# Determining needed change in lens

Discussion in 'Exposure Discussion' started by loowens, Sep 14, 2009.

1. ### loowensMember

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All, I am new to the forum and was reading a few posts about a problem I am trying to solve. Everyone seemed helpful and well mannered which is rare for a forum so I decided to join to ask my question.

I have a captured image taken with a 12.5mm lens on a 1/2" format CCD camera. Given that image is there a mathematical method to determine the lens that would be needed to see a specific portion of that image?

Basically I have an image of a roadway, but I want to know what lens at the same position would be needed to only see a specific part of the roadway in the original image.

Shane

2. ### Bruce WatsonMember

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The concept you are looking for is called angle of view.

3. ### ic-racerMember

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If you mean the diagonal of the sensor is 1/2" then that is about 12.5mm (12.7 to be exact).
So any film/lens combo where the focal length is the same as the film diagonal will duplicate that angular view for you.

So, 43mm lens on 35mm camera, 105mm on 6x9cm camera 150mm on 4x5" camera etc....

4. ### Q.G.Inactive

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The ratio of the size of the portion of the image to the size of the complete image is the same as the ratio of the focal lengths needed to produce both, but inversed.

If the portion is, say, 1/3 of the total image's size, the lens needed to record it on the entire frame is 3x as long as the one used to take the image the portion is 1/3 of.