need 300DPI to email photos. How do I change it from 72

Short answer: the dpi is set when you decide what size you're printing at.

Long Answer: Dpi means nothing in the digital world of your computer. There are no "inches" to have "dots per..." Size is measured in pixels. That's the same on your camera. It doesn't take 10 x 8 or 6 x 4 shots. It takes shots measured in megapixels. For instance 4,000 x 3,000 is a 12 megapixel camera.

Using that example, that shot from that camera has 12 million pixels. So that's how many "Dots" there are. To decide the ratio of dots per inch, you now need to decide the "inches" part. And that's printing. Print at 10 x 8 and the dpi will be 4,000/10 or about 400 dpi. At 6 x 4 then it's 4,000/6 or 660 dpi. Work the other way: Print at 300 dpi and the resulting image will be about 13 inches on the longer side.

So, your photo as a fixed number of pixels. Changing the dimensions of the print will vary the dpi, changing the dpi will vary the dimensions of the print.

For more see http://www.rideau-info.com/photos/mythdpi.html

Regards

TD

You need an image that is 1800 pixels x 1200 pixels so when printed at 300 dpi it will give you a 1800/300 = 6 inch x 1200/300 = 4 inch image.

First crop the image in iPhoto to 4 x 6. Next export out of iPhoto via File ➙ Export ➙ File Export with the following settings:

This will produce the image at 1800 x 1200 pixels. When viewed in Preview the Info pane will show this:

Preview sets the dpi at the screen resolution of 72 dpi but the image, if printed at 300 dpi, will print at 6 x 4. Dpi does not mean anything until a print size is selected for that image file.

Some apps will keep the dpi set at 300 dpi whereas others will convert it to 72 dpi for displaying on a computer screen. It's still the same image of 1800 x 1200 pixels.

If you must present the file so it reads 300 dpi when opened by some other app you'll need a 3rd party image editor that can set the dpi to 300 and save it without changing to the screen resolution.

The only apps that I'm familiar with that can do that are Photoshop, Photoshop Elements for Mac and GraphicConverter. The demo version will do it rather well. Use the the Picture ➙ Resolution menu option and set DPI to 800 without resampling.

For more on DPI read The Myth of DPI