Hi to everybody and welcome to OptoTubeChannel, the first Italian YouTubeChannel dedicated to Optometry Today let’s talk about a little comparing between visual acuity evaluated by snellen chart and edtrs Let’s see them in the specific. In the snellen chart there are some features, for example the evaluation made in decimal, and there are simbols which were taken without a specific criterius. Then the problem in this chart is that there isn’t a constant crawding, so we have for example in 1/10 row one letter, then in 2/10 row we have 2 letters, in 4/10 we have 3 or 4 and so in this way… so there isn’t a constant crawding and so about letters number. Then the great negative feature is the aritmetic progression, infact the letters dimension change in a rapid way because passing from a row to another happends an halving of the letters and in this way we cannot have an evaluation of visual acuity very attendible and precise also from a psychophysic point of view. While using edtrs chart what’s happend? This chart use a logaritmic progression which is much more gradual compared to aritmetic progression. Result is wrote down in logMAR or logaritmic units which are the snellen fraction logarithm, for example the logarithm of 10/10 is 1 Then another important thing in the etdrs is that crawding is constant, so there are 5 letters for row and the crawding coefficient is generally 1 but could be different. It is the distance between letters distance so when there isn’t this distance, the coefficient is 0, when the distance is the width of a simbol the coefficient is 1, so for half width is 0.5… Also when we pass from a row to another the progression is much more smooth because that space is the height of the previous simbol, so this chart is much more more reliable and accurate the evaluation of visual acuity performed by etdrs chart. Let’s come back to the crawding phenomenon because there’s both for snellen and etdrs and represent a main aspect, often underestimate and less talked. What is pratically this crawding? As well as the distance between a simbol and another, crawding is an intrinsic phenomenon of our visual system, in particular from visual cortex, a phenomenon that regolarized and simplifies peripheric target perception with a consequence that’s a little fall of visual acuity about the central target which we are going to identify, so what’s happend? Simply there’s an acuity difference if we evaluate visual acuity of a single letter for example a black ” E ” placed on a white ground, isolated from the other target and if I evaluate visual acuity for a target which is placed in a row of letters there will be the crawding phenomenon, in which I won’t have a redistribution of spatial attention but a redistribution more gradual of visual acuity with a little decreasing of the acuity of the central target compared to peripheral target. But why we should consider a chart in which is considered crawding? The reason is simple! Today as yesterday in daily life we don’t see an isolated target from the ground we know to see by ground-figure model but this model is not used with a 1/1 ratio! Today better than yesterday this ratio is not proportionated, maybe it could be 1.000.000.000 / 1 or more, caused there are too target and only one ground, and for this reason is important to evaluate crawding, to recreate echological condition to know how visual system works in daily life. And this is the reason too etdrs chart results much more accurate So this is all for this video, if you liked leave a like and subscribe channel! Let’s see at the next opto-video!!! OPTO-STYLE!!!