Excellent explanation Chuck.
Dick
Chuck Norcutt wrote:
> Bob, you apparently don't understand the resolution number either. The
> 72dpi (or any dpi number for that matter) is not something that's
> physical until it's tied to a print size. There may be a tag in the
> EXIF image data that says 72dpi but that's all it is... a tag. You can
> change it to anything you want but it has absolutely no physical effect
> on the image. It's an advisory value to a display or print system
> (should such system choose to honor it) that says print or display this
> thing at whatever physical dimension you get by dividing the horizontal
> and vertical pixel counts by the resolution value.
>
> Your camera does not shoot images at 72dpi. It shoots images at
> whatever the native image size is... in pixels. Assuming you've chosen
> the full size image it shoots images that are 2592 x 1944 pixels.
> Whether the resolution tag says 72dpi or 7200 dpi doesn't change the
> image one bit... it's still 2592 x 1944.
>
> That said, one needs to be careful in PhotoShop with that dpi number.
> If you change it in the image size dialog *and if* the "resample image"
> option is checked it will have an effect on your image. If you enter
> that dialog with an image that is 2592 wide with a resolution value of
> 72dpi the print size of the image will be shown as 2592/72dpi = 36"
> wide. If you then change the resolution to 144dpi without changing the
> print size and also have the "resample image" option checked you will
> increase the size of the image to 5184 pixels. It will double the
> number of pixel columns by interpolation.
>
> Just remember that the image itself is nothing but *dimensionless*
> pixels. Dimensions only come about when we decide at what density those
> pixels will be diplayed or printed. Photoshop will happily add or
> delete pixels if you allow it to in order to meet your request for a
> certain size image at a given resolution.
>
> Chuck Norcutt
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