressed, resulting in less visual distinction and separation between the foreground, middle ground, and background.

Focal Length Crop Factor

Often people will tell you what the lens would be in a 35mm equivalent by using a "crop factor" which is the size of your camera's sensor in relation to a 35mm frame of film. In terms of today's DSLRs, most sensors are the APS-C-sized format, smaller than that of a 35mm (ie full) frame.

What this results in is a magnification of a lens's focal length, whose value is determined by the sensor's size in comparison to a standard full frame. Manufacturers refer to this magnification as the "crop factor" - or, how much of the image is cropped due to the smaller sensor.

The APS-C format, measuring around 24x16mm, is smaller than a full-frame sensor by a factor of 1.5x. So, the focal length of a lens must also be multiplied by this amount to arrive at its effective (35mm equivalent)focal length - so a 28mm lens becomes a 42mm lens, a 50mm lens become a 75mm lens and so on.

Lens Compression

Have you ever wondered how photographers are able to make their subjects seem so close to objects in the background? Perhaps you've seen photos of a moon that seems impossibly big against a skyline and thought it must be Photoshopped. Well, you can actually achieve this effect without any computer manipulation, using a technique known as lens compression.

Lens compression is an effect that comes about by moving the position of the camera in relation to the subject you are photographing. As you move further away from the subject, you will usually zoom in on your subject to make it larger. For example, if you are taking a portrait of a person from further away, you will want them to fill more of the frame, so you will zoom in. This has the effect of making the subject appear closer to the background.

Here's an example to give an idea of lens compression in action. The first image is shot with the camera close to the subject using a wide angle. The second image is shot using a telephoto around 200mm with the camera placed much further back from the subject. Notice how the trees in the background look much closer to the subject in the second photo.

Thoughts?

Can you think of anything else that photographers ought to know about focal length, field-of-view, and composition? If so, please let us know in the comments. I'd like to hear your thoughts.

 

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