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FORUMS Canon Cameras, Lenses & Accessories Canon EF and EF-S Lenses 
Thread started 10 Aug 2009 (Monday) 00:43
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Focal length (XX mm) question

 
Guts311
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Aug 10, 2009 00:43 |  #1

Hey all, quick question about focal lengths.
I've researched this and while I've found many technical details of focal length and angle of view and all that, I haven't found a definitive definition of what the millimeters in the focal length is actually a measurement of.

-Is it the exact mm measurement between a certain part of the lens elements and the focal plane (sensor/film)? Is it the measurement of any obvious thing viewable by your own eyes relating to the camera/lens/subject??
Does it have to do with converging/diverging of light? It must be a measurement of something obvious and simple right?

Obviously, these focal lengths in mm are very very short distances (e.g. 50mm equals just about only 2 inches).

So what is it?!


EDIT: I got this from Wiki, but I still don't understand exactly what the measurement is:
"A system with a shorter focal length has greater optical power than one with a long focal length; that is, it bends the pencil of rays more strongly, bringing them to a focus in a shorter distance."




  
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SkipD
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Aug 10, 2009 01:14 |  #2

The focal length of a simple lens such as a one-piece-of-glass magnifying glass is the distance between the center of that lens and where the image is created behind it. In a more complex lens, the definition gets a lot more technical (and I'm not really qualified to answer that question without researching it).


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Daniel ­ Browning
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Aug 10, 2009 02:56 |  #3

Guts311 wrote in post #8430778 (external link)
-Is it the exact mm measurement between a certain part of the lens elements and the focal plane (sensor/film)?

That part is easy: the focal length of a lens is the distance from the rear nodal point to the sensor. The hard part is finding out where the rear nodal point is. ;)


Daniel

  
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macroimage
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Aug 10, 2009 05:20 |  #4

Daniel Browning wrote in post #8431291 (external link)
That part is easy: the focal length of a lens is the distance from the rear nodal point to the sensor. The hard part is finding out where the rear nodal point is. ;)

This distance only equals the focal length when the lens is focused at infinity.


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Guts311
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Aug 10, 2009 08:46 |  #5

Right, I read that that's only when it's focused to infinity too.

So no one has an easy answer for what the mm focal length is actually a measurement of?? C'mon pros, help me :p




  
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Aug 10, 2009 09:01 |  #6

macroimage wrote in post #8431599 (external link)
This distance only equals the focal length when the lens is focused at infinity.

For a simple lens (single element) the FL is the distance from lens optic to its focal plane. For a 'normal' design lens with complex optics (multiple elements), it is like the simple lens.

For a 'retrofocus' lens (typical WA for SLR/dSLR) the distance from lens to focal plane is far longer than the actual FL, as the rear node is placed behind the rear optic, leading to a longer distance to focal plane. This is to permit the reflex mirror sufficient space to swing upward without striking the lens.

For a 'telephoto' lens (typical long FL for SLR/dSLR) the distance from the lens to focal plane is shorter than the actual FL, as the rear node is forward of rear optic, leading to a shorter distance to focal plane. This is to permit a more compact lens than if it were a 'long focus' design.


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nureality
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Aug 10, 2009 09:14 |  #7

Way back in the early days of lenses (circa 1920) the FL would have been the focal length from the film plane to the front element - because there wasn't much else involved except for a single element. In order for a lens to be longer, it needed a longer barrel. This is because such things as higher magnification glass and corrective glass and multi-element lenses had not yet been invented.

Once complex lens systems came out, they needed to correlate focal lengths by their equivalent in the one-element system. i.e. what FOV does this lens look like had the photo been taken with a one-element optic?

Its basically carried over ever since.

If you look back to the 50's, 60's and 70's - the early days of the SLR, you notice the Tele's and Super Tele's were big long lenses, 1000mm (that wasn't a mirror lens) was give-or-take 1000mm long. These are very simple optics... there isn't much in the way of internal magnification, its 1000mm of power because its 1000mm of length. By comparison you look at 400mm lenses today and they are incredibly compact (even more than they would have to be to compare to a 400mm of those times) - yet much sharper and of much higher quality... technology marches on.

In short, today's XXmm calculation is always an approximation, AT INFINITY, of what that XXmm would look like when compared to a lens of yesteryear of the same XXmm.


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Focal length (XX mm) question
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