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Originally Posted by StuHaluski
I have since realized while I made my smaller image at 20 dpi it still views as if it were 72dpi, so in that respect photographerC is correct.
However one cannot just ignore dpi because it still affects size.
It is a very confusing thing to learn.
Sorry if I added to the confusion, Im sure I did.
Stu
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I'm not sure how folks get confused on this one but they do. I've encountered several posts where folks had long discussions about dpi in images, when images don't have a dpi.
Maybe the easiest way to make that point is ask you where you set the DPI in your camera. You don't. Your camera is going to generate a certain number of megapixels. The image size coming out of my camera (when set to maximum) is 3008 x 2000 pixels. DPI means nothing because the resolution is the width times the height.
If I went to print this image at an 8" x 10" size on a 300 dpi printer, it would be okay because 10" x 300 = 3000. I.E. my image has enough resolution to cover the area. Okay but my Epson printer boasts that it has 14400 DPI. So when I go to print my image - 10" x 14400 = 144,000. Whoops. I'm about 143,700 pixels shy of maximum output. Is it a problem? No. The computer will work everything out and print the image. The quality factor is often determined by what the eye can see not what the printer can produce.
The problem comes in when I have an image that is 100x100 pixels and someone wants to print it out 16"x20" at 100DPI (or 2000 pixels on the longest side). It's going to look like crap because I don't have the resolution to print it at that size at that DPI. You can make the image bigger but the computer is inventing the information contained in the extra pixels. The image will look pixelated when you go to look at it (like looking at an image at 400% on your screen).
The reason I mentioned 100 DPI is not because that's the optimal setting. Remember most of the time the DPI is going to be the result of fitting your image to the desired output - not the other way around. 100 DPI is what most folks agree is what the eye is capable of seeing and is the minimum you should have for a quality image at all sizes, except for very large images where you can count on the natural viewing distance to compensate for lower resolution (like billboards).
So using that as a benchmark the largest image I could make from my camera would be 30" x 20" and expect to have reasonable quality. Larger than that and you run the risk of seeing pixellation if you are not at a natural viewing distance from the image. If you have a D200 which is a 10MP camera you can expect to go even higher and achieve the same quality result
So, my camera which has no DPI can print to a 30"x20" print on a printer (that prints at 14400 DPI) and still look fine because the resolution of my image works out to be about 100 DPI. The printer is still printing at it's hardware resolution - it's just resizing the image to fit.
So DPI is only significant when printing but when sizing an image for the screen - it's irrelevant. You just use the actual pixel count to determine size. Unless you have a .5 megapixel camera, you will almost certainly be downrezzing your image to fit the screen (which rarely means a loss of quality).
As far as editing your image goes, you should almost always work with the full image from the camera unless you need to crop for reasons of distortion or composition.
Checking the DPI is sometimes necessary, but setting it never is.
Hope this helps and happy shooting.