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Thread: Question about vertex/effective power

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    Question about vertex/effective power

    I understand that as vertex distance increases, the effective power of the lens system becomes more plus. As vertex distance decreases, the effective power becomes more minus. But- I am a myope. If I increase vertex distance with a plus lens, the image indeed appears larger. HOWEVER, when I increase vertex distance with a minus lens, the image appears smaller. What gives?

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    What is the opposite of "magnify"? ______(8 letters)
    Eyes wide open

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    Do you really think I would have posed this question at all if I didn't know that a minus lens minifies?

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    What's up? drk's Avatar
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    I think he's pointing out how as minus lens becomes weaker, more minification occurs, which is counter-intuitive ASSUMING THAT MAGNIFICATION IS DIRECTLY RELATED TO POWER!

    So, it's obviously not. It seems to be related to vertex distance, too.

    This is how formulas are made!

    Obviously, the power of the lens matters. Plus lens magnify, minus lens minify. So the power is in the equation.

    But you notice that vertex is directly related to magnification and minification. It's usually termed "h".

    So, it would look like this, roughly:

    Spectacle magnification = h x F

    Now, what is it, exactly? http://www.opticiansfriend.com/artic...ations.html#SM
    Spectacle Magnification Formula:
    Magnification (M) = (Shape Factor) * (Power Factor)
    Shape Factor = 1 / (1 - ((c * D1) / n))
    Power Factor = 1 / (1 - z Dv)
    D1 = front surface power (base curve) in diopters
    c = center thickness in meters
    n = index of refraction of lens
    z = vertex distance in meters
    Dv = back vertex power
    • Flatter base curves decrease magnification
    • Steeper base curves increase magnification
    • Thinner lenses decrease magnification
    • Thicker lenses increase magnification
    • Higher vertex distances create more magnification
    • Shorter vertex distances create less magnification
    • Higher index yield less magnification
    • Lower index yield more magnification


    That's too exact for my liking!

    Magnification (M) = (Shape Factor) * (Power Factor)
    Shape Factor = 1 / (1 - ((c * D1) / n))
    Power Factor = 1 / (1 - z Dv)

    Let's forget about the shape of the lens. For eggheads only.

    Magnification (M) ~= (Power Factor)

    OR

    Magnification (M) ~= 1 / (1 - z Dv)

    So, if you put a higher number in the denominator, you get a higher magnification/minification effect. Higher vertex (z), higher effect. Higher power Dv, higher effect. Plus or minus lens determines whether a mag or min effect.

    YOU ARE SIR ISSAC NEWTON IN TRAINING!

    (I will correct the graphic above re: vertex. It doesn't "increase or decrease magnification" but image size change! It increases magnification AND minification!)
    Last edited by drk; 08-14-2014 at 01:32 PM.

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    Compulsive Truthteller OptiBoard Gold Supporter Uncle Fester's Avatar
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    I'll give it a shot but we're going back to school days before the wheel was round!

    A plus lens creates a real focus behind the back side of a lens. The greater the power the larger the image size.

    A minus lens creates an imaginary focus in front of the lens so as plus power is added the object would move further away thus smaller.

    At least that's my stab at it and someones welcome to correct me if I'm wrong.



    edit-Well I guess drk gives the better answer but mine is how I "imagine" it.
    Last edited by Uncle Fester; 08-14-2014 at 01:48 PM.

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    Quote Originally Posted by cowboytelemark View Post
    Do you really think I would have posed this question at all if I didn't know that a minus lens minifies?

    Actually, YES.....your profile/posts have you saying you are in training, and the mentor/teacher in me says it never hurts to review!

    It is a good question, and drk gave you a comprehensive answer! (insert applause here).

    The answer I'm after is diminish.
    Eyes wide open

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    Compulsive Truthteller OptiBoard Gold Supporter Uncle Fester's Avatar
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    Quote Originally Posted by Uncle Fester View Post
    I'll give it a shot but we're going back to school days before the wheel was round!

    A plus lens creates a real focus behind the back side of a lens. The greater the power the larger the image size.

    A minus lens creates an imaginary focus in front of the lens so as plus power is added the object would move further away thus smaller.

    At least that's my stab at it and someones welcome to correct me if I'm wrong.



    edit-Well I guess drk gives the better answer but mine is how I "imagine" it.
    At the risk of taking myself to the woodshed am I wrong in my more simplistic answer?

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    What's up? drk's Avatar
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    You'd have to look at some serious optics to do ray tracings and stuff. I think you're only partially right.

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    Compulsive Truthteller OptiBoard Gold Supporter Uncle Fester's Avatar
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    Thanks Doc!

    What brought my response to mind was a school project where we all were given some lenses and using a film slide image to start had to calculate where to place a series of lenses on a board to create an in focus image at the end. As I recall some images focused larger and some smaller than the original.

    Don't know who got the better laugh as we worked the problem. Our teacher Dr. LaBelle or Mr Gauss.

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    drk, thanks for the great reply. Someday soon I will have to play with these formulae, if my optical ever slows down enough!

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    ATO Member HarryChiling's Avatar
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    "z" is actually the distance to the stop which in ophthalmics is the pupil. An accepted common measure is to add 4mm from the corneal vertex to the pupil plane, so.

    z = vertex + 4mm, which would provide more accuracy.

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