View Full Version : A question about saving an image.

04-14-2007, 04:47 PM
Hey all!
My camera shoots at 180dpi, after editing the image is there any advantage to saving it at 240dpi? On the one hand I feel it's just a waste and a larger file, on the other I think maybe it interpolates the info and higher dpi may be beneficial.

04-14-2007, 05:55 PM
Actually your camera does not shot at any DPI. DPI is a term that really only relates to output. Until you print an image it is just an imaginary value that relates the number of pixels in your image to a length, in this case inches, from Dots Per Inch. Your camera has a fixed resolution of pixels. It only has a physical dimension when printed. If you take picture that is 1000 pixels wide and print it 4x6 then it is 167 dpi in the wide dimension. The same picture at 8x10 is only 100 dpi.

Think about it if it was really related to your camera's image and it records at 180 Dots per Inch even a VGA camera (1280x768) would have to have a sensor 7 inches wide to record those pixels. Your sensor on the camer is probably lesss than an inch across. So the actual dpi of the recorded image is more like 2000-3000 dpi. Now think about this further, is the DPI of the recording based on the area of the sensor that the image hits or is it relative to the area of the object being recorded? If it is relative to the actual subject then it would different for a landscape from a mountain top, where the edges of the image might be miles apart, and an image of a close-up of a fly that might be 1 inch wide.

04-14-2007, 06:37 PM
I'm still confused:confused: ... how about this approach.
When I take an image from my camera to the PC directly, then look at the properties it tells me the image is 2816 x 2112 with a resolution of 180dpi.

Is there any positives or negatives to saving it at 240 dpi which is the default setting on the editing software? Or for that matter will it have any effect at all?

04-14-2007, 06:50 PM
2816 x 2112 with a resolution of 180dpi..

The 180dpi is a default value that does nothing to alter the fact your photo is 2816 pixels x 2112 pixels.

The final size of paper you print the image on causes the pixels per inch to be calculated as kgosden points out.

The only other thing is affects is when you open the image in an editor, the size of the image in inches (or centimeters) will be shown depending on this value.

Ie open it in photoshop (or whatever) and the document size will be shown as 2816/180 = 15.6 inches by 2112/180 = 11.7 inches

Of course as soon as you tell your editor to print that photo on a 6x4 then the ppi changes totally and the 180 figure is ignored.

If you get the image printed at a shop or online its possible that poorly written software or a half trained basic wage "print technician" will see the 180 figure and go .. sorry not big enough to print..

Ie for your image you could print it at 6x9 inches and get 312ppi, however some software may not bother to do the calculation and just read 180 and refuse to print it or the assistant may just read the 180 figure and not understand what that means ..

04-14-2007, 08:20 PM
Thanks! So I guess the bottom line is... it doesn't matter. I wonder why there is even an option to change the number in the software then?

04-14-2007, 10:36 PM
The value really is not in the software, but in the file header of the image. It is there just like many other fields, to convey information about the image. In the case of DPI it is there as part of the standard to help some software that might use that value. There is a value for which camera took the photo, but that really doesn't have anything to do with the final print either. If you are aware of the printing process you will be using setting a DPI that 'fits' that processes requirements might help you during the editing process if you tell the software to show you calculated output dimensions in inches.