1. Viewing distance: a trade-off of reserves
Jim Sheedy

2. When viewing smaller text, what factors determine the habitual viewing distance?
Hypothesis
A balance is determined between 2 countervailing factors
The desire to increase the angular size drives towards a closer viewing distance
Acuity reserve
Resistance to near viewing is caused by accommodation (and convergence).
Accommodative reserve

3. Acuity Reserve
the ratio of a given print size to the visual acuity threshold for identifying letters or characters.
essentially quantifies the amount by which given text exceeds threshold

4. Optimal acuity reserve
The minimum acuity reserve that optimizes performance
2:1 ratio determined by Whittaker and Lovie-Kitchin for low vision patients
4:1 and 5:1 ratios determined for young adults with normal vision (Bowers et al, Yager et al)

5. Optimal acuity reserve – where the curves flatten

6. Optimal Acuity Reserve
Significance levels for adjacent visual angles. Shaded areas reflect the angular size at which reaction time significantly increases. * * * * * * * * * *
The larger size of each shaded box is treated as beginning of flat curve

7. Acuity reserve calculation
The sizes at which the curves become flat generally range from 20/40 to 20/62.
Comparing these values to the threshold acuity levels for each condition and averaging across all conditions gives a mean acuity reserve ratio of 2.93:1 (range of 3.93: :1).
Rounded value: 3:1

8. Comparison to typical text
We flirt with 3:1

9. Measured viewing distances

10. Comparison to 3:1 acuity reserve

11. Maintaining the 3:1 reserve would require considerably more accommodation
Accommodative demand of the measured viewing distances for hard copy text, and the accommodative demand that would be caused if viewing distance were adjusted to provide a 3:1 acuity reserve.

12. Accommodative lag increases with shorter viewing distances
…an indication of accommodative resistance?
…effect of pupil constriction?
Data from the Grand Seiko calibration study

13. Age and accommodation
Age-related amplitude of accommodation as determined by Donders (1864) and Duane (1912)

14. Reading small text
Reduced viewing distance inadequately compensates text angular size
The need for acuity reserve drives reduced viewing distance
Accommodative stress likely plays a role in mitigating the reduced viewing distance
Other factors
Criticality of task
Convergence
Task interaction
Compromises result

15. Acknowledgment
These studies are supported by Microsoft Corporation