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Photography Question 
Linda Sandbo
BetterPhoto Member Since: 10/29/2010

How does DPI relate to KB or MB?

I am trying to understand the relationship of DPI to a digital image size. What does it mean in KB or MBs??
For example, if an image is 1.2mb, what is its DPI???

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4/23/2011 10:30:23 AM

Jessica Jenney
BetterPhoto Member Since: 12/2/2005
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  Depends on your camera. Look at your images and the DPI should all be the same number. You can also change it if you need to.

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4/23/2011 11:19:59 AM

Gregory LaGrange
BetterPhoto Member Since: 11/11/2003
  Kb/Mb is file size. That's different than image size. You can increase a file size by doing a whole bunch of Photoshop things to it, but not do anything to the image size.
Your DPI is your resolution. So you can take a 5mb file with 500 dpi. And we'll say that image at that resolution is a 4x6. You can increase the image size all the way to a 20x30 just by typing in new dimensions, but your resolution will go way down to about 100dpi. But the file size will stay the same.
Different example: You have a 5mb image that you want to upload to a website. Your website displays images at 480x720 pixels. What you do is downsize the image by getting rid of pixels that aren't needed. In that case, you're getting rid of information in the file, making the file smaller. And also making the image smaller.
Third example: Same 5mb file, but you can retain the image size. And you can reduce the file size by saving it, and in the window where it has the 1-10 option of saving the file, you pick 2 instead of 10. Your new file will have the same image size, but a smaller file.

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4/23/2011 12:44:02 PM

Linda Sandbo
BetterPhoto Member Since: 10/29/2010
  I think I got it thanks to you and Jessica... I did not understand the basic principles of dpi. Someone had requested an image from me and said must be at least 300dpi.. So, I thought I would go to the best source to explain that to me... Thanks to you both.. I get it now..and can apply that to print size... I learn something helpful almost each day on this site... You guys are great!!!!!!!!!

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4/23/2011 1:32:27 PM

Lynn R. Powers   The only place DPI is important is when printing. Your meta data will tell you the dimensions of the photo in pixels. For the ideal prints, the DPI will be from 300-360dpi. I found that I can print down to 200dpi and still have a good but not ideal print. I tried once, only, at 140dpi and it was terrible.
Take the pixel count for each dimension and divide by 300 and that will give you the size print that you can make at 300 dpi.
Example: Pixel 3750 x 2500
3750/300 = 12.5 inches
2500/300 = 8.33 inches
That is the largest photo you can make at 300 dpi. I can routinely get that with a Canon 20D with an 8MP sensor even though the photo size 9.375 MP.
If you are unable to get the size photo that the customer wants at 300 dpi, it will be necessary to either down-rez or up-rez your photo. It can be done with Elements or CS, but I do not know how, so I send it out to get it done to a pro lab - NOT WallyWorld, Costco, or the local one hour labs. There are also programs such as Genuine Fractals that you can have as plug-ins that make it much easier.
By the way, that is generally the DPI required as the minimum resolution for a print when they are to be used commercially.

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4/23/2011 5:06:47 PM

Linda Sandbo
BetterPhoto Member Since: 10/29/2010
  Thanks Lynn, I've never had to worry about "dpi" before and I really appreciate the input from all of you.

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4/24/2011 4:53:01 AM

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