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02-03-2010, 10:27 AM   #1
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How is lens magnification calculated?

I know pretty much every 50mm lens has a magnification ratio of 0.15:1 or 1:6.6666...
But I don't get how you get these numbers. I've tried everything logical to me: 50mm:45cm, 5cm:45cm, 45cm:5cm and so on. Nothing comes even close.

So how is this calculated?

02-03-2010, 01:04 PM   #2
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I recently read that the magnification ratio is the actual size of an object to the actual size projected on the camera's sensor. A coin 1cm in diameter shot with a lens having 1:4 magnification would project a 1/4 cm diameter object on the actual sensor or frame of film.
02-03-2010, 01:46 PM   #3
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Hmmm thanks, but that isn't my question. Determining magnification by simply measuring it is easy. However this is not what I meant.

My question is if you have a lens' focal length and minimum focus distance how do you get its reproduction ratio?

I chose the normal 50mm lens because I know its magnification - but still I can't reconstruct how the values are obtained.
02-03-2010, 02:04 PM   #4
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Ok, I think I see where you are going now. You want the equation that relates focal length and min focus distance to magnification correct? I will do a quick Wikipedia search as its not in my Optics textbook.

02-03-2010, 02:08 PM   #5
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QuoteOriginally posted by Egg Salad Quote
My question is if you have a lens' focal length and minimum focus distance how do you get its reproduction ratio?
Here is a calculator and below it the theory and equations I think you are looking for.
Lens Magnification and Depth of Field Calculator

By coincidence its on a server maybe not far from you.
02-03-2010, 02:38 PM   #6
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i just thought it was the focal length divided by 50mm
02-03-2010, 04:04 PM   #7
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I quote from the Link to the magnification calculator, that imtheguy provided:

"However, it still behaves similar to a single thin lens, which collects incoming parallel light rays (from the sun) into one single focal point. Its behaviour can thus be approximated by the well-known lens equation
1 / f = 1 / g + 1 / h (1)

where f is the focal length of the lens, g is the object distance (between object and lens), and h is the image distance (between lens and sensor). "

Obviously this is a bit theoretical, as it uses the simple "thin lens" equation. Modern lenses, especially those with internal focusing (IF) should, I think, not be looked at, as being thin lenses. Not the least, because the focal length changes with focusing distance!

Dimitrov actually illustrates that with Pentax lenses: Focal Length Experiments

The explanations following the above mentioned calculator are, by the way, quite good.

Ben
02-03-2010, 05:08 PM   #8
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Yes, now this is what I'm talking about.

No, really. That's what I was after.
Yes, I know that calculation only refers to one lens-lenses (isn't there a differentation between one lens and the whole thing - the lens?) but this is also what manufacturers use so it's good enough for me.

Now I could just use the formula but since I'm quite lazy and the internet supports my lazyness I'll just use this calculator.

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