S Afr Optom 2009 68(1) 38-48 The effect of pinholes of different sizes on visual acuity under different refracting states and ambi- ent lighting conditions* Abdul N*, Meyer N*, van Bosch M*, van Zyl A*, Viljoen M* and Carlson AS** **Department of Optometry, University of Johannesburg, PO Box 524, Auckland Park, 2006 South Africa **<[email protected]> Received 18 February 2008; revised version accepted 6 March 2009 Abstract gave more positive results, but also when the pin- hole was placed in front of the subjects. Accord- Purpose: The purpose of this pilot study was to ing to Borish, the pinhole does improve VA by a investigate the effect of different size pinholes on straight forward process of elimination produced visual acuity (VA) at near (0.4 m) while various by spherical aberrations of the eye. However, this factors were altered. The alterations made involved did not seem to be the case in this study under con- accommodation and illumination. ditions of low illumination. Method: Four subjects were selected and their ages Conclusion: The pinhole effect offers improve- ranged from 7 to 14 years. Subjective refractions ment in VA at near by reducing the amount of blur were performed and then near visual acuities were presented to the retina. This study showed that all measured at 0.4 m under two different lighting con- subjects had an improvement in VA of at least two ditions (460 lux and 1 lux) by adding stigmatic (or lines in high illumination. Pinhole size did not alter spherical) lenses of positive and negative powers in the results sufficiently thus suggesting that a pin- front of pinholes of diameters 1.5 mm and 2 mm. hole, no matter the size, should increase the VA. Results: The results showed that there was an im- However this was not the case when illumination provement in VA in most, however, the size of the was decreased. pinhole played a minor role. Low illumination did have a drastic effect on our results not only by de- Key words: Pinhole, blur circles, illumination, creasing the amount of lenses used for near that visual acuity Introduction the eye are less significant. Residual refractive errors, Many people experience an improvement in their para-central corneal scars, irregular astigmatism, ec- visual acuity (VA) with pupil constriction in bright centric opacification of the posterior lens capsule and light conditions due to the pinhole effect. This implies non-cortical central cataracts are examples of clinical that when the pupil is small clinical irregularities of irregularities of the eye. The smaller the pupil, the less * 4th year students; research project performed as part of the requirements for the course Optometry 3. ** Research supervisor 38 The South African Optometrist S Afr Optom 2009 68(1) 38-48 Abdul N, Meyer N, van Bosch M, van Zyl A, Viljoen M and Carlson AS - The effect of pinholes ... lighting conditions defocus occurs from spherical aberration (the pinhole of intermediate focal distance8. There are a few con- effect)1. This is a single reason why subjects with troversies regarding whether accommodation is the large pupils, either from dilation or other reasons may foremost factor contributing to the pupil response at see “ghosting” around an otherwise sharp image.2 near given that it has also been suggested that pupil Subjects whose VA cannot be improved to 20/20 with constriction may be the consequence of other factors pinhole are referred to as having non-refractive errors such as target misalignment and proximal cues such and may require referral to an ophthalmologist3. as size, fusional vergence and near effort9. In a clinical study done by Borish1, it was found The purpose of this preliminary pilot study was that the most effective pinhole varied in diameter be- to investigate the effects of different sized pinholes tween 0.94 mm and 1.75 mm. When changing the di- on proximal VA at 0.4 m while various factors were ameter of the pinhole from 1 mm to 0.2 mm it caused altered. The alterations involved accommodation and the print to be too blurred to be distinguished and illumination. The accommodative response was in- when the aperture was even further reduced it caused cluded in this study for the following reason; the level the page to assume a uniform greyish cast1. of accommodation affects the amount of spherical ab- errations of the human eye and the pinhole is used to Effects of the near synkinesis on pinhole eliminate these spherical aberrations. The eye of a young person can quickly change its Methods optical power by altering the shape of the crystalline Four subjects were selected for participation in this lens. The change of power is called accommodation study. Their ages ranged from 7 years and 