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Understanding Image Resolution - Printer Resolution - Pixel Size - File Size - File Formats
DPI = Dots per inch
PPI = Pixels per inch
MP = Megapixels
MB = Megabytes
JPG = image file format
TIF = image file format
PSD = PhotoShop file format
RAW = Camera brand specific file format
DPI and PPI are often used interchangeably but really are 2 very different components. Image resolution has a number of contributing factors but simply refers to the level of detail in an image and is expressed as pixels per inch (PPI). For the purpose of printing, the greater the PPI, the greater the detail and the better quality of the image.
First, 72 PPI is all the resolution necessary for viewing images on your computer screen, tablet or smartphone. Since this results in smaller image file sizes, they open, email or upload much faster on all of your devices. This is almost always the default resolution your camera and/or phone uses.
Your camera's quality is primarily judged by the number of megapixels (MP). Always make sure to set your camera on the highest resolution possible. This will allow you to take the largest and best quality photos. On an iPhone under Settings - Camera - Format is where you can choose to either take more pictures at lower quality or fewer pictures with the best quality from your camera. Also, when taking pictures with your iPhone set it to HD (high definition) mode. Older generation iPhones have an 8 Megapixel camera, and newer models (iPhone 6s, 7, 8, X) are equipped with a 12 Megapixel camera.
An 8 Megapixel camera produces a file size that is 2448 x 3264 PPI. The image dimensions are about 8" x 11" at maximum quality (2448/300 & 3264/300). That same image can be safely printed at 16" x 22" with good quality (2448/150 & 3264/150) as the larger image size would typically be viewed from a further distance, and the human eye can only detect so much detail from a distance. Similarly, a 12 Megapixel (3000x4000/ 10,000,000) camera creates an image that is approximately 10" x 14" at the best possible print quality.
Megapixels refer to the maximum number of pixels that your camera is capable of capturing per shot. Each megapixel is roughly the graphic resolution of one million pixels. Typical late model Nikon or Canon DSLR or Mirrorless cameras have 16-45 MP. More pixels means that the size and quality of your photo will be better and sharper. However, keep in mind that the number of pixels is not the only factor in taking good quality photos. Unsteadiness, focus accuracy, ISO settings and/or poor exposure can also adversely affect image quality.
DPI (dots per inch)
This number strictly refers to the number of ink droplets a printer can place on paper or canvas per inch. The print quality gets better when there are more dots of ink printed on the surface. Our 11 color, fine art printers are capable of 2400 x 1200 dpi. That translates to 2.8 million droplets of ink per square inch. Each individual droplet can be as small as 4 picoliters. To give you some reference, an average drop of rain contains hundreds of thousands of picoliters! To summarize, a higher DPI printer places more ink droplets per pixel and results in the most accurate color rendition and detail.
PPI (pixels per inch)
This is the number of pixels per inch in your image. All that the PPI number will affect is the quality and maximum printable size. If there are too few pixels per inch, then the pixels will be very large and you will get a very pixelated image (jagged edges). EXAMPLE: An 8x10 image at 300 PPI = (8x300) X (10x300) = 2400 ppi X 3000 ppi is a total of 7.2 million pixels. Therefore, you need at least a 7.2 MP camera to create a best possible quality printed 8x10 image. That said, this "rule" is not written in stone. Since we view larger prints (20" x 30" or larger) from a further distance than you would a small print, you can get away with a lower PPI and still have the image quality look good to the eye. The larger your desired print size, the lower the PPI necessary for good quality. Large images printed at 100-150 ppi will still have good quality when viewed from a foot or more away.
What can you do to increase the printable size of a photo?
Interpolation is the process where your imaging software (PhotoShop, etc) artificially adds pixels to your file to increase resolution. It accomplishes this by "resampling" the original pixels and creating similar pixels . By doing this, your file size (MB) will definitely get larger but will not necessarily make your image look better at a larger size. Although resizing and resampling is possible in software like PhotoShop, we use more advanced software designed for this one particular purpose that produces a much better result. Unless one is attempting an extreme jump in size, it is better to simply print at a reduced PPI to get to a larger image.
The term megabyte (MB) refers to a unit of measurement which describes the size of a digital file. One megabyte is comprised of 1,024,000 bytes, or 1,024 kilobytes of digital information. When a picture is captured on a digital camera the resulting file must be stored on the camera's memory. Depending on the resolution of the captured image, and the file format being used (JPEG), an image will take up a certain number of megabytes of memory. For example, an 8" x 10" image size at 300 PPI will take up about 35MB of memory space unless compressed.
The 2 most common and universally accepted file formats for digital printing are JPEG (.jpg) or TIFF (.tif). RAW files are camera specific; (Nikon is .NEF, Canon is .CR2. etc.). If your camera does take pictures in RAW, you have the option of "capturing" the utmost data possible which will not be "interpreted" when being saved to your memory card. However, this format does require the user to first do some "pre-editing" of the image using PhotoShop or the software provided by the camera maker before saving to a more universal format. In this stage, you are working with every possible byte of data the camera "saw", allowing the most precise adjustments to color, size, clarity, tone, brightness, vibrance, etc. PhotoShop has its own specific format, ".PSD". It cannot be opened by most imaging software such as iPhoto, Paint, etc. but perfectly fine to send to us for printing.
Most consumer cameras save files as JPEGS to their memory card or hard drive space. The JPEG format was created for a specific purpose; It reduces (compresses) the file size to save storage space, whether saved to the camera or computer. This compression is accomplished by discarding some of the data that comprises the image. IMPORTANT: Every time a JPEG is opened and re-saved in JPEG format WITH THE SAME EXACT NAME, the image quality deteriorates from the compression as it is it essentially discarding pixels to reduce the file size. To prevent image degradation whenever you make changes to a photo, save it using a variation of the file name to avoid overwriting the original file.
For example, an iPhone X takes a photo that is 4032 x 3024 PPI. The JPEG file size saved to the camera takes up only 3MB of memory space but when uncompressed and opened in a program such as PhotoShop, the actual file size and amount of data captured in your image is about 35MB. A 3MB file can be sent via email, a 35MB file cannot.
In contrast, a TIF file is generally saved uncompressed and that same 35MB of data will take up 35MB of memory card or hard drive space. A TIF file can be edited and re-edited without any loss of image quality even if saving overwrites the previous version. It is possible to compress the file size of a TIF but we do not recommend doing this.
At Digital Arts Studio, our goal is to provide the best possible results from any image sent to us for printing. We are always available and happy to answer your questions or review a file or two at no charge. You can always directly upload (see top of toolbar menu) or email a file to us to evaluate and quote on any adjustments you'd like to see made. A "soft proof" can be sent via email for your approval prior to making a final print.