What is Focal length: Definition and 236 Discussions

The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative focal length indicates that the system diverges light. A system with a shorter focal length bends the rays more sharply, bringing them to a focus in a shorter distance or diverging them more quickly. For the special case of a thin lens in air, a positive focal length is the distance over which initially collimated (parallel) rays are brought to a focus, or alternatively a negative focal length indicates how far in front of the lens a point source must be located to form a collimated beam. For more general optical systems, the focal length has no intuitive meaning; it is simply the inverse of the system's optical power.
In most photography and all telescopy, where the subject is essentially infinitely far away, longer focal length (lower optical power) leads to higher magnification and a narrower angle of view; conversely, shorter focal length or higher optical power is associated with lower magnification and a wider angle of view. On the other hand, in applications such as microscopy in which magnification is achieved by bringing the object close to the lens, a shorter focal length (higher optical power) leads to higher magnification because the subject can be brought closer to the center of projection.

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  1. blank44544

    Finding focal length from di v. M graph

    I tried di= f +fM, but that would mean slope and y int is the same when it’s not.
  2. Lotto

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  3. Lotto

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  4. ilovemycat

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    So I have plotted all the given data points on a graph, putting 1/di on the y- axis and 1/do on the x-axis. I have to now find focal length from the graph? I'm not sure how, I'm thinking I have to rearrange the equation 1/f = 1/do + 1/di into y = mx + b form then go from there, but I'm totally...
  5. Lotto

    What is a focal length of a thin convex lens?

    I think that there might be several solutions. I drawed one possible situation: I think that this is just geometry, but I don't know how to solve it simply. I had an idea that if the beam was going through the black axis, then it would be easy to calculate, and that would be aslo solution for...
  6. ab200

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    Putting the questionable nature of my professor's problem-writing aside, I found this question pretty confusing. I assumed that 60 m was the focal length, and that R1 was infinity since it is a flat surface. This gives me: 1/60 = (1.5 - 1)[(1/∞) - (1/R2)] This gave me R2 = 30 m, but that isn't...
  7. D

    How Do Contact Lenses Correct Myopia and Alter Near Point Vision?

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  8. F

    I How to calculate focal length?

    Hello, I am new to this forum and know very little about it. But perhaps somebody can help me. I have an Epson 2040 projector. The beam it projects fits into the screen at a distance of about 4.5 meters. But logistically I need to put it at 8 meters from the screen, so the projected images are...
  9. thementalist123

    What is the Focal Length of a Lens for a Perfectly Fitting Image on a Sensor?

    My attempt: m=-v/u=-f/(-f-u) -1/0.5= -f/-f-0.10 -> -f-0.10=0.5f -> f=0.20 m
  10. Aymangh994

    I Total focal length and magnification

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  11. Aymangh994

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  12. Melbourne Guy

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  13. bluesteels

    How do you know if the focal length is positive or negative?

    In this problem, it doesn't tell you where is the focal length. So how do you know if it is in front or behind the object?
  14. VVS2000

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  15. Istiak

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  16. Horizont22

    I Shorten the lens distance while having same focal length?

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  17. B

    I How to calculate the focal length and focal point of collimator?

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  18. S

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  19. S

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    ##\frac{1}{f}=\frac{1}{u}+\frac{1}{v}## ##f = \frac{u.v}{u+v}## Let: u + v = w → w = 250 mm ± 8 mm Percentage uncertainy of v = (3/50) . 100% = 6% Percentage uncertainty of u = (5/200) . 100%=2.5% Percentage uncertainty of w = (8/250) . 100% = 3.2% Percentage uncertainty of f = 6 % + 2.5 %...
  20. K

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  21. C

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  22. O

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  23. B

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  24. fernelau

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  25. F

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  26. J

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  27. S

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  28. G

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  29. N

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  30. TheSodesa

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  31. Phil123

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  32. T

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  33. A

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  34. N

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  35. P

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  36. W

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  37. N

    How Do Sign Conventions Affect Focal Length Calculations?

    Homework Statement 1) A double convex thin lens has a radius of 25 cm for the first surface and a radius of 40 cm for the second surface. If the glass has a refractive index of 1.55 the focal length is a) 28 cm, b) 9.93 cm, c) 121 cm, or d) 63 cm. 2) An object is placed 16 cm in front of a...
  38. Q

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  39. M

    I Determining Appropriate Focal Length

    So I'm working on a project where we're deciding a new lens to use for our laser diode. I need to determine the appropriate focal length based on the equation: 1/ƒ = 1/μ + 1/σ where ƒ is our focal length, μ is the distance from the lens to image and σ is distance from lens to object. For the...
  40. P

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    Does the orientation of a plan convex lens affect it's focal lenght? If I put il like in the first example in the photo and makeep the calculation using the lens maker equation I get f positive What happens if I turn the curved face on the right? Using the lens maker equation now the radius...
  41. N

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  42. R

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  43. R

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  44. T

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  45. G

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  46. J

    B Variable focal length TV / screen

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  47. K

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  48. RabbitWho

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  49. M

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  50. F

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