> ----------
> From: "Mauricio Bonifacino" <[EMAIL PROTECTED]>
> Date: Thu, 9 Aug 2001 15:49:40 -0300
> To: <[EMAIL PROTECTED]>
> Subject: focal lenght
>
> Dear Friend:
>
> I work with plants and i am needing to build a ruller that allows me to
> determine the size of small flowers. I need a ruller that measures 1/10 mm.
> I decided doing a reticule model at 1:20 and taking a picture and using the
> design printed on a slide as a ruller. I need to know the relation between
> the film position, the focal lenght of the lens and the object distance. I
> will be working with a 80 mm lense and I will place the object at 1,6 m from
> the camera in order to have a 1:20 scale.
In principle something like this is possible, but you would have a hard
time getting it accurate, as all measurements of distances would have to
be relative to the primary and secondary principle points of your lens.
And there positions are generally not known. But still, you can find all
the relevant formulas in
<URL:http://www.photo.net/photo/optics/lensTutorial.html>. One
possibility that might be more reproducable is to use the closest focusing
setting your lens allows and take a picture of a ruler. Afterwards, you
photograph all flowers at exactly this focusing distance and compare the
image size of the ruler with the image size of the flower. No need for
math containing unknown variables ...
cheerio
ralf
--
Ralf Stubner <mailto:[EMAIL PROTECTED]>
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