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Science Forum Index » Optics Forum » Focal length
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| Maurizio |
 Posted: Sat Feb 23, 2008 8:58 pm |
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Guest
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Hi,
I've a question about Focal length for a Thin lens.
For all kinds of thin lens I can found f (Focal length) using equation
found here:
http://en.wikipedia.org/wiki/Focal_length
In details:
* Front focal length (FFL) or Front focal distance (FFD) is the
distance from the front focal point of the system to the vertex of the
first optical surface.
* Back focal length (BFL) or Back focal distance (BFD) is the distance
from the vertex of the last optical surface of the system to the rear
focal point.
I'd like to consider the distanse f from the center of the lens to the
principal foci ( front focal point and rear focal point)
For a thin lens I can consider the tickness = 0 so in module |FFD| = |
BFD| = |f|
Is it correct?
For simmetic lens ( biconvex and biconcave ) the FFL and BFL is the
same so |FFL| = |BFL| = |f|
For a plane-convex or plane-concave lens |FFL| is still equal to |
BFL|?
For every kinds of lens ( biconvex,biconcave,plane-convex,plane-
concave etc. ) the distance |f| from the center of the lens to the
principal foci is equal?
I'd like to thank you for your help!!
Have a nice day!
Maurizio |
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| Guest |
| Posted: Mon Apr 07, 2008 3:38 pm |
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In a single thin lens the there is no difference between the Front or
Back Focal Length.
You typically only talk about the FFL being different from the BFL
when you have multiple elements. When you would have to take into
account the distance from the front focal plane to the front principal
plane as your FFL and the the back focal plane to the rear principal
plane as your BFL. Don't confused this with Front Focal Distance (FFD)
which is measured from the focal point to the vertex. Of course
different professors may have different notation.
Confusing isn't it? |
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| Guest |
| Posted: Mon Apr 07, 2008 4:13 pm |
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Never mind I was wrong. Typically the FFL will not equal BFL when the
object and image side are in different indice of refractions. Sorry
for the confusion. |
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