Decoding Snellen's Chart: Testing Your Visual Acuity Made Simple! | NEET PG | MBBS | Dr Vivek 

Yes!

@@tamunotonyeedward2544 Thanks…!

Sure. Thanks.

Visual acuity does not increase beyond 6 M distance. Any distance from 6 M upto infinity, visual acuity remains the same. If the Snellen's chart is placed at, say 7 meters, the person will be able to read it the same way as with 6 meters.

Yes. Correct. Thanks!

Alp..?

@@kumarkool987 yes bro. U too?

Yes bro..mine on July 1st.. You can try this app at home play.google.com/store/apps/details?id=com.shadinaga.OptoCharts

What is the dimensions of 6/6 line letter..?

@@kumarkool987 Bhai medical ho gaya? Kaun sa hospital me ? Aur knock knee check kiye Kya?

Of course you are right, and i have said the same. Whole letter will subtend 5’ and its components (like, the two vertical lines of H ) will subtend 1’ , only then they can be seen separately. [1’ or more than 1’ is the necessity to resolve the details.]

Can you please elaborate on the question? (The entire letter subtends an angle of 5 minutes and its individual components, any two components, will subtend an angle of 1 minute.)

Because it is like that. When Snellen notation is 6/6, the minimum separable is 1 divided by 6/6, that is, 1 minute. So, if two points subtend 1 minute angle, they can be seen separately. If the vision is 6/24, then the minimum separable is 24/6, that is 4 minutes. In this case, two points have to subtend 4 minutes' angle (or more), only then they can be seen separately. That is how these letters in Snellen's chart have been constructed.

It’s not about small things. It is about ability to see two points separately. That ability is greatest on the fovea centralis. Large objects will make image on other parts of retina and will be visible.

@@VivekSirsPhysiology sir I checked wikipedia.it says visual acuity checks only foveal vision which is like 1 percent of the retina.the remaining 99 percent is for peripheral vision which only sees movement and contrast.it is also mentioned that we can see only twice the width of our thumbnail with foveal vision.how do we determine the acuity of peripheral vision sir

@@sriramgovindarajan2585 we don't have to check visual acuity of peripheral retina. Our eye (vision) is designed in such a way, that fovea centralis has the best visual acuity. And hence, whenever we want to observe details of an object, we have to rotate our eyes and "look toward" the object. The rays coming from the object will then fall on retina and details can be deciphered. Rays falling on any other part of retina will give other info about the object. In short, peripheral retina is not designed for visual acuity in the first place. It is designed for visual threshold. (It has more and more rods, so when light is minimum, it can be detected by peripheral retina).

So in short, different functions ascribed to central retina/fovea, and peripheral retina. And it suits the vision. Not all objects in our field of vision will be 'objects of interest'. So, we have to actively look at the object of interest, so that rays coming from it will fall on fovea and we will know the details. This way, there will be sharp focus on such an object.

At most places, the 6 meter distance isn't available. A mirror is placed at 3 meters, and Snellen's chart is placed just where the subject is seated. Thus, 3 meters (incident rays) and 3 meters (reflected rays) - the distance becomes 6 meters. 6 meters is the boundary between near and far vision. So, yes; the chart shouldn't be placed within 6 meters , to avoid accommodation of the eyes while reading letters.

@@VivekSirsPhysiology but 3 meter Snellen chart is also available it means it can't check distant vision??

It can be checked, but after paralysing the accommodation. The point is - one doesn't want the contraction of ciliary body, while testing visual acuity. So, 6 m is best. 3 m can be used, but there will be accommodation in the eyes.

@@VivekSirsPhysiology but even at 3 meters there should b a letter of particular size that we can read without accomodation like in Snellen chart it's the last line that we can read without accomodation but if we move at 3 meter then the letter should b smaller in such a way that its vertical lines subtend an angle of 1 Min at nodal point? Is it like that

There are two things in this ~ 1. What is the boundary between near vision and far vision? When would you call an object a “distant” object? The answer is 6 meters and beyond. The reason is precisely what is explained in the video. An object at 6 meters and beyond would be seen automatically, without any need for accommodation. An object within 6 meters would start making an image beyond the retina and hence changes of accommodation would be necessary to focus its image on the retina. When we are checking distant vision, we don’t want these changes to happen. In fact, ophthalmologists often even paralyse the accommodation before testing the distant vision. 2. How can we check a ‘3 meters’ vision? Yes. You have answered it. If we want to check the vision for 3 meters , the Snellen’s letter will be constructed such that its details should subtend an angle of 1’ at the nodal point.

Jitna hum dhur hote jayenge image utna bara denge.. lekin tb angle same hi bnega i.e 1 min...

Ok. Thanks for the comment/feedback. It will motivate me to try and improve; learn more and more, and overcome the inadequacies. Regards.