Multiple distances and variable contrast

Download from Apple Store

Click on any of the options in the navigation menu below for accessing to all the details of the App


Multiple distances and variable contrast

Download from Apple Store

Click on any of the options in the navigation menu below for accessing to all the details of the App


VisionC is an application developed for presenting high and low contrast optotypes at multiple distances. With this app, you will not need to have multiple printed charts of optotypes, each one designed for being performed at one distance or in a specific condition of contrast. VisionC has two presentation modes: 

01 ETDRS CHART (Early Treatment Diabetic Retinopathy Study). This chart is considered the Gold Standard for measuring Visual Acuity.

02 SLOAN LETTERS with Crowding effect. Designed for achieving visual acuities with big size optotypes, which is not possible with an ETRDS configuration due to the screen size limitation of the iPad.

The versatility of this application for being conducted at multiple distances it makes it very useful for measuring high and low contrast visual acuity in patients submitted to any multifocal procedure, either Multifocal Contact Lenses, Multifocal Intraocular Lenses or Premium Laser Refractive Surgery for Presbyopia. In the same way, it has a specific configuration for measuring Low Vision patients at 1 m of distance.

In the bottom image, you can see how the letters are reproduced in an iPad Retina in comparison with the equivalent area of an ETDRS cabin (selected by a black square)

ETDRS Charts


The ETDRS charts use the SLOAN letters (C,D,H,K,N,O,R,S,V,Z) with angles and curvatures of specific designs for each letter. The reason because these letters are used is for minimizing the readability differences between them. In the ETDRS test, the letters are grouped in rows of 5 letters whose sizes are reduced as the visual acuity is reduced in logMAR notation (incremented in decimal scale). This scale is obtained by means of the logarithm of the minimum angle resoluble and is named logMAR progression. 

  • ETDRS charts to be presented at 40 cm, 67 cm, 80 cm, 2 m, 3 m y 4 meters. The charts are composed by 5 letters per visual acuity row considering line steps of 0.1 logMAR according to the international standard. 
  • 3 packages of different letters for avoiding memorization.
  • Contrast variation from 0 to -2.3 log units in -0.1 log unit steps for measuring visual acuity at different levels of contrast.
  • For knowing exactly the ranges of Visual Acuity and Contrast that can be measured at each distance you can see the tables that are shown at the bottom of this section. (The tables are not visible if you are acceding to the web page by a smartphone).

Slide the finger over the screen in the vertical direction for changing the contrast level or in the horizontal direction for changing the letters package. The visual acuity corresponding to each line will appear in logMAR and Snellen notation at the left side of the screen and in decimal notation at the right side as it is shown in the bottom image. The contrast level in log units or the equivalent in percentage is shown at the bottom left of the screen.



Table 1. For each presentation distance the minimum and maximum visual acuities that could be measured are shown.

Distances   Maximum VA     Minimum VA  
  LogMAR Decimal Snellen LogMAR Decimal Snellen
40 cm 1 0.1 20/200 0 1.0 20/20
67 cm 1 0.1 20/200 -0.3 2.0 20/10
80 cm 1 0.1 20/200 -0.3 2.0 20/10
2 m 0.7 0.2 20/100 -0.2 1.59 20/13
3 m 0.6 0.25 20/80 -0.3 2.0 20/10
4 m 0.5 0.32 20/63 -0.3 2.0 20/10

The letters definition are determined by the pixels per inch of the screen. This is why the smaller letters presented at near distances (40 cm - 80 cm) could not be perfectly defined. (1)

Table 2. Contrast levels from 0 to -2.3 log units. Contrast sensitivity is the same value in positive notation.

Contrast Log Units

0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8 -0.9 -1.0 -1.1 -1.2 -1.3
Contrast Percentage (%) 100% 79% 63% 50% 40% 32% 25% 20% 16% 13% 10% 8% 6% 5%

The conversion table shows the equivalent from log units to percentages of contrast just up to -1.3 logC although the application can present up to -2.3 logC.

(1).- Rodríguez-Vallejo M. Comment on: “Effectiveness of a smartphone application for testing near-visual acuity.” Eye. 2016:Advance online publication 4 March.

SLOAN Letters


Although we recommend using the App in the ETDRS configuration, in patients with Low Vision the size of the iPad screen is not enough big in order to present 5 letters per row. In this case, we can select the SLOAN option for presenting isolated crowded letters for achieving higher values of visual acuity.

Other interesting clinical applications of the SLOAN letters is the possibility to measure the contrast sensitivity for a particular size of the optotype. This configuration can be obtained by setting the desired percentage of contrast and returning back to the visual acuity measuring configuration by pressing in a bottom as it is described in the instructions described at the bottom.

