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iphone 5 screen size and converting points to inches

literature says the screen size is 568 x 320 points
i assume this because of the compactness of the retina display points and the typical conversion of 1/96 pixel = 1" does not apply

if i convert points to inches 1/72" = 1 point

568 x 320 points
7.888 x 4.444 inches

thats obviously not right what am I doing wrong?

need to know the absolute screen size (mm, cm, inches, etc) so I can draw a rectangle in photoshop respresenting the actual screen size.
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glenn_r
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glenn_r
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GaryCommented:
Screen size = 1136 x 640 px
Pixels per inch = 326

1136/326 = 3.4846625 inches
640/326 = 1.963190 inches
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glenn_rAuthor Commented:
then how do they get 568 x 320 points
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GaryCommented:
How does who get? The screen resolution is 1136 x 640 px.
Maybe some confusion between what is a retina display
The higher resolution will be used when for example an image is displayed - you would have two copies of an image, one for normal displays and another that is twice the dimensions for retina displays.  On a retina device both images would be the same dimension but the retina version will have a lot more detail due to the doubling of the pixels.
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glenn_rAuthor Commented:
most illustrations show the size of the iphone 5 display at 568 x 320 points
that where i went wrong
if you convert those points to inches i produces different numbers they you gave above
any ideas on how they converted those points to inches?
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GaryCommented:
If you want to use those dimensions then you need to halve the ppi - 326px becomes 163px
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glenn_rAuthor Commented:
Not sure if I follow you on that one

ppi is 326 so what the calculation?
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GaryCommented:
To the point.
The screen resolution is 1136 x 640 px - there is no if's and but's about it, that is what it is.
There are 326px per inch.

If you want to make your calculations based on a non retina display resolution then you halve everything including the ppi.
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glenn_rAuthor Commented:
got the ppi explantion 100%

confused at why you halve everything for the point conversion
i understand that ppi is lxw and if so a square root of that number would produce the x & y size
can you articulate your work from a different angle?
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GaryCommented:
I don't know how better to explain it.
On a retina display 1 pixel is actually 4 pixels - 2 across and 2 down

Take an image that is 1136 x 640 px (this would be how you would size an image for a retina display) - every pixel of the image uses 1 pixel on the display.

Take another image that is 568 x 320px - for every pixel of this image there are 4 pixels used.
This would also be the case for text and anything else that is not retina ready.
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glenn_rAuthor Commented:
got it
keep thinking in terms of lxw or 2 px horizontal and 2 px vertical
make sense now
thanks for the explanations
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