When I put images from my digital camera to iphoto they register as jpegs with only 72 dpi. How can I get them to be 300dpi?

Question

I have a mac osx 10.6.8. iphoto 11 version 9.2.3.
My camera is a Canon Power Shot SX700HS.
I want to send high res. photos to a company who require 700dpi.
When I attach my camera to my mac they turn into jpgs and then
when I try to send them by email they come in at 72dpi.
Can anyone advise me on the best way to send these photos?

Answer 1

I notice that images on Image Capture come in at 180dpi.
180 dpi isn't terrible, but defined edges that aren't perfectly vertical or horizontal will show jaggy pixel stepping. Not real obvious, but you'd see it. Overall, the picture would have a slightly pixelated look at that resolution.
The maximum size picture your camera can take is 4608 x 3456 pixels. If you set the resolution to 300 dpi, that makes it 11.52" x 15.36". So without scaling/resampling the image up to keep it at the vendor required output resolution of 300 dpi to get larger sizes than that, your camera is best for 11" x 14" images or smaller.
Whatever image editor you use to size your images, you need to finalize them at 300 dpi, which is what your service requires. If you were to send them for example an image sized to 16"x20" at 120 dpi, what their system will automatically do is scale the image to 300 dpi before printing it, which will cause pretty noticeable softening and pixelation on the final print.
For that camera, it makes more sense that it comes off the camera as 180 dpi. That must have been Mail that was scaling them down to 72 dpi what added as an attachment.
So how do you change the resolution? One way is with Preview. Here I'm working with a test image to show the difference in what happens depending on how you size it. I've purposely changed this image from it's original 300 dpi setting to 120.
Open the image and choose Tools > Adjust Size.
This image is 1816 × 1419 pixels. At 120 dpi, that makes it 15.13" x 11.83". If I reset the resolution to 300 dpi, not resample, the size changes to 6.05" x 4.73". It does that because I turned Resample image off. When you do that, the image doesn't physically change one bit. It still has the same number of pixels in the height and width it started with. All you've done in such a case is tell the output device to use 300 of the pixels per inch instead of 120. Since you're using more of the same original number of pixels in less space, it becomes effectively smaller.
Now let's say I want this image to be output large enough for an 11" x 14" print, but also need it to be 300 dpi for the vendor. To do that, I need to resample it.
There isn't enough pixel information in the image to do that without resampling up. Or said another way, adding more pixels than existed to begin with. Notice the scale percentage is 232.6, which directly translates to adding over twice as many pixels to the image (232.6 percent) as it started with in order to achieve 300 dpi at 11" x 14". Notice too of course that the size of the file will go from 7.8 MB to 55.7 MB (sizes are normally displayed as how it would be for an uncompressed format such as TIFF). The image will now have 4223 x 3300 pixels instead of its original 1816 × 1419. Such large resampling going up also makes most images soft and pixelated looking. Though since CS6, Adobe drastically changed the scaling algorithm. Larger up-sampling of images is much better than it used to be. You can scale most images up quite a ways before they start looking really bad.
I know this is a lot. Hopefully, not too confusing.