# A&P Can someone explain attachable lenses to me?

Discussion in 'Lifestyle' started by Metaphysical, Mar 3, 2005.

1. ### MetaphysicalWith a hundred ways to do a dozen things, why not

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i know that the mm number is the range of the dof right??? what do the other numbers mean, and what else should i consider when buying a new glass?

2. ### raptor_talonWho is the master? Sho'nuff!

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wrong. the figures in mm are the focal lengths of the lense

http://www.photoxels.com/tutorial_focal_length.html
http://www.geometer.org/beginner/lenses.html

Imagine cutting a rectangle out of a card, holding the card in front of you, and looking at the world through the hole. Imagine that your photo will be what you see through the hole. If the card is held close to your eye, you'll see a great deal, and this is what happens with lenses with short focal lengths, which are also called "wide-angle" lenses.

As you move the card away from your eye, this is equivalent to looking through lenses with longer and longer focal lengths, and when the card is quite far from your eye, you get a view similar to what you'll see through a telephoto lens.

3. ### CRCNew Member

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Wow, that's an awesome analogy. Noob question, but what does the mm actually measure?

4. ### bosox*

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" The focal length of a lens is defined as the distance in mm from the optical center of the lens to the focal point, which is located on the sensor or film if the image is "in focus"."

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