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shreyas srivatsa
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# Posted: 16 Apr 08 14:55
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I know that focus is adjusting the distance between the lens and the object in order to get a sharp and clear image on the focal plane, bring in the right amount of light as well to capture the oject in natural conditions.
But i am getting confused with the values given with the lens specs. For example,
"Canon EF 70-300mm lens" 70-300 mm refers to the range of focus you can set using the camera, practically what does 300mm mean! Is it the distance between the lens and object?
Need some clarity on this, guess there is problem at the basic level. Need some help!
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Vidar S
Member
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# Posted: 16 Apr 08 15:22 - Edited by: Vidar S
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I'll try to explain, but experts may feel free to correct me! 
Try looking through a cylinder with a diameter of 50 mm. You now have a 1:1 magnification. Convert the idea to a lense on your camera. If you use a 100 mm lense, you have magnified your view 2x, or you could also say that you have shortened your distance to the object with half the distance your standing because of the 2x magnification. So 300 mm is 6x magnification,.
Because of the size of the sensor on DSLR you will usually have to mulitply the lense measurements with 1.5
My standard lense is a Nikon 18-70 mm, but when multiplayed with 1.5 it's actually 27-105 mm
Hope my english was useful!
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shreyas srivatsa
Member
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# Posted: 16 Apr 08 15:42 - Edited by: shreyas srivatsa
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Thank for the reply vidar.This one leads to the next question.
Does it mean the values refer to zoom?
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Vidar S
Member
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# Posted: 16 Apr 08 15:46
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You're welcome, and yes it does.
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shreyas srivatsa
Member
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# Posted: 16 Apr 08 16:23 - Edited by: shreyas srivatsa
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 oops!!!
That was a big misunderstanding. 
Thank you very much.
Do you know any website or any manual what i can refer to?
I would like to get my basic concepts right.
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Vidar S
Member
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# Posted: 16 Apr 08 16:25
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http://en.wikipedia.org/wiki/Zoom_lens
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CatNut
Member
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# Posted: 20 May 08 01:30
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The value in MM (70 to 300mm) is the distance from the Film (or sensor) to the first bit of glass in the lens system, that is closest to the film.
In a 35mm film camera 50mm is the same "Power" (1 to 1) as your eye.
Less than 50mm gives less magnification and a wider field of view. Hence "wide angle".
Greater than 50mm gives more magnification and a narrower angle of view.
Most Digital cameras have a sensor smaller than the old 35mm film, so the measurements of focal length are correspondingly less.
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