40 cm Working Distance: Does it Work Always?

Vipin G, M. Optom

Project Optometrist, Elite School of Optometry, Chennai, India

 

In clinical optometry, 40 cm has long been the standard distance for near vision testing. ⁽¹⁾ This measurement was traditionally believed to represent a typical working distance for near tasks like reading. However, emerging evidence and modern visual habits suggest that this standard may no longer reflect real-life patient needs.

The Origins of the 40 cm Standard

The consensus in vision care is that 40 cm approximates the distance at which most individuals perform near tasks. ⁽¹⁾ This standardisation facilitates consistent testing across clinical settings. Yet, there is limited scientific evidence that 40 cm represents all individuals or tasks. Factors such as arm length, posture, and task type greatly influence the preferred working distance, making a single universal distance less practical. ⁽²⁾

Evidence Against the 40 cm Standard

Recent studies challenge the universality of the 40 cm standard:

• Smartphones & Tablets: Studies from the United States and China show average smartphone viewing distances around 34–36 cm, often closer than the traditional 40 cm. ^(1,3)
• Reading Distance in Children: Children, due to shorter arms, hold reading materials much closer, with mean distances of 27 cm or less. ⁽⁴⁾
• Dynamic Viewing: Tasks like reading print, using smartphones, and laptop work require varying distances and gaze angles, none of which may match 40 cm. ⁽⁵⁾

These findings highlight the need for testing methods that better reflect real-world behaviours, especially as near-vision demands become increasingly diverse.

Implications for Presbyopia Management

Current near vision lens prescriptions for presbyopic patients assume that they will perform most near tasks at 40 cm. ⁽⁶⁾ However, pre-presbyopic individuals frequently work at closer distances, especially with handheld devices. ⁽⁷⁾ This discrepancy raises questions about whether additional near powers should be adjusted to accommodate closer working distances, ensuring greater comfort and visual efficiency.

• Testing should match the actual working distance of patients. Detailed case histories should include daily visual tasks and habits. ⁽⁶⁾
• Children require unique considerations, including testing distances that reflect their naturally closer working positions. ⁽⁸⁾
• Near-vision evaluations must evolve to reflect diverse visual demands such as screens at different distances and angles. ^(5,8)

Why Change Is Necessary

With rapid technological advancement, visual requirements are shifting dramatically. ^(5,8) Not all patients function at a working distance of 40 cm; individual habits vary. While a fixed 40 cm simplifies clinical testing, it risks overlooking the true diversity of near-vision behaviours.

Aspect	Traditional View (40 cm Standard)	Emerging Evidence / Challenges	Clinical Implications
Smartphone/Tablet Use	Not considered when the standard was set.	Average viewing distance ~34–36 cm, often closer than 40 cm.	Prescriptions may under- or over-correct for device users.
Reading (Children)	The same 40 cm applied across ages.	Children hold material ~27 cm or less.	Testing at 40 cm overlooks natural working distance.
Dynamic Visual Tasks	One fixed distance assumed adequate.	Tasks involve varying distances and angles.	40 cm cannot represent all near activities.
Presbyopia Management	Near additions prescribed for 40 cm.	Pre-presbyopes work closer with handheld devices.	Misalignment reduces comfort and efficiency.
Overall Clinical Practice	Simplified, one-size-fits-all.	Increasing diversity of tasks and needs.	Risk of suboptimal correction if 40 cm is always used.

Table 1: Limitations and Evolving Perspectives on the 40 cm Near Vision Standard

Conclusion

The 40 cm standard, while historically useful, no longer reflects the complexities of modern near-vision demands. ^(5,6,8) Optometrists should consider adopting individualised testing distances to ensure assessments are relevant and effective. Tailoring testing protocols to real-world visual habits can improve comfort and visual performance for patients in their daily lives.

 

References

1. Bababekova, Y., Rosenfield, M., Hue, J. E., & Huang, R. R. (2011). Font size and viewing distance of handheld smartphones. Optometry and Vision Science, 88(7), 795–797.
2. Fekadu, S., Assem, A., & Mengistu, Y. (2020). Near vision spectacle coverage and associated factors. Clinical Ophthalmology, 3121–3130.
3. Lan, M., Rosenfield, M., & Liu, L. (2018). Cell Phone Viewing Distance and Age in a Chinese Population. Optometry & Visual Performance, 6(5).
4. Rosenfield, M., Wong, N. N., & Solan, H. A. (2001). Nearwork distances in children. Ophthalmic & Physiological Optics, 21(1).
5. Rosenfield, M. (2024). Why do we test at 40 cm? Ophthalmic and Physiological Optics, 44(3), 481–482.
6. du Toit, R. (2006). How to prescribe spectacles for presbyopia. Community Eye Health, 19(57), 12.
7. McDonald, M. B., et al. (2022). Classification of presbyopia by severity. Ophthalmology and Therapy, 11(1), 1–11.
8. Buch, J., Riederer, D., Scales, C., & Xu, J. (2023). Tear film dynamics of a new soft contact lens. Ophthalmic and Physiological Optics, 43(5), 1070–1078.