Focal length magnification

Focal length magnification describes the phenomenon, where the use of different sized film- or CCD-formats (or the use of lenses designed to be used with other formats) changes the field of image that is captured and its magnification factor.

This happens in the same way as it would when choosing a longer or shorter focal length.

 

The focal length

Let's address the focal length again.

As explained in detail in the field of view article, the focal length of a lens is responsible for the captured field of view of an image.

As a rule of thumb we can say, that shorter focal lengths lead to a wider field of view being captured (wide angle shots), whereas longer focal lengths (for example when using tele-lenses) capture a smaller part of the scene (but zooms in on this part to fill the whole frame with it).

Digital cameras are especially demanding in this context, since CCD-chips have (for the most part) a much smaller format than the field of image of a 35mm film.

Anyone who wants to have a closer look at focal length and the way it can be calculated, and isn't afraid of a little bit of mathematics, will find out about the calculation of the focal length and the so-called "standard" focal length here.

When speaking of lenses with a standard focal length, we are referring to lenses that approximately match the human eye with its viewing angle of 47° (50 mm lenses for small image format).

image

If the the size of the field of view is changed (and the diagonal of the field of view), the focal length seems to change. But in fact it is not the focal length, but rather the angle of the field of image.

"If, for example, a 35mm lens is mounted on a medium format camera, the focal length does not correspond to the standard focal length anymore, since a larger field of image is captured. (Of course it is not always possible to change the lenses of different camera formats, since the lenses are naturally designed to fit a certain type of camera).

The other way round, a lens with standard focal length for medium format will appear as a telephoto lens when mounted on a 35mm camera."

image

In the “digital world” we have to deal with very different image formats due to the different CCD sizes used.

Generally the chips are smaller than the size of a 35mm film camera's, which leads to an extension of the focal length when using lenses which are composed for film cameras. In order to measure it, the focal length is calculated as if would be on a 35mm camera. This is called 35mm equivalent.

The effect, that a lens with a focal length of 50 mm (SI), mounted to a camera with a smaller frame size has a smaller angle of view, is called focal length magnification.

Using the so-called magnification factor it is possible to calculate the focal length. These factors normally lie between 1.4x and 2x.

If I buy a 50 mm lens and mount it on a camera that is denoted as having a extension factor of 1.4, this yields a focal length of 1.4 x 50 mm = 70 mm.

 

Advantages and disadvantages of Focal length magnification

The phenomenon of focal length magnification for smaller formats has its advantages as well as its disadvantages.

One advantage is that it is possible to build compact telephoto lenses, since a lens that would have needed a focal length of 200 mm for a 35mm camera, only needs a focal length of 100mm when used with a suitable (smaller) chip.

One disadvantage of the focal length extension comes where wide angle lenses are to be build, since the focal lengths cannot be shortened arbitrarily. To, for example, construct a SI equivalent 28mm lens for a camera with an extension factor of 2, one has to use a 14mm lens. In the last years, there have been big advances made in the construction of lenses with short focal lengths, but this development is limited by the fact that flaws in the image like the vignetting (darkening of the edges) are very hard to correct in this area of focal lengths. Another important point that has to be considered when designing a lens is its resolution.

Photoworld Articles

* This content is taken from the konicaminoltaphotoworld.com website that was unfortunately taken offline. I take full responsibility for offering this here on dyxum but I honestly believe that this contests SHOULD be available to all Konica Minolta customers; this is the least they can do !

mladen sever

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