DPI is an abbreviation for “Dots per Inch”. DPI might also be called PPI (pixels per inch). Simply put, it’s a measurement of the number of dots in a square inch of an image. (You can find a more detailed explanation of DPI and PPI on Wikipedia.)
You will need to have at least 300 DPI in an image when you print it so that it comes out clearly. An image printed at less than 300 DPI will come out somewhat blurry. You cannot add more resolution to an image (unless you have the original physical photo and can scan it at a higher resolution). You can, however, resize an image to make it smaller with a higher DPI. Basically, what you are doing is restructuring the placement of the dots so that they are more densely situated in the smaller image version.
One confusing factor about images on monitors is they will only appear at a screen resolution (generally 72 DPI) even if you save them in a higher resolution. This is why it doesn’t make sense to put higher resolution images on a website. You would just be wasting space on the server (that is, the “big” computer storing your website files). Whoever runs the server you are using will appreciate your taking the extra effort to keep images around 72 DPI for blogs and websites.
Photoshop is a robust application (program) used for editing photographs among other things. You could spend a lifetime learning all the ins and outs of Photoshop. It is a wonderful program if you need to get into some very high-tech editing.
This application is also quite expensive. It would not be considered a “consumer level” product. This is not to discourage you from learning and using Photoshop, but if you just want to do simple editing, consider using one of a few more simple products. Some of these are free. These include but are not limited to Photoshop Elements, Picasa, and Gimp. If you just want to get rid of red eye, straighten a horizon, and adjust the lightness/darkness of a photo, these programs might be what you’re looking for.
A PDF, portable document format, is a great format to save documents that you would like to send to someone else. The great thing about PDFs is it saves the formatting, including the fonts, layout, photo placement, etc. Normally, when you save a document in a format like .doc (Word’s file format), someone viewing your file on their computer must have the same fonts that you have used also stored on their computer in order for it to appear properly. If that person does not have the font, then their computer will choose a replacement font.
A PDF is basically a picture of a file that you have created. You can also choose which resolution you would like to use when you save your file as a PDF. You would want to save it at 72 dpi if someone was just going to view it on their screen monitor. You would want to save it at 300 dpi if someone was going to print it.
How to change the resolution of an image
Although different programs might handle this slightly differently, most will have you go to your menu bar and select “Image size,” which you will find after clicking on “Image” in the main menu bar. You will see a text box to enter the desired resolution. You will want to keep the proportions of the image the same so make sure that the ‘constrain proportions’ box is checked. Selecting that option will ensure that your photo does not get stretched and distorted. (NOTE: in Word 2007 click on the image and then follow Picture Tools – Format – Compress Pictures)
Changing the resolution restructures the way that the pixels (dots) are placed within the image. Increasing the dots per inch will move the dots closer together, which will increase the quality of the photo. The photo will be smaller because the dots have moved closer together.
Please note that you cannot simply “add quality” to a photo. For instance, if you have a 4 X6 inch photo that is 300 DPI, you cannot simply type in 450 DPI in the text box. You can’t simply add DPI to an image. Even if the computer accepts the higher number the photo won’t actually increase in DPI. If you’d like to have a higher DPI at a 4X6 size, you will need to either re-take the photo with a higher setting in your camera or re-scan the photo at a higher DPI.
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