The subject of digital photos can be confusing. This FAQ is based on the information on this website and actual questions I've been asked. Just click on a question below to go to the answer:
Q. A Magazine wants my pictures at 300 dpi - what do they mean?
- A magazine wants my pictures at 300 dpi - what do they mean?
- A magazine has rejected my photo because it's not 300 dpi - why?
- My new camera takes pictures at 72 dpi, my old one was 180 dpi, why the difference?
- What does a camera's "Megapixel" number mean?
- I tried to change the dpi on my photo and now it's huge and blurry - why?
- The filesize of my new "saved as" photo is smaller/larger than the original - why?
- How many pixels per inch do I really need for photographic quality?
- Why do people think DPI is a measure of digital image resolution?
- When I have my pictures printed, part of the picture gets cut off, - why?
- When change the DPI number in my photo it gets bigger/smaller.
A. The request for a 300 dpi picture is in itself meaningless (there is no such thing as a 300 dpi, or any dpi digital photo). What they want is digital image of sufficient pixel dimensions to be able to deliver 300 pixels per inch at the dimensions of the printed image. So, for the "300 dpi" request to be meaningful, you'll have to know the intended output (paper) dimensions. For details see What Printshops Really Want
Q. A magazine has rejected my photo because it's not 300 dpi - why?
A. Either because it really isn't big enough (in terms of pixels) or they don't understand that the dpi setting in a digital photo is meaningless in terms of the digital photo itself. To see what pixel size they really need, see: What Printshops Really Want. If the photo is actually of sufficient digital pixel size and they just don't understand the dpi setting, then simply change the dpi to 300 (without re-sizing the photo) and send the "new high-res version" back to them. See Changing the DPI of a Photo
Q. My new camera takes pictures at 72 dpi, my old one was 180 dpi, why the difference?
A. Camera manufacturers seem somewhat arbitrary when it comes to the dpi setting of the original photo (possibly because they know it's a meaningless figure). They could park any number in there, 72 is often chosen since that's the original "screen resolution" of computer monitors (although the dpi setting a digital photo has no impact on that). The real digital "resolution" of your photos are its pixels, the total of those is expressed as megapixels (see next question). If your new camera is 8 Mp and your old camera was 6 Mp, then your new camera is taking higher resolution photos than your old camera (regardless of the dpi setting which is irrelevant). See the What Is a Digital Photo page for the description of what digital photo resolution actually is.
Q. What does a camera's "Megapixel" number mean?
A. A camera's megapixels are its largest photo's horizontal pixel dimension multiplied by its vertical pixel dimensions. So, for example, if your camera can take a photo that is 3504 pixels wide by 2336 pixels high, the total number of pixels in the image is 8,185,344 (3504 x 2336). This would be referred to as an 8 megapixel (or perhaps 8.2 megapixel) camera. Megapixels are the "resolution" of a digital photo. See the What Is a Digital Photo page for the description megapixels and digital photo resolution.
Q. I tried to change the dpi on my photo and now it's huge and blurry - why?
A. Because many software programs default to re-sizing the photo when the dpi number is changed. So when you changed the setting from 72 to 300, it re-sized the photo by a factor of 4.2 (72 x 4.2 = 300). If the original photo had 3000 horizontal pixels, it now has 12,600 pixels. Digital photos don't take well to this amount of enlargement without special techniques. To change the dpi without re-sizing the photo in the process see: Changing the DPI of a Photo. To actually change the pixel size of the photo see: Changing the size of a Digital Photo
Q. The filesize of my new "saved as" photo is smaller/larger than the original - why?
A. You are likely saving the file in either a different file format (i.e. RAW - JPEG - TIF), or, if the same file format, your software program is using a different file compression than the original. Most commonly this is with JPEG. A high quality JPEG from a camera is being saved with a quality factor of about 97 (approx. Adobe scale 11). The new save from your software program might be defaulting to a quality factor of about 75 (approx. Adobe scale 6). If your original photo was say 3 megabytes (Mb) in file size from the camera, saving that exact same photo (with no change to pixel dimensions) at quality 75 would result in a 0.6 Mb file size. This will also result in loss of photo quality. See the Digital Image Filesize section on the Digital Photo File Types Page
