How to find dpi for photos

I need to send some photos a particular way: 5"x8" at 300dpi.
First, how do I find out at what dpi and size a current photo is?
2nd, how do I change the original photo if it is not the dpi and size I need?

Thank you for any light you can shed on this. I'm tech challenged and don't speak 'computereeze", so I'd appreciate answers in layman terms for someone who was born before home computers were common.

To find the DPI: If you open it in Preview, click Tools > Show Inspector. The information you seek will be in the resulting popup window.

To resize: Open in Preview and click Tools >Adjust Size

ETA: If you're interested in basic image editing, you might find this page helpful.

DPI info

Thank you!
I need a clarification regarding changing the sixe of the photo in Preview.
When entering the width and length, I can't just put in 5" x 7" because the boxes automatically seem to adjust to other dimensions. If I "unlock" it, then the result looks squished.
I'm obviously doing something wrong....

DPI has nothing to do with anything except for when printing. DPI=dots per inch. It is the value needed to imput for your printer. PPI on the other hand, is pixels per inch. That is the value used for calculating what is going to be on a display. Most if not all displays default to 75 PPI anyway, so this is not the value you should be concerned with if you're sending someone a photo via the web etc. However, if you output a photo at something like 300 PPI, what that means is that whomever has this photo can enlarge and print it without it pixelating. If you tried to enlarge a 75 PPI photo, it would pixelate, obviously.

You should be concerned with the dimensions/resolution, if sending a photo to someone, digitally. So then, if you know the resolution of their monitor, you can resize the photo to fit that. For instance, my MacBook Pro's screen resolution is 1440x900. Typically, a camera will save a photo at greater resolutions than that, so you have to resize to fit.

Doug

Sounds like he's submitting something for print. That's about the only time I've seen a per inch requirement.

Ah, well if it IS for print, then that's fine. Setting output to 300 DPI is absolutely recommended for very large prints, but it also depends upon other things as well. 240 and even 200 DPI would be fine, depending on the size and how close one would be standing in relation to the print.

The BEST and most accurate way to get a good print is to physically crop your photo to the size of the image you want.

Here's an interesting discussion, though it goes in circles a bit: DPI for printing? - Tech Support Guy Forums

I recommend doing some research on this topic before anything else. This is a subject that even professional graphic techs and artists get totally botched up. Not very black and white nor is it easy to grasp.

Doug

Doug b said:

I recommend doing some research on this topic before anything else. This is a subject that even professional graphic techs and artists get totally botched up. Not very black and white nor is it easy to grasp.

Click to expand...

So true. I have to study it up every time. I made a book for my wife from photos she took, using Apple's system in Aperture. Fun and worked out well, except two images blurred in print, though looked fine on screen. The software warned of likely print quality problems in some, and the warning disappeared when when I used a higher resolution, larger image. It missed those two. So even their software expert checks can flub this.

DPI v PPI

I am having a similar issue - I need to provide images in 600 dpi. When I go to tools > Adjust size, I only see a way to change the PPI. If I understood the advice in this thread, I should go to "Resolution" and adjust it to 600. There are only two options, however; pixels/inch and pixels/cm. I have never seen 'dots' as a measurement before, I'm not sure how well it correlates with pixels. Therefore, my question is whether adjusting PPI to 600 will be the same as 600 dpi? Thanks.

Same Problem

Same problem. I have a number of photo's that I sent to my publisher for inclusion in a forthcoming book. They range from 72 to 150 DPI. Their photo people say they need 300 DPI. When I go to to "preview" and change to 300 the image becomes extremely large. How would I handle this and send them images sized at 300 DPI?

Larry

@iholcombe, what, exactly, do you mean by "becomes extremely large?" If you don't change the display zoom (PPI) and double the DPI, the image will expand to four times the size. Each "dot" or pixel becomes four. So on your screen, if you hold the same zoom level, the image will now be four times as large, in area, as it was originally.

In any event, you are going to have an adventure with what you need to do unless you have the original pictures in a higher resolution. The challenge with going from lower DPI to higher DPI is that you cannot add information to a picture that isn't there. Consider this: if you increase from 150 DPI to 300 DPI and try to hold the same size image, what goes into those extra 150 dots in each inch? What the software will try to do is to use the existing dot/pixel and simply duplicate it in each axis. It will end up expanding each dot to four dots. But the new "dots" now are larger, so you get what is called "pixelation." Each pixel (picture element on the screen, or "dot") now appears to be four times as large as it was. So if your publisher needs 300 DPI images and you send them 150, or 150 stretched to 300, the printed images could be pixelated and show "jaggies" or rough edges, in the printed picture. It all depends on how large the printed picture ends up in size. (The same argument applies for the 75 DPI images, except that each dot/pixel now expands to 16 dot/pixels (four in each axis).)

Thanks for the information and what you told me is what I expected. I took a photo that I made with my iPad4 and noted that the DPI was only 72. When I changed it to 300 instead of a photo of two people all I got was an arm. Then reading your reply I understand more clearly the problem. They will have to make out with what they have.

Thanks for your help.

Larry