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Science Forum Index » Optics Forum » focal length larger than expected. why?
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Message |
| Guest |
 Posted: Sun Jan 06, 2008 2:23 pm |
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Hey Guys,
I have bougth a cylindrical lens. Its focal length is supposed to be
4mm with a 1% tolerance.
HOWEVER:
I observe the focal length to be about 4.5mm (The procedure described
below). Anyone has had the same problem or know the reason?
Thanks,
Pooria
Procedure: I mounted a microscope objective on a traverse. A simple
laser beam was directly passing through the lens. Hence, the focal
point should be where the beam has the minimum waist size. Once I
found that point with the objective, I moved it towards the lens. One
expects the objective surface to touch the lens surface, when it has
been moved 4mm. However, when the objective wah moved 4mm from the
focal point towards the lens, there was still a distance between the
lens and objective. |
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| Dave Martindale |
| Posted: Sun Jan 06, 2008 10:18 pm |
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Guest
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pooria_sattari@yahoo.ca writes:
Quote: Procedure: I mounted a microscope objective on a traverse. A simple
laser beam was directly passing through the lens. Hence, the focal
point should be where the beam has the minimum waist size. Once I
found that point with the objective, I moved it towards the lens. One
expects the objective surface to touch the lens surface, when it has
been moved 4mm. However, when the objective wah moved 4mm from the
focal point towards the lens, there was still a distance between the
lens and objective.
It seems you are mixing up several different measurements.
Quote: I have bougth a cylindrical lens. Its focal length is supposed to be
4mm with a 1% tolerance.
That number will be its effective focal length. The effective focal
length of your lens determines the size of an in-focus image. It's
also the distance from one of the "principal planes" of the lens to the
image focal plane when the subject is at infinity.
But it is *not* the distance from the rear surface of the lens to the
focal plane, since the rear principal plane is almost never located at
the rear surface. The distance from the rear surface of the real
element to the focal plane (for a subject at infinity) is the back focal
length, which is generally *not* the effective focal length.
The BFL is often, but not always, smaller than the EFL.
Also, suppose that the BFL was actually 4 mm, and suppose that your
experimental setup had accurately determined the position of the laser
beam waist. Then when you move the microscope objective forward exactly
4 mm, you'd expect the dust on the rear surface of the lens to come into
focus. You would *not* expect the objective to touch the rear surface
of your lens.
This is because the distance between your lens's rear surface and the
objective's front surface when the beam waist is narrowest is the sum of
the back focal length of your lens *and* the front focal distance of the
microscope objective. The latter is always at least equal to the
thickness of a slide cover glass, and much more for low-power
objectives.
I think you should (a) figure out for yourself the difference between
EFL, BFL, and FFL, and (b) find a better way to measure EFL, if that's
the parameter you care about.
Dave |
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| Salmon Egg |
| Posted: Mon Jan 07, 2008 3:00 am |
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Guest
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On 1/6/08 4:23 PM, in article
f5d005ca-daba-438b-839c-df02bdcd8f31@m77g2000hsc.googlegroups.com,
"pooria_sattari@yahoo.ca" <pooria_sattari@yahoo.ca> wrote:
Quote: Hey Guys,
I have bougth a cylindrical lens. Its focal length is supposed to be
4mm with a 1% tolerance.
HOWEVER:
I observe the focal length to be about 4.5mm (The procedure described
below). Anyone has had the same problem or know the reason?
Thanks,
Pooria
Procedure: I mounted a microscope objective on a traverse. A simple
laser beam was directly passing through the lens. Hence, the focal
point should be where the beam has the minimum waist size. Once I
found that point with the objective, I moved it towards the lens. One
expects the objective surface to touch the lens surface, when it has
been moved 4mm. However, when the objective wah moved 4mm from the
focal point towards the lens, there was still a distance between the
lens and objective.
To things off the top of my head. BTW, I find it difficult to understand
your words without a diagram. Realize you know the details of what you are
talking about, but I do not..
You must be sure about where the reference (principal) plane from which
focal length is measured.
I think that your assumption that minimum beam size or waist marks the focal
point may be in error. You really should check that out using the ABCD ray
trace matrices.
Bill |
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| Eugene A. Pallat |
| Posted: Sun Jan 27, 2008 6:19 pm |
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Guest
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Be aware that the focus of the lens is from principal plane 2 (pp2) to the
focusing point. Also in some lens designs, pp2 can be in front of pp1.
Gene Pallat
Orion Data Systems
Orion Forensics
<pooria_sattari@yahoo.ca> wrote in message
news:f5d005ca-daba-438b-839c-df02bdcd8f31@m77g2000hsc.googlegroups.com...
Quote: Hey Guys,
I have bougth a cylindrical lens. Its focal length is supposed to be
4mm with a 1% tolerance.
HOWEVER:
I observe the focal length to be about 4.5mm (The procedure described
below). Anyone has had the same problem or know the reason?
Thanks,
Pooria
Procedure: I mounted a microscope objective on a traverse. A simple
laser beam was directly passing through the lens. Hence, the focal
point should be where the beam has the minimum waist size. Once I
found that point with the objective, I moved it towards the lens. One
expects the objective surface to touch the lens surface, when it has
been moved 4mm. However, when the objective wah moved 4mm from the
focal point towards the lens, there was still a distance between the
lens and objective.
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