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Does any one have any information about the width of the sharp focal area of the retina (the Macula)? I am trying to get an understanding of how this could impact the ideal length for a line of copy.

Is there a formula that would give me (for someone with good vision): If you are ZZcm from the screen then the line length should be XX and the font size should be YY.

I know there are a lot more variables involved such as Pixel Density and Font type, I am just trying to understand the main biological principles of readability online.

*Apologies if any of the terminology is wrong.

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Some useful links on these two questions for you: this one and this one –  JonW Jun 19 '13 at 14:35
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Does any one have any information about the width of the sharp focal area of the retina (the Macula)? I am trying to get an understanding of how this could impact the ideal length for a line of copy.

The fovea at the centre of the macula is the area of greatest visual acuity - the high resolution stuff that you need for reading.

However - it's pretty much irrelevant for deciding on line length. The fovea only covers about 2 degrees of the visual field. If you want to visualise that hold your hand out at arms length and stick up your thumb. The top-half of your thumb is about the size of what the fovea sees in high-res.

(Go read http://en.wikipedia.org/wiki/Fovea for more if you're interested).

So for normal body text you will find that at any particular point the brain will only "see" a single word at best.

The act of reading needs movements of the eye for it to work. See http://en.wikipedia.org/wiki/Eye_movements_in_reading for some more detail.

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Thanks good answer and obvious now I sit and think about it! Bit of a follow on: In that case is there any information regarding the width the eye can easily scan? I know there is lots of information about optimum line length but I wanted to understand if that is related to the physical makeup of the eye or primarily by other limitations such as visual processing capacity? –  Sheff Jun 20 '13 at 8:20
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@Sheff – if you want dive into science you could refer to cognitive psychology researches, there are a lot of theories and experiments. But they mostly are not expressed as rules-ready-to-use for UX design. That's why in practice experimental researsh like A/B testing are highly popular. –  Alexey Kolchenko Jun 20 '13 at 9:36
    
Thanks again. I think your right, I have gathered some psychological papers together on the subject, but wanted to check that the information didnt already exist in a "rules for ux format". –  Sheff Jun 20 '13 at 9:45
    
As is normal with cognitive psychology; you want to think in terms of data in the brain effecting incoming environmental data - and not the other way around. So it kind of depends what you already know: dummies.com/how-to/content/… –  PhillipW Jun 20 '13 at 9:53
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