Newbie question

[drew2285]

Hey guys,
I'm fairly new to photoshop and scanning.  However, I was working on a project and was scanning in 8x10 images from a book.  I want to double their size so I scanned them in at 720dpi.  When I open up the images in photoshop it shows the images as the size of 8x10 at 720dpi.  My question since I scanned them in at 720 already can't I change the ppi in photoshop to 360?  If I doubled their size to 16x20 in photoshop and printed at 360 would that still give me the right resolution?  
thanks

[BernardLanguillier]

Hey guys,
I'm fairly new to photoshop and scanning.  However, I was working on a project and was scanning in 8x10 images from a book.  I want to double their size so I scanned them in at 720dpi.  When I open up the images in photoshop it shows the images as the size of 8x10 at 720dpi.  My question since I scanned them in at 720 already can't I change the ppi in photoshop to 360?  If I doubled their size to 16x20 in photoshop and printed at 360 would that still give me the right resolution? 
thanks
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You can use the PS "size" function to change the DPI of the image without doing any re-sampling.

1. Just un-check the lowest box in the Size panel.
2. Change de DPI from 720 to 360

-> The will double the output size to 16 x 20.

Regards.
Bernard

[Gabe]

Well, 16x20 is 4 times the size of 8x10 to start with...

Mike.
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What a completely unhelpful answer to a very straightforward question.

Given that 16x20 is a 200% enlargement over 8x10, I'm curious to know what size of print you suggest the OP make instead.


drew: as Bernard says -- just make sure you have "Resample Image" unchecked and you'll get the results you're expecting.

[jbrembat]

I suppose you have a 5760x7200 image.

 8x720=5760 pixels
10x720=7200 pixels

The image PPI does not exists. An image is a grid of values, so an image has a width and a height (in pixels units), but no density.
The density comes in when you want to see an image. The values must be layed down on a device and PPI acquires a sense.

Lets go on a bit. The images are stored in files of some format (jpg,tif,bmp...). Well, all these formats provide a place to store a PPI value.
The stored value is "for documentation purpose". The print format is selected on printer driver.
If you change the stored value, you are not changing the image grid.
Remember an image has width and height.
PPI are very significative when you have to print, but the right value is fixed by the device and used by the driver software.
In other words: you can fix the print quality (and bordless print if supported), you cannot chose the PPI, printer software fix it depending on your request. If you send an image to a printer with a different PPI value, the driver will resample the image to get the right PPI value.

For more information  http://www.photoresampling.com
You can find an explanation of PPI and DPI and download for free "Printer data" utility.

Jacopo