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Author Topic: lens ratio... what is it?  (Read 3781 times)

Lorne Hastings

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lens ratio... what is it?
« on: December 29, 2005, 03:01:37 pm »

I know there is an equation to figure out the throw distance of a lens. Could someone please post t. For example a 1.3 what do these numbers translate to?
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Mike Noth

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Re: lens ratio... what is it?
« Reply #1 on: December 29, 2005, 03:59:46 pm »

Lorne,

Are you referring to projector or camera lensing?

If projection, the formula is (Image Width x Focal Length)/Aperture.

Most Projectors that I have run across have an aperture of .8 - 1.2, so I have found that about 95% of the time, I can simplify the formula to Image Width x Focal Length.

Showing my work:

I've got a 9 x 12 screen and a projector with a 1.2 to 1.8 Zoom Lens.  My projection distance would then be 14.4' to 21.6' (12 x 1.2 & 12 x 1.8 )

Check the manuals of your projectors to confirm that the aperture won't make your margin of error way out of whack, or if the situation requires extra rigor.

If you are referring to camera lenses, I'm not sure of the formula, I order based on experience.  I have just learned over the years when I will need 33x vs. 55x.

Hope that helps.

Mike Noth

(Edited typo)
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Mac Kerr

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Re: lens ratio... what is it?
« Reply #2 on: December 29, 2005, 04:01:15 pm »

Lorne Hastings wrote on Thu, 29 December 2005 15:01

I know there is an equation to figure out the throw distance of a lens. Could someone please post t. For example a 1.3 what do these numbers translate to?
It is the ratio of the distance to the screen, and the image width. With a 1.3/1 lens you need a 13' throw from the screen to the lens to get a 10' wide image. In almost all cases the throw will be longer than the width is wide. There are 0.8 ratio lenses, but they are rare.

Mac
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