IOL Calc Quiz: Why are the eyes so different?

This patient has similar cataracts in both eyes and is preparing for cataract surgery with you. The biometry has been done by the ophthalmic technician and is shown in the picture above. The right eye calculates to need a +9.5 D IOL for emmetropia, whereas the left eye calculates to +17.5 D for the same result. What is going on and why are the eyes so different?

The patient has a best corrected vision of about 20/100 in both the right eye and the left eye. The patient recalled having good vision with glasses about 5 years ago, but then the vision started to decrease.

The axial length of the right eye is 27.76 mm and the left eye is 25.31 mm on optical coherence biometry. The average keratometry is 42.97 D right eye and 42.35 D left eye. Anterior chamber depth is very similar in both eyes. The big difference between the eyes appears to be the axial length.

What is your next step?

  • choice A: repeat all of the measurements
  • choice B: obtain more history from the patient
  • choice C: book the surgery and go with the above calculations

For choice A: We repeated all of the measurements and they came out the same. The axial length was also measured with A-scan ultrasound and the same difference between the eyes was found.

For choice B: The patient reports that her eyes have always been different and that she did some patching for amblyopia of the right eye as a child. She also brought in her old glasses from 10 years ago. These spectacles corrected her vision to 20/40 OD and 20/20 OS.

The glasses show a spherical equivalent of -9 D in the right eye and -4 D in the left eye. When we look at the spectacles we can see that the right lens is much thicker than the left lens and of course, this is verified by the lensometer reading.

Her old glasses show 5 diopters of anisometropia and the biometry / lens calcs show 8 diopters of anisometropia. Why? Because of the vertex distance. Remember that 1 D at the corneal or spectacle plane does not equal 1 D at the IOL plane.

In general, 1.5 D on the IOL will give about 1 D at the corneal/spectacle plane.

In this case, 8 D of anisometropia at the IOL plane will be about 5.3 D of anisometropia at the spectacle plane. And that checks out with her prior history.

For Choice C: We did the cataract surgery for the right eye and used the +9.5 D IOL which resulted in a post-op refraction of close to plano. The left eye did receive the +17.5 D IOL and it also was close to emmetropia. The vision of the right eye was limited to 20/40 due to the underlying amblyopia, but the left eye achieved 20/20 vision. The patient is very happy.


  1. Thankyou Professor for the informative article.
    Could you please elaborate “In general, 1.5 D on the IOL will give about 1 D at the corneal/spectacle plane.”? I have been trying to understand the reason behind it, does it have to do with the refractive index?

      1. Is there an article or journal/text that you could recommend where I can find the derivation? I only find “1.5 D on the IOL will give about 1 D at the corneal/spectacle plane” or “1D will give 0.7D on corneal plane”.. Thankyou very much in advance

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