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Old Sep 29, 2004, 8:51 PM   #11
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In the most basic terms the focal length of a lens is the distance that from the glass lens to the point where parallel rays of light will converge to a single point. If you take a simple magnifying glass, and focus the sun's rays to a point on the ground, then the distance between the glass and the ground is the focal length.

Camera lenses are more complicated because the lens is made up of a number of glass lens elements. The number of elements are used to correct the different paths of light through a lens(for example for different colors, for the lens surfaces being spherical, etc)

These lens elements are also used to increase or decrease the lens length for a given focal length. For example, a 200mm lens SHOULD be 200mm long, but they are generally not. Lens designers use a lens shortening design called "telephoto" to deduce the length and size of the lens, to make them easier to handle while still having an OPTICAL FOCAL LENGTH of 200mm. Another example is the wide angle lens. If an SLR camera has a 24mm lens, the lens needs to be too close to the film or sensor plane for the mirror to operate. In this case the lens needs to be "lengthened", using a design call "retrofocal". This lengthens the distance from the back element to the film plane, while retaining an OPTICAL FOCAL LENGTH of 24mm.

Since 35mm film was the standard format for many years, lens focal lengths are easier to relate to if they are quoted as focal length for 35mm cameras. A "standard normal" lens is 50mm for 35mm film, giving a "normal" field of view of about 40 degrees.35mm frames are all a particular relatively large size. With the introduction of digital cameras the "frame" size(CCD) can be and is smaller. A "standard normal" sized lens for a certain size CCD can be 6mm (smaller CCD, the lens can be closer to the CCD, but still retain a 40 degree field of view. If the CCD is larger, then a "standard" focal length for a 40 degree field of view will larger

Because a focal length of 6mm doesn't tell us anything if you don't know the relative CCD size, it's often best to quote in terms of "equivalent 35mm focal length". The "so many times zoom" is useful to give a brief idea of the zoom's range.... e.g. 3x optical zoom, 6x, 10x, etc. However, to give a reasonable indication of the actual magnification it's best to quote the "equivalent 35mm focal length".

e.g. the Olympus C-770 has a 10x optical zoom, while the Panasonic LZ-10 has a 12x zoom. But that's not the whole story. The Panasonic seemingly has an extra 2x up it's sleeve for telephoto, but it's not actually the case!!!!

Olympus C-770 has a 10x optical zoom: 38mm-380mm

Panasonic LZ-10 has a 12x optical zoom: 35mm-420mm

The Panasonic is more like an 11x at the top end compared to the Olympus, but it has a wider wide angle at the bottom end.

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Old Sep 29, 2004, 9:29 PM   #12
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Thanks, Dnas;

It all makes sense, though as I have said earlier, it's prpbably not necessary to understand it all...chapter and verse... for my kind of digicam. Still, it's good to know. One thing I would like to know, though, is what effect my add-on lenses have on my exposure. (though again, my camera's TTL metering makes this a moot point)

Thanks again.


Tom, on Point Pelee, Canada
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Old Sep 29, 2004, 10:09 PM   #13
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In general, if you have a converter lens on the front, and it's the same size (same width of glass), but 2x magnification, you lose 2 F stops. e.g. you only get a quarter of the light..... if 1F stop is half, and another F stop is half again: 1/2 x 1/2 = 1/4

If the glass is the same size, but it's 1.7x, then it's (1/1.7) x (1/1.7) = 0.34 or about 1/3 the light.

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