10 months and is measured in dioptres (the inverse of the focal to 12 years and 3 months. The effects of various fac- length measured in meters). The amplitude of accom- tors were investigated in this study including the ef- modation is the dioptric difference between the near fect of adding positive and negative stigmatic lenses and far points of vision4. It declines at approximately as well as the effect of illumination on the perception 0.25 D per year5. When one focuses on a near ob- of letters at 40 cm without and then with pinholes of ject activation of accommodation, convergence, eye diameter of 1 and 1.5 mm respectively. Each subject movements as well as pupil constriction takes place. was tested in the afternoon (after school) on differ- Accommodation causes an increase in the optical ent days over a period of two weeks. All the tests on power of the eye to facilitate focus on the near object each subject were completed on the same day. The while convergence eye movements direct the eyes full set of measurements per subject took about 45-60 towards the near object. Pupil constriction, which is minutes. also part of the near synkinesis, increases the depth Subjective refractions were measured on both of focus and will thus assist accommodation in pro- eyes of each subject first and then their amplitudes viding a clear image of the near object on the retina. of accommodation were measured using the nega- Depth-of-focus of the eye is the range of focusing tive lens method (#19) through a phoropter, viewing errors where the image appears not to have changed the standard Snellen near acuity chart. The expected in contrast, clarity or form6. This provides important amplitudes were determined by using the following information of the range of clear vision where differ- mathematical equation: Aexp = 15 – 0.25 Age. The ent monocular depth cues like target size, blur and compensated distance VA for all subjects was 6/6 and proximal information are present under natural view- the actual amplitudes ranged from 8.25 D to 10. 75 D. ing conditions7. The depth of focus of the eye can For this study, only the right eyes were used. be altered by spherical aberration of the human eye, Illumination was measured using a Sekonic Lumi which changes as a function of the level of accommo- Model 246 illuminometer which measures the amount dation. Studies have shown that spherical aberration of light emitted off the chart used at 0.4 m. Two spe- becomes more positive with far accommodation and cific illuminations were used in this experiment, high negative with near accommodation. Thus, this implies (460 lux) and low illumination (1 lux). The various that the depth of focus increases towards the direction illuminations were obtained with the use of a rheo- 39 The South African Optometrist S Afr Optom 2009 68(1) 38-48 Abdul N, Meyer N, van Bosch M, van Zyl A, Viljoen M and Carlson AS - The effect of pinholes ... lighting conditions stat on the light switch that could be adjusted to the was always in the back part of the trial frame while illumination required. The VA of each subject was the lenses were added in the front. measured under these lighting conditions while lens- es of positive power were added in 1 D steps with the Results and Discussion aim of relaxing accommodation. While this was done Table 1 shows the four subjects’ ages, subjective the Lighthouse Near Acuity Chart was first viewed refractions, expected and actual amplitudes of accom- without a pinhole and then through the pinholes with modation. Similar results were shown by all subjects diameters of 1.5 mm and 2 mm respectively with the so only results for Subject 1 will be included here in purpose of seeing the effect that these pinholes would Tables 2 to 5. Table 2 shows the effect of lenses of have on VA. Lenses were added until a VA of 6/60 positive power on VA for Subject 1 viewing a Light- or less was obtained. After this was done the exer- house Near Acuity chart at 40 cm without a pinhole cise was repeated with lenses of negative power, thus and through pinholes of diameter 1.5 mm and 2 mm stimulating accommodation. respectively with the target illumination at 460 lux The conditions were kept constant during the meas- while Table 3 shows the effects with the target illu- urements for all the subjects. The conditions included minated at 1 lux. Tables 4 and 5 show a similar effect all participants being tested in the same examination for Subject 1 but here lenses of negative power were room thus maintaining the identical setting to avoid used. Figures 1 to 12 show the results for all the sub- external distractions and the placement of the pinhole jects for the various experimental conditions.