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01-31-2009, 02:58 PM   #1
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Lens Question

I just bought the K2000, first camera ever for me, and so far I like it. I'm already looking into getting a 2-3x telephoto lens, and maybe a 200mm lens. I'm looking to take pictures of objects far away, is 200mm good enough? Something like 500-1000 ft? Is there some formula to configure how much of a lens you'll need based on distance?

01-31-2009, 03:10 PM   #2
Ole's Avatar

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Yes there is.

If D is the distance to the object, O is the largest dimension of the object, S is the sensor width in mm (24mm for an APS-C sensor), then to fill the frame you need a focal length F (in mm) of:

F = S * D / O = 24 * D / O

D and O must be expressed in the same unit (e.g. ft).

This formula is an approximation and is good only for larger distances like you asked for.

Last edited by Ole; 01-31-2009 at 03:20 PM.
02-01-2009, 03:47 PM   #3
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Super Ole,
thanks for this simple formula. Maybe I'll remember :-)
I was trying to snap people with slight OOF background with a 3.5/135mm lens, and there the longest I can get with OOF blur (to my liking) is about 35ft.
To capture a standing person with some leeway I'd say I need a 7ft. stretch. To ease things up I'll round the sensor from 24 to 25.

25*35/7 = 125mm

I'd rather go for a faster aperture or shorter focal length or shorter persons :-)
Thanks for sharing,
Georg (the other)

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