Consider the pinhole
Consider a large dilated pupil. As light passes through it, the light comes to a sharp point on the retina. It’s very clear, but also very small.5 There is no depth of focus. A clear object exists but at a very specific plane. If that object moves at all, clarity is immediately lost. However, if we make the pupil smaller, that results in a larger depth of focus: if an object moves closer we’re still within that depth of focus and can see the object clearly.4
Ideally, a small pupil or pinhole will improve our depth of focus, and if we can find the right balance, maybe we won’t have to compromise peripheral vision.4
Pinholes, planes, and peripherals
Next, we need to think about where to place the pinhole. If we hold it at the spectacle plane like in clinic, the light passing through the pinhole through the center of the pupil is in focus.5 It’s clear and on the retina. But the light further out peripherally will hit the iris and no longer reaches the retina.5 We’ve now lost that part of the image, so there is a huge trade-off in the restriction of peripheral vision. If we move the pinhole to the corneal plane, then we see a little wider peripheral vision, but a significant portion is still blocked.5
How about if we moved even further, into the iris plane? Now no light is blocked, and peripheral vision is still intact.5
Optimal pupil size: there’s no magic number
What is the optimal pupil size we need to reach for our patients? This is a trick question! There is no set optimal size because pupil size will change depending on the environment and a patient’s normal pupil size.6 We need to instead think about what percentage change in pupil size will give the best vision.
Optimal size is a percentage of the patient’s natural pupil and will vary from patient to patient rather than being a single magic number. A graph of pupil size in bright light versus image quality shows that optimal distance and near vision occurs between 40 and 50% of pupil size, and this occurs across all light conditions including mesopic and low light.6
If the pupil isn’t small enough, there won’t be any near vision improvement. If it’s too small, then we’ve lost our distance vision, but if the pinhole isn’t at the iris plane then we impact peripheral vision. Thus, we need to place the pinhole at the iris which will extend our depth of focus without impacting our peripheral vision, and the optimal pupil size needs to be dynamic so we can have maximum image quality at not just near, but also at distance.6
- Chuck RS, Jacobs DS, Lee JK, Afshari NA, Vitale S, Keenan JD. Refractive Errors & Refractive Surgery PPP - 2017. American Academy of Ophthalmology. https://www.aao.org/preferred-practice-pattern/refractive-errors-refractive-surgery-ppp-2017. Published July 9, 2018.
- Moarefi MA, Bafna S, Wiley W. A Review of Presbyopia Treatment with Corneal Inlays. Ophthalmology and therapy. https://www.ncbi.nlm.nih.gov/pubmed/28390052. Published April 7, 2017.
- Alio del Barrio JL, Vega-Estrada A. Accommodative intraocular lenses: where are we and where we are going. Eye and Vision. https://eandv.biomedcentral.com/articles/10.1186/s40662-017-0077-7. Published June 26, 2017.
- Rocha KM, Gouvea L, Waring GO, Haddad J. Static and Dynamic Factors Associated With Extended Depth of Focus in Monofocal Intraocular Lenses. American Journal of Ophthalmology. https://www.sciencedirect.com/science/article/pii/S0002939420301823. Published April 23, 2020.
- Charman WN. Correcting presbyopia: the problem of pupil size. Wiley Online Library. https://www.onlinelibrary.wiley.com/doi/pdf/10.1111/opo.12346. Published December 28, 2016.
- Xu R, Thibos L, Bradley A. Effect of Target Luminance on Optimum Pupil Diameter for Presbyopic Eyes. November 11, 2016.