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Thread: Limitations of concave-mirror projection?

  1. #1 Limitations of concave-mirror projection? 
    Forum Ph.D. Leszek Luchowski's Avatar
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    Hello;

    I am reading this book on art history and it mentions 16th-century artists who used concave mirrors to project an image of their object (such as a basket of fruit) onto canvas, so they could make a sketch and get the proportions right.

    The book says that an image obtained in this way cannot be larger than 30cm in diameter, regardless of the size of the mirror. It gives no reason for this, just states it as a fact and proceeds to discuss how artists stuck many such small images together to make a larger painting. Later on in history, this 30-cm limit was removed by the use of lenses instead of mirrors.

    I am puzzled. Why 30cm? (that's approx. 1 foot if you aren't metric). Why any limit at all? I should think a properly shaped mirror would project images just like a lens does - except of course it reverses the direction of the light so the image is on the same side of the mirror as the object. And a small lens can project a big picture, as anybody knows who has been to a cinema.

    Any clues anyone?


    Leszek. Pronounced [LEH-sheck]. The wondering Slav.
    History teaches us that we don't learn from history.
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  3. #2 Re: Limitations of concave-mirror projection? 
    . DrRocket's Avatar
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    Quote Originally Posted by Leszek Luchowski
    Hello;

    I am reading this book on art history and it mentions 16th-century artists who used concave mirrors to project an image of their object (such as a basket of fruit) onto canvas, so they could make a sketch and get the proportions right.

    The book says that an image obtained in this way cannot be larger than 30cm in diameter, regardless of the size of the mirror. It gives no reason for this, just states it as a fact and proceeds to discuss how artists stuck many such small images together to make a larger painting. Later on in history, this 30-cm limit was removed by the use of lenses instead of mirrors.

    I am puzzled. Why 30cm? (that's approx. 1 foot if you aren't metric). Why any limit at all? I should think a properly shaped mirror would project images just like a lens does - except of course it reverses the direction of the light so the image is on the same side of the mirror as the object. And a small lens can project a big picture, as anybody knows who has been to a cinema.

    Any clues anyone?
    Your analysis seems to be correct.

    The only reason for a limitatin that leaps to mind is that as the image becomes larger, it also becomes fainter. That might [put a practical limit on the size of the image that could be projected onto a canvas with enough intensity to allow to be traced.


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