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Author Topic: Confusion over DPI in new camera purchases  (Read 1719 times)

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« on: December 27, 2006, 17:21 »
0
Just a question for you who have become digital masters.

My old Canon 300D transfers photos to Photoshop at 350 dpi.

I have noted that some cameras claiming to be 10 megapixels at RAW only have 180dpi. So surely, this is simply a 5 megapixel camera with interpolated pixels. Am I right.

I may be wrong, but if my camera captures 3072 x 2048 at 350dpi, this means i can double the image size and still have a print quality photo at 175dpi?

Can someone explain this for me ... make it the Idiot's Guide to Pixels please. I ain't real smart.


w7lwi

  • Those that don't stand up to evil enable evil.
« Reply #1 on: December 27, 2006, 19:04 »
0
For some reason, this topic seems to confuse many people, so don't feel bad about asking.

The primary source of confusion seems to be a misunderstanding of what DPI actually means.  It has nothing to do with the image size in the camera.  That number is fixed by the width and height of the sensor.  For example, your camera has a sensor that is 3072 wide x 2048 high.  These numbers represent individual pixels on the sensor.  Thus you have a 6.3 megapixel camera or, expressed differently, 6.3 million pixels in each image (3072 times 2048).  Short of cropping your image, you will always have this many pixels, regardless of how large or small the final displayed image may be.

DPI, on the other hand, simply means how many of these pixels will be displayed in a one inch length.  This in turn tells you how large (or small) the picture will be.  You will always have the same number of pixels in the image (6.3 million).  You just have more or less of them in a one inch long space. 

Again using your camera as an example, if you displayed an image at 300 DPI (that is 300 pixels displayed in a one inch length) your picture would be approximately 10" wide (3072/300) by 7" high (2048/300).  If you were to change this to 150 DPI, now you only have half as many pixels in the same space, but since you still have the same 6.3 million pixels to work with, your image is now 20" wide (3072/150) by 14" high (2048/150).  The downside to large images is they tend to lose resolution as fewer pixels are compressed into a given space.  That's why the vast majority of commercial print agencies specify 300 DPI as their standard.  That results in an image that is crisp and clean (assuming it is to begin with).   :>)

Hopefully you were able to follow this and I didn't confuse you even more.

« Reply #2 on: January 22, 2007, 06:42 »
0
thanks for the clear explanation w7lwi

it seems there is no end of confusion on this matter, and you put it very nicely. Much better than i could have.


 

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