Image format and resolution
In almost all cases, you will be working with photos created by a digital camera. It is rare to use scans of physical photographs, negatives or slides from an analogue camera.
A digital camera has a light-sensitive sensor (the CCD or CMOS sensor) with thousands of tiny cells that convert light into pixels. The maximum number of pixels produced by the CCD/CMOS sensor is expressed in megapixels (millions of pixels). The latest digital cameras can capture images of up to 50 megapixels.
If a digital camera with a sensor with a maximum resolution of 5616 x 3744 pixels (= approximately 21 megapixels) takes a photo on the highest quality setting