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  • OT : measuring focal length of a lens

    Is there a relatively simple way of measuring focal length of a convex lens? I have a box full of lenses collected from different device, which I'd like to identify.

  • #2
    Typically, it's the diameter of the focused image at the "film plane".
    Lynn S.

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    • #3
      Typically it's the distance from the lens of a plane with a focused image of an object at infinity.
      On a practical basis you could measure the distance of a screen from the lens to get a focussed image of clouds or a sunset, you could build something with cardboard tubes & tracing paper that would do the job adequately,
      Regards,
      Nick

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      • #4
        Lens focal length.

        Originally posted by rotate
        Is there a relatively simple way of measuring focal length of a convex lens? I have a box full of lenses collected from different device, which I'd like to identify.
        Hi Rotate.

        I don't know if this relevant, but some while ago I had to see and Ophthalmologist regarding damage to my eye during a "road-rage incident".

        I saw one of his Optometrists prior to seeing the Ophthalmologist so that he would have some solid "back-ground info" on me.

        The Optometrist just put my current (bi-focal and reading) glasses in what looked like a calibrated simple clamp device and so help me she just read my current glasses prescription right off!! I though tis was so simple and "magic".

        I had occasion to see my local Optometrist a little later as follow-up to the referral and he smiled and just said "Thats right - easy".

        So, why not ask your Doctor or perhaps call in and see your local Optometrist. They might be able to do your job or they may not. But at least it might be worth a try.

        There are people who make their own lenses as a hobby - Astronomers and "star-gazers" and the like - and they take it real seriously - so its almost certain they can help you in one way or another.

        I'm sure that others on this thread can "wise me up" on this matter.

        I hope this helps.

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        • #5
          The gadget the optometrist uses doesn't measure the focal length. It produces a diopter reading by comparing the radius of curvature difference on the inside and outside of the lens. This is an approximation that only applies to lenses that are thin compared to the radius of curvature and isn't accurate for lenses with short focal lengths (high curvature).

          A further complication for trying to predict the focal length in Rotate's case is not knowing the lens material. Even assuming the lenses are singlets without knowing the type of glass and the refractive index the focal length cannot be predicted even if the curvature to thickness ratio is known. The only way to characterize the lenses is to test them, keeping in mind that for a singlet lens the focal length is specified for green light. It will be shorter for blue light and longer for red light.
          Free software for calculating bolt circles and similar: Click Here

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          • #6
            I always thought you could put it in sunlight ..parallel rays..
            And see where the focus is by starting a fire !
            Or is that twice the focal length
            Rich

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            • #7
              That will indeed show where the focus is and therefor the focal length. I have an interesting lens that is a bit more difficult to test. It is a very short focal length and fast F ratio lens. The light is reflected back out the front as the back side of the lens is silvered.
              Free software for calculating bolt circles and similar: Click Here

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              • #8
                Yeah, I like the way the way Rich does it. Might as well have a bit of fun while you're at it. I had one lens that I built that was about 20 inches across-that was fun to test. It was made from two pieces of plexiglass heated and bulged with pressure to form the shape, then they were glued together then filled with water. I nearly set my house on fire by accident one day- came home to find the lens focussing sunlight onto the wall. It was burning a little track along the surface as the sun went down.

                Bigger being better, I just had to rig up something else, so it became the 4 ft diameter mirror made using a half sheet of particle board with a built-up edge, and space blanket material stretched over it. Sucking a bit of vacuum pulled the space blanket into a nice curve. I actually had to seal the particle board because it wouldn't hold vacuum for more than a few seconds. That mirror would set a 2x4 on fire the moment a fucus was had. The spot would literally explode into flame.

                Sometimes I get a rough idea of focal length of a lens by putting an led behind it and adjusting the spacing to get a bright spot on the wall that's the same diameter as the lens. This works best if I sand away most of the plastic in front of the led, leaving a flat surface. The led die is usually a pretty small spot source, so it's kind of like using the sun, only backwards. By the way, if you do modify an led to use like this, you should have two or three on hand to start with. The first one I sanded the lens off of, I went a little too close to the die, and it died instantly. There was a wire coming off the top of the die and looping through the plastic to one of the leads- I didn't see it until the deed was done. Newer leds may not be like this, I don't know.
                I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                • #9
                  Darryl,

                  If you like burning ants you are welcome to come visit here. I have plenty of ants to fry, lots and lots of ants. Here is one ant hill down the side of my hill in the back.



                  Come and visit anyway if you are in the area.
                  Free software for calculating bolt circles and similar: Click Here

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                  • #10
                    Echo:-

                    Typically it's the distance from the lens of a plane with a focused image of an object at infinity.
                    On a practical basis you could measure the distance of a screen from the lens to get a focussed image of clouds or a sunset, you could build something with cardboard tubes & tracing paper that would do the job adequately,
                    Regards,
                    Nick

                    Comment


                    • #11
                      I have to go to town right now to pick up supplies but I just did a quick test. Later today, probably tonight, I will show how to make a very simple focal length measuring jig using a laser. It has the advantage of directly reading on a calibrated scale.
                      Free software for calculating bolt circles and similar: Click Here

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                      • #12
                        Ohmygosh Evan! Those ants have eaten some poor soul. There's nothing left but his hat.
                        Lynn (Huntsville, AL)

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                        • #13
                          The thing I don't understand about focal length is where on the lens is the measurement taken? Some lenses have many elements, so from the front to rear surface they can be very thick. The focal length is measured from the focal place to what point on the lens? Wikipedia attempts to answer the question, but it's not easy to understand.

                          http://en.wikipedia.org/wiki/Focal_length

                          Orrin
                          So many projects. So little time.

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                          • #14
                            Ahh, at last to the meaty part. As you've guessed, there are many "focal lengths". From the vertex of the rear element to the focal plane we have the back focal length. From the surface of the mounting flange we have the flange focal length (imaginative names, huh?). For simple lenses measuring from the center of the lens to the infinity focus will give the focal length to within a couple of percent. Unfortunately, there are many times when none of these describe how the lens will behave in reality, such as the multi-element case. What really needs to be known is the effective focal length. Two methods are commonly used for this, one is the nodal slide and the other is a collimator and measuring microscope. The nodal slide is used to locate the nodal points and principle points of a lens, one of the definitions of focal length is the distance from the second (or rear) nodal point to the focus. Google "nodal slide", there are probably detailed explanations there. A collimator is essentially a projector that makes a target appear to be at an infinite distance. If the angular size of the target is known, then measuring the size of the image of the lens in question enables you to calculate the magnification of the lens, and its focal length. Without a collimator you can do a similar job if there is an distant object which you know the angular size of, the sun or moon, for example. Focus the moon, which is a little more than 1/2 degree in diameter, on a piece of paper. Measure the diameter of the image. Divide by the sine of 1/2 degree (0.0087). The result is the focal length in the units you measured the image in. For more accuracy you can look up the moon's (or sun's) actual diameter on the particular day, and use more decimal places in the sine. This method gives the best accuracy to effort ratio, limited by your ability to measure the image.

                            Joe

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                            • #15
                              Proof that people in BC ARE crazy....

                              OK, so there we have it....Some guy in Prince George with bloody Tigers...a recently retired Xerox repairman & computer guru in Williams Lake with his own ant hill collection.
                              Evan, teach the Beardog to eat ants ? Burn them with the tiger torch....
                              My Jack Russells have done in most of the rats around here, the squirrels are still fun. They also eat spiders.....
                              Are those ants or termites ???
                              Sell them on EBay ?
                              Protein....

                              Rick

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