  • Isolated Sloan Letters with crowding effect for being presented at distances of 4, 3, 2, 0.80, 0.67 o 0.40 m. The visual acuity range from 1.0 to -0.3 logMAR [20/200 to 20/10 in Snellen][0.1 a 2.0 in decimal].
  • Contrast variation from 0 to -1.9 log units in -0.1 log unit steps for measuring visual acuity for different conditions of contrast. 


At the bottom left of the screen, you will find a button for changing between the different settings of the test: You can measure the visual acuity for a particular level of contrast or you can measure the contrast for a particular size of the optotype. 

If the button is switched off, by means of sliding down the finger over the screen you will see that the optotype size decrease in 0.1 logMAR steps in such a way you can measure the visual acuity.


If you switch on the button, the same sliding down gesture over the screen will not change the optotype size but the contrast will change in 0.1 log unit steps.

If you want to measure the contrast sensitivity for a particular size of the optotype you only have to go to to the desired value of visual acuity, switch on the button and to start to measure the contrast sensitivity. 

Low Vision


The application has a specific setting for Low Vision patients that can be conducted with an ETDRS chart or with SLOAN letters. Moreover, the low vision exam can be only performed with the iPad at 1 meter of distance. 

  • ETDRS chart at 1 meter of distance. Visual acuity range measured from 0.2 to 1.1 logMAR [20/32 to 20/253 in Snellen][0.08 a 0.63 in Decimal]
  • Contrast variation from 0 to -1.9 log units in -0.1 log unit steps for measuring the visual acuity at different conditions of contrast. 
  • Isolated Sloan Letters with Crowding effect for measuring visual acuities over the possible values with the ETRDRS limited by the screen size. Possible measure up to 1.6 logMAR [20/800].

At the bottom left of the start screen, you will find a button. After switching on, you will be able to measure patients in normal conditions (letting to select the presentation distance). With the button switched off, the test will be able to be conducted in Low Vision mode. A message will be shown in the screen indicating: "Low vision, please present the test at 1 m of distance". 


Version 1.0.1
    • New design of the start screen.
    • Insert the brightness directly from the app.
    • New presentation distance of 2 m. 
    • Reconfigured the visual acuity lines by means of removing those with higher alliasing due to screen limitations.
    • The labels which indicated the visual acuity have been removed to improve the visibility.
    • Contrast in percentage or log units.
    • Bugs from previous version corrected.


Any visual test used in clinical practice must have demonstrated its validity and reproducibility for the purpose for which it was designed. The Apple Store is full of Apps to evaluate the visual performance that has not been validated but they are used by professionals. None app that has not been validated should be used by a vision professional.

We have conducted early studies with the VisionC (at 85% of screen brightness) in order to evaluate the agreement at 3 meters versus a cabin used for clinical trials presented at 4 meters in 38 eyes of healthy subjects measured with both tests. The mean differences were -0.02 logMAR which means that the VisionC underestimates the visual acuity in just one letter in comparison to the cabin. Furthermore, the 95% confidence interval was 0.11 logMAR which means that with a 95% of probability, we are not going to find differences of more than one line of visual acuity between tests. This bias can be explained by the reproducibility of the ETDRS procedure for which it is normal to found up to one line of visual acuity difference between days (1).

With regard to low contrast visual acuity measurement with VisionC, it is important to note that the test has been designed considering the bias between different iPad models (2). Currently, it has been conducted an early agreement experiment for the comparison of the low contrast (25%) visual acuity measured at 4 meters with an optotype cabin and measured with the VisionC (85% of screen brightness) in 36 eyes obtaining an under-estimation of the low-contrast visual acuity of 0.06 logMAR (3 letters) with the VisionC at 3 meters. With the 95% of confidence, we can find a difference up to 2 lines in the measurement of low-contrast visual acuity.


We have conducted early studies with the VisionC on which we have demonstrated its reliability in the Visual Acuity measurement, however, these findings have not been reported in the research literature yet. With regards to the low contrast visual acuity, there are higher differences that we can consider normal for any contrast sensitivity test.


1. Rodríguez-Vallejo M, Llorens-Quintana C, Furlan WD, Monsoriu JA. Visual acuity and contrast sensitivity screening with a new iPad application. Displays. 2016; 44: 15–20. 

2. Rodríguez-Vallejo M, Monsoriu JA, Furlan WD. Inter-display reproducibility of contrast sensitivity measurement with iPad. Optom Vis Sci. 2016; 93: 1532–6.