What does the 28mm on my 28 – 135mm lens mean? And, why does my point and shoot say “35mm equivalent = 28 – 135mm”?
Focal length is a technical photographic term which describes one of the physical attributes of a lens. It can be translated into several non-technical definitions including how it describes the magnification of a lens, the lens’ angle of view, and how a subject might be distorted or portrayed.
Focal Length – technical definition
From two respected photographic resources:
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One way of thinking about focal length is that it describes the physical length of a lens from the focusing point inside the lens to the film plane or recording surface. So a 50mm lens is physically about 50mm in length and a 200mm lens is about 200mm in length. With a zoom lens, when set to 28mm the distance from the focusing surface inside the lens to the film plane is about 28mm (1.10 inches)
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Focal length is a technical photographic term which describes one of the physical attributes of a lens. It can be translated into several non-technical definitions including how it describes the magnification of a lens, the lens’ angle of view, and how a subject might be distorted or portrayed.
Focal Length – technical definition
From two respected photographic resources:
- Focal length - The distance from the lens to the focal plane where the lens is focused on infinity. The longer the focal length, the greater the magnification of the image.
- One of the most important characteristics of a lens is its focal length. Technically, this refers to the distance from the rear nodal point of the lens (usually close to the aperture plane) to the plane where subjects at infinity come into focus. Knowing the focal length is important not only because it identifies the distance of the lens from the film (for distant subjects), but also because it provides a measure of the image size and subject area in relation to the film format. If you use a short focal length lens to photograph a subject, you will obtain an image of greater area of the subject, and each part of the subject will be smaller in the photograph, than if you use a longer lens. When I first became aware of the lens and the image it forms in relation to the subject, I thought of the lens as “embracing the outer world”
One way of thinking about focal length is that it describes the physical length of a lens from the focusing point inside the lens to the film plane or recording surface. So a 50mm lens is physically about 50mm in length and a 200mm lens is about 200mm in length. With a zoom lens, when set to 28mm the distance from the focusing surface inside the lens to the film plane is about 28mm (1.10 inches)
Three different perspectives of the Painted Hills Unit of John Day Fossil Beds National Monument, OR
Focal Length – magnification
Focal length is often used as a way of describing how magnified a subject might appear. For most modern non-professional digital slr cameras, a “normal” lens is about 35mm in length; meaning that photographs taken with a 35mm lens will have about the same magnification as we see with our normal vision.
Digital slr lenses which are longer than 35mm are called telephoto lenses. A telephoto lens brings subjects closer than they appear to the naked eye. A 70mm lens would be a 2x lens because things appear twice as close as they do to the naked eye.
Long telephoto lenses, such as 400mm or 500mm lenses, can be compared to binoculars in terms of their magnification. A 400mm lens brings things 11.5x closer than normal vision, rivaling a pair of binoculars.
Digital slr lenses which are shorter than 35mm are called wide angle lenses. Wide angle lenses make subjects appear further away than they would appear with normal vision. A 17mm lens is ½ as long as a 35mm lens, so subjects will appear twice as far away as with the naked eye.
Focal Length – angle of view
Focal length can also be used as a way of describing the angle of view of an image. Wide angle lenses produce images of vistas which appear wider and more inclusive than the scenes may appear to the naked eye. By including more in a scene, wide angle lens sometimes create stories and relationships which wouldn’t be evident otherwise. In digital photography an 18mm lens can show an angle of view equivalent to about 75°
Telephoto lenses can single out a narrower perspective of a scene than the eye can see. Telephoto lenses can be used to eliminate distractions or subjects from an image by ‘cropping them out’ from the field of view. A digital 200mm lens sees less than 18° of a scene.
Focal Length – lens qualities or distortions
In practical application, focal length also gives us an idea of how an image will look depending on what lens we choose. Each lens has its own look and signature which is a combination of its focal length, construction materials and quality, and whether or not it is a zoom lens or a fixed length like a 100mm lens.
In general wide angle lenses make an image appear to have more depth. There is an appearance that objects in the image have space between them and that there is a clear differentiation between the foreground and the background. A wide angle lens can appear to exaggerate the perspective of a building or subject shot at close range.
Wide-angle lenses can create apparent distortion of subjects. Because photographers are often forced to shoot at closer distances with wide angle lenses, subjects closer to the lens appear larger than those further away. Also, objects at the edges of the frame may appear angled or even warped when photographed with a wide angle lens. Be mindful of this when photographing large groups of people at short distances with a wide angle lens. People on the edges can appear unnatural and unattractively shaped.
Telephoto lenses can appear to flatten an image so that there is little sense of depth between objects that may not actually be close to one another; they compress depth, bringing foreground and background together.
Focal Length – “35mm equivalents”
In the world of digital photography, different cameras use different sized sensors to record photographs. Point and shoot cameras, such as Canon’s SD series may use a sensor that is about 5.8 x 4.3mm, while the digital Rebel and 7D use a 22.3mm x 14.9mm sized sensor. Full frame cameras use a 36mm x 24mm sensor.
With all these different sized sensors, discussions about focal length become difficult and head-banging. After all, we don’t really care how long a lens is; we care about the look it creates.
In the interest of reducing confusing (and obfuscating how small some sensors actually are), digital camera manufacturers have chosen to use the term “35mm equivalents”. The theory is that many photographers have enough experience with 35mm film photography that presenting the focal lengths with this system will make deciding which lens to use easier. It also gives everyone a level playing field when describing what a shot will look like.
Photographing with 35mm film, 50mm is considered a ‘normal’ lens. Lower numbered lenses are wide-angled. Lens with longer mm numbers are telephoto lenses. A 100mm lens is a 2x lens; a 400mm lens is an 8x telephoto.
Focal Length – other web resources
Canon Focal Length Simulator – one of several on-line tools where one can compare the magnification and distortion of different lenses taken from the same position. Part of Canon’s lens education website.
Differences between Wide angle Lens and Telephoto Lenses
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