The focal length of a simple lens is the distance from the lens to the focal plane where the light is focused and an image is formed. Camera lenses are usually marked with the focal length and focal ratio. Magnifying Power The focal length determines the magnification of the lens when it is used alone on a camera, without any additional optical devices such as a teleconverter.
A lens with more focal length, and more magnifying power, will make objects larger in the image than a lens with a shorter focal length. If we have a selection of lenses with different focal lengths available, we try to match the focal length to the size of the object we are photographing. For example, if we want to shoot the entire constellation of Orion, we might use a 35mm lens on a DSLR. If we want to zoom in and shoot just the Orion Nebula, we might want to use a lens or telescope with a focal length of 1,000mm. Focal Length and Field of View When we use shorter focal lengths, the field of view gets wider. With longer focal lengths, the field of view gets smaller. The size of the field of view that we get is also determined by the size of the sensor in our DSLR cameras. Some cameras have a larger sensor. Most of the DLSR cameras that we will be using have a rectangular sensor that is about 22mm x 15mm in size. With a lens of a certain focal length, that sized sensor will give us a particular field of view. Some professional cameras have a larger sensor that is about 36mm x 24mm. If we use exactly the same focal length lens with a camera that has a larger sensor, it will give us a larger field of view. This brings us to the "focal length multiplier myth". The Focal Length Multiplier Myth Many reviewers and DSLR camera users mistakenly think that a smaller sensor gives some type of magic "multiplier" effect to a lens' focal length. For example, they will say that a Canon DSLR with a 22mm x 15mm sized sensor has a lens "multiplier" effect of 1.6x. Nikon's roughly 24mm x 16mm sensor would have a "multiplier" effect of 1.5x.
They say this means that your 100mm lens will act like a 160mm lens on a 1.6x sensor. Even the camera manufacturers do not discourage this concept. This would mean that a 100mm f/2.8 lens somehow turned into a 160mm f/2.8 lens. Suddenly you would have more aperture for free! Wouldn't it be great if this were true? Unfortunately, it is not. The focal length of a lens causes an image of a certain size to form at the focal plane. This size is fixed by the focal length, not the size of the sensor in the camera. What is really happening is that the sensor is defining how much of that image, and field of view, is recorded. If you take an image formed by a lens of a certain focal length and you use a larger sensor with it, then you have a wider field of view. If you use that same lens with a smaller sensor, you have a smaller field of view. It is like sitting on your couch in your living room and looking out of the front window. A larger window will give you a wider picture. In the case of both the camera and the picture window, the size of the objects outside have not changed, and the size of the object on your retina or on the DSLR sensor has not changed, only the field of view has changed. The true description of the effect of a different sized sensor on the field of view is that of a crop factor. The image is cropped more with a smaller sensor. The crop factor of a 300mm lens with a 1.5x sensor is to give you the effect of a field of view of a 450mm lens. However it does not give you the magnification of a 450mm lens.
|
|||||||||
|
Back | Up | Next |
|||||||||
