# Professional and Educational Organizations > Professional and Educational Organizations Discussion Forum >  Accommodation demands in hyperopia, comparing contacts and glasses

## Shiva

I am confused in solving a problem asking to calculate the amount of accommodation needed for a +7 D hyperope , viewing an object in 20 cm distance- considering vertex distance about 13 mm-
should I consider the near vergence + 12 or + 2?
Actually I cannot understand the philosophy of calculating vergence at spectacle plane for near and distance to calculate contact lens advantage.
I would be grateful for responses

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## tmorse

You must calculate the vergence power difference at the cornea plane for both eyeglasses and CL to determine the CL to eyeglasses advantage for hyperopes, rather than just at the spectacle plane. Example...

  +7.00D GLASSES AT INFINITY:

  Rays at infinity have 0.00 vergence and strike a +7.00D lens... they exit this lens at L=+7.00 or l=1/L = 1/+7.00= +0.143m vergence. But they must also travel 0.013m before striking the corneal plane. 

  Vergence formula at corneal plane l2 is = l'-t/n = +0.143m-0.013/1= +0.130m or +7.69D vergence at corneal plane using eyeglasses.

  GLASSES AT NEAR:
  Rays from 200mm object for near have -5.00D vergence. These rays strike our +7.00 lens and exit eyeglass lens L= +2.00D vergence. But rays must still travel vertex distance of 0.013m before striking corneal plane.
  Using vergence formula, l= 1/L= 1/+2.00D= +0.500m. 
  Thus l2=l'-t/n= +0.500-0.013/1= +0.487m. Incoming vergence at corneal plane for eyeglasses at near is 1/+0.487=+2.05D 

  The rays at corneal plane have vergence difference between distance @ infinity and near of (+.7.69)-(+2.05)= +5.64D using eyeglasses.

                                                  ***********************
  CONTACT LENSES AT INFINITY:
  +7.00 glasses power 13mm out from cornea has contact lens (CL) effective power of only +6.41D power at the corneal plane (plus lens moved OUT is effectively stronger and moved IN is effectively weaker). So to compensate we must order a stronger CL lens power of +7.71D
  since CL lenses need this +7.71 power at corneal plane to see clearly.

  CONTACT LENSES AT NEAR:
  Object 200mm from CL results in an incoming near vergence at CL of -5.00D. But since vertex distance in CL is 0.00mm, we can combine incoming near vergence with corneal CL +7.71D. These near rays at corneal plane have a total vergence of (+7.71D)+(-5.00D) = +2.71D

  The vergence difference using CL lenses for distance and for near is (+7.71)-(+2.71)= +5.00D. 

  In this example 0.64D more accommodation is required using eyeglasses compared to contact lenses, and this can result in a delay a presbyopia RX for hyperopes  approaching 40 years of age.

  Further, hyperopes wearing eyeglasses for distance produce a base-out effect when looking at near, which places the near object further in and requires more outward muscle pull effort for proper fusion. But since a plus CL stays centered on the eye at all times including at near, no extra muscle strain is needed for near vision when wearing contact lenses. 
  Thus the adage when transitioning from eyeglasses to contact lenses, HYPEROPES are happy and MYOPES are miserable.

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