MAR and Magnification (was Re: Some useful stuff)

Hi Wayne, and fellow LVTF members, (and regrets I was unable to be on the call last week, I was stuck on location.)


Just as an FYI, multiplying viewing distance & font magnification factors can lead to some misunderstandings, and I don’t recommend it for reasons I will outline below. 

“Magnification” can also be misleading as a way to describe stimuli size, which is described in most research as MAR which (essentially) directly tells you the magnification factor relative to normal vision.

In regards to the posted link of acuity equivalences, MAR is missing (though logMAR is shown), and “foot” is best labeled as Snellen-Imperial and “metre” is best labeled Snellen-Metric. 

Please see this link for an in depth discussion and chart:  
 https://webvision.med.utah.edu/book/part-viii-psychophysics-of-vision/visual-acuity/


Definitions:

MAR is “Minimum Angle of Resolution” in arc-minutes (60 arc minutes per degree), in other words the total angle of view of a particular stimuli size, regardless of distance.

Snellen is the more “common knowledge” acuity measurement that the public readily understands. The numerator 20 in Snellen Imperial is feet, and the numerator 6 in Snellen Metric is meters. 

Snellen directly relates to MAR - just divide the denominator by the numerator. So for Snellen imperial (where the numerator is set as 20 feet) 20/70 is MAR 3.5

Low Vision: The World Heath Org defines vision equal to or worse than Snellen 20/70 as Moderate Low Vision.

Perfect vision: for humans the theoretical best (limited by cone density) is 20/10 (an Eagle’s eye has about twice the resolving power IIR). For humans the PSF (Point Spread Function) is ideally about 1 arc minute.

“Normal” vision: often understood to be 20/20, however a healthy young adult with no impairment will usually be closer to 20/15. Moreover, most standards (such as ANSI) imply a “normal” vision up to about 20/60 (that is, the standard is intended to accommodate up to 20/60 or so which is considered “near normal"), and 20/40 is the common threshold for a non-commercial driver’s license in most states. (20/30 is used for a non-commercial pilot’s license).

Decimal Acuity is 1/MAR

logMAR: Because MAR increases geometrically, logMAR is often used especially for charting purposes.


Be Cautious Using the Term “Magnification.”

The reason to avoid the term magnification is it can be misleading — if you magnify something with a magnifying glass,  you may also be changing the focus-point on the retina! If you “magnify” my moving something closer, you may also impair the focus point on the retina. 

The reason that increasing magnification by decreasing distance is problematic is due to presbyopia, i.e. age related farsightedness wherein decreasing distance moves the stimuli outside the ability of the ciliary muscles to pull the natural lens into focus.

As a result, while decreasing distance may result in “magnification” it may ALSO reduce acuity!

It should be no surprise then that the cited paper indicates that most were “ agnifying" by increase the size of stimuli on the screen more often than just by moving “closer”.

20/200 is specifically ten times the stimuli size in arc minutes as 20/20. 

20/200 is MAR 10, and 20/20 is MAR 1, thus:
20/40 indicates a stimuli that is 2 times larger than 20/20
20/70 indicates a stimuli that is 3.5 times larger than 20/20
20/200 indicates a stimuli that is 10 times larger than 20/20


For reference:




Regards,

Andy



Andrew Somers
Color Science Researcher
Voice/Text: 213-448-4746   Fax: 323-466-8779  
P.O.Box 1867   •••   Hollywood, CA 90078
Andy@GeneralTitles.com <mailto:Andy@GeneralTitles.com>





> On Jul 22, 2019, at 4:58 PM, Wayne Dick <wayneedick@gmail.com> wrote:
> 
> Dear LVTF,
> Attached is the URL of a table that lists all the visual acuities in the various scales.
> http://nosetothepage.org/AcuityScales.html <http://nosetothepage.org/AcuityScales.html>
> Foot, Metre, Decimal and logMAR. It should make reading research articles easier.
> 
> A recent study shows that most people with low vision like to enlarge their text a lot. They also prefer making their text large. Of legally blind, 20/200 that enlargement is achieved as follows: they shorten the viewing distance by a factor of 2.8. That is they view at 1/2.8 the normal distance. Also they enlarge the text by a factor of 6. Taking this together that means that people at 20/200 on the average prefer magnification of 2.8x6=16.8. 
> 
> Also, as we observe people with low vision like reading on desktop monitors.
> 
> I have also included 
> 
> Here is the article abstract:
> How People with Low Vision Achieve Magnification in Digital Reading.
> Granquist C, Wu YH1, Gage R1, Crossland MD2, Legge GE1.
> 
> Abstract
> SIGNIFICANCE:
> Digital reading displays provide opportunities for enhancing accessibility of text for low vision. How are these displays used by people in their daily lives?
> 
> PURPOSE:
> Subjects responded to an online survey concerning their vision history, reading technology, display preferences, and reading habits. Here, we report on findings concerning acuity and magnification.
> 
> METHODS:
> The survey asked subjects to arrange a text passage for typical reading and to report viewing distance, screen dimensions, and the number of characters per line. Seventy-five adult subjects (most with early-onset low vision, few with central field loss) completed all survey questions relevant to the analysis of acuity and magnification. Mean acuity was .92 logMAR (range, 0.1 to 1.6), and mean age was 44.8 years (range, 18 to 71 years). Twelve normally sighted controls reported the same information while viewing the passage on cell phones, tablets, and computers.
> 
> RESULTS:
> The controls had a mean viewing distance of 38.7 cm and a mean x-height of 1.38 mm. For all three types of devices, angular x-height was 0.21° (close to laboratory estimates of the critical print size for reading). Low vision subjects showed decreasing viewing distance and increasing print size with larger values of logMAR acuity. Most of the low vision subjects achieved their desirable magnification by a combination of reduced viewing distance and increased physical letter size. The majority (54 of 75) relied more on letter-size magnification. Relative to the controls, regression analysis revealed that a typical low vision subject with logMAR acuity of 1.0 reduced viewing distance by a factor of 2.8 and enlarged physical print size by a factor of 6.
> 
> CONCLUSIONS:
> Our survey shows that people with a wide range of acuities are engaged in digital reading. Our subjects achieved desirable magnification primarily by enlarging physical character size and to a lesser extent by reducing viewing distance.
>