Q. How many pixels per inch (ppi) do I really need for photographic quality?
A. This is completely in the eye of the beholder, its a subjective figure. Magazines usually ask for 300 ppi, I think that 200 ppi is generally fine, you may find that 100 ppi is okay for you. It's affected by your perception of quality and viewing distance (a poster, which is viewed from a distance of 6 feet doesn't need as many ppi as a magazine photo that is viewed at a distance of 1 foot). Quality is not linear, 300 ppi is not "twice as good" as 150 ppi. For the professional, quality includes shadow details and native file level of detail (resolving power) - these demand higher ppi. The ppi discussion includes other factors such as the quality of the recording device (your camera's optics & sensor or a scanner's sensor); the size (in pixels) of the digital image; the digital format it is stored in (lossless vs lossy compression); and the technical proficiency and the "eye" of the photographer. But even with those factors at their best, determining a required ppi is not cut and dried. For some discussion on this see The Arithmetic of DPI
Q. Why do people think PPI (or DPI in older software) setting is a measure of digital image resolution?
A. Mostly it's the fault of software manufacturers who have labelled the PPI setting in their program under the title of "Resolution" within their "Image Size" dialog box. The PPI figure is used as a conversion factor by some software to either re-size the photo and/or set its ppi output to paper. These are legitimate (and can be very handy for people such as graphics designers), but with the logic of 2+2=5, people have erroneously concluded that because PPI can change the photo in these ways, that if the same photo had two different PPIs, the higher PPI photo would represent more "resolution." In reality, if two digital photos have the same pixel dimensions, they are identical in digital resolution no matter what the DPI setting in the photo. PPI, unless used to re-sample (re-size) the photo, does not create "resolution" - but today many people will still think that if they have two photos of the same pixel dimensions, one set to 72 ppi and one set to 300 ppi, that the 300 ppi photo is "high res" and the 72 ppi photo is "low res" when in fact they are "same res". This high res/low res misconception is one of the reasons I started this website.
Q. When I have my pictures printed, parts get cut off. Why?
A. This is due to the difference between the aspect ratio of your digital photo and the paper size of the print. The aspect ratio is the horizonal dimension of a photo related to its vertical dimension. For instance, a 6" by 4" photo has an aspect ratio of 1.5:1 (or 3:2 in whole numbers). A 7" x 5" photo has a ratio of 1.4:1. With a digital photo the aspect ratio is calculated using its pixel dimensions. A 2048 x 1536 pixel size photo has an aspect ratio of 1.33:1. One with a 3504 x 2336 dimension has a ratio of 1.5:1 (which, not coincidently, is the ratio of 35mm film). If an image with a ratio of 1.33:1 is printed on 6" x 4" paper (1.5:1), to fill the 6" x 4" paper the printer can either add whitespace around the photo, or crop (cut off parts) of the photo to make it fit. Most printing services do the latter. To edit the photo yourself so that it fits onto the paper size you wish to use, without cropping (by adding whitespace), do a Google search for "photo aspect ratio" and you'll get lots of tips. For more information see the Aspect Ratio of a Photo on the Changing Size page
Q. When change the DPI/PPI number in my photo it gets bigger/smaller.
A. One of two things could be happening here.
(1) If your software is set to re-sample (change digital size) of your photo when you change the PPI number then it is in fact getting bigger or smaller. For instance, if the PPI is at 72 and you change it to 300 with the re-sampling (re-sizing) option of your photo's software turned on, it will enlarge the photo 4.17 times (for instance, go from 1000 pixels to 4167 pixels). Note that this is exactly the same as leaving the PPI at 72 and enlarging the photo from 1000 pixels to 4167 pixels. See changing DPI/PPI for more info.
(2) If your software is set to not re-sample (either re-sizing is turned off, or the ppi is being set independent of re-sizing) it may show in a section called document or paper size a change of size. Note that this is not a change of digital size, your software is simply doing the math of pixels per inch (see Adobe Photoshop Image Size Dialog Box). So, for instance if your original photo is 1000 pixels in width with a ppi set to 72, the document/paper size box is going to show 13.889 inches (1000 pixels divided by 72 pixels per inch). If you change that 72 to 300 (leaving the image at 1000 pixels) that box is going to show 3.333 inches (1000 pixels divided by 300 pixels per inch). Your photo has not gotten smaller, the digital photo itself has not changed, your software is simply showing the printed size results of using different pixels per inch. Note that if you printed your 1000 pixel image with its ppi set at 72 to paper at 3.333 inches in size, it will be printing at 300 ppi (no matter what the dpi setting in the photo is). For more info on this math, see the arithmatic of dpi.