Filmwasters
Which Board? => Articles => : Francois March 06, 2011, 10:26:38 PM
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After seeing Ed's glorious lens hacking job, I thought I could bring a tad bit of science to the equation. Though I'll admit it's way less fun than just winging it but I think knowing what's at work under all this can be interesting. For this I decided to concentrate mostly on magnifiers mainly because they’re the most likely object to be scavenged as a camera lens.
Since science is about information, let's start with what we can find out. On most loupes, you have the magnification indicated. While the magnification level can vary a bit depending on the viewer’s eyes, it tends to be quite exact for our purpose. Magnification level can be converted to the focal length in inches by using a simple formula.
10 / magnification = focal length in inches.
But it's easier to simply refer to the following chart…
5x = 51mm
6x = 42mm
8x = 32mm
10x = 25mm
15x = 16mm
So, we now have a bit of information we can use.
Now since we're exposing a photo sensitive material, we're still missing a bit of primordial information: the f/stop.
While we can guess this information, it is wise to know that the f/stop value changes when the optics have more than one element. But since we're talking about magnifiers here, we're lucky to have a simple formula. Now, the f/stop depends on the physical size of the lens and its focal length. That's why that sexy Leitz Noctilux is so big.
The f/stop value is calculated by the following formula:
f/stop = focal length / lens diameter.
So, you need to get out your ruler now.
As an example, I have an 8x magnifier. Its lens is 24mm across.
So, we have a 32mm lens / 24mm diameter = f/1.3
Not too shabby!
Now, it’s your turn to hack a lens!
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Cool.
See also this thread on flickr: http://www.flickr.com/groups/homemadelens/discuss/72157622612617105/ (http://www.flickr.com/groups/homemadelens/discuss/72157622612617105/). johnnyoptic is definitely one of my flickr heroes.
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Thanks, Francois. Next step is for me to measure my current setup and see if my estimation of f2,8 is correct. This information has been a huge help though and you've taken it exactly up to the level after which I lose interest in the theory (read, "hard facts" ;)) and just dive in and make loads of mistakes.
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And have a lot more fun...
I take most of my optical information from a book titled "Popular Optics" by the Edmund Scientific presses. Old but written in simple easy to understand language.
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ohyeah, I neer wrote anything here.. THANKS francois! haha, and you too DS :D this is a great start for digging into lens hacking~
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I believe that if you focus the image of the sun on a piece of cardboard and measure
the distance between the lens and the cardboard, then divide that into 10, that will give you the magnification. Also if you know the diopter of the lens, then divide by 4 to get the magnification.
DaveO
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Actually,
1000/diopter= focal length in mm.
As for magnification, it varies with the optical system used...
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It's probably 250/diopter= fl in mm
DaveO