constant aperture lens

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what does it mean by constant aperture lens? is it very useful? Why do some flashes have built-in af assisting light but some do not? thank you

-- tokamak (sir-tokamak@hongkong.com), May 27, 2001

Answers

Constant aperture lens is with zoom lenses. They remain a contant aperture through their focal length, i.e. a 28-70 f/2.8. A variable aperture lens does not, i.e. 35-80 f/4.5-5.6. The contant aperture lenses cost more as they are generally faster lenses to beign with, and the variable aperture lenses are generally used on consumer and some mid-level zooms. Some good contant aperture zooms are the L series glass that Canon makes; good amature variable aperture lense are the 28-105 and the 35-135 (I thin that is the range). Obvious prices is a clue to the difference, but really a constant aperture lens is easy to use wide open becuase you know that you will never change aperture when youzoom in and out. Just one less thing to think about.

-- Chris Gillis (chris@photogenica.net), May 27, 2001.

With regard to your question about why some flashes have AF assist lights while some other's don't have them, this is often simply a matter of design, and in general, more advanced flashes tend to cost more because they have more useful features, such as more power; tilt/swivel heads; more sophisticated controls; an AF assist light; ability to accept an auxiliary power pack; diffuser panels; more rugged construction, etc.

-- kurt heintzelman (heintzelman.1@osu.edu), May 27, 2001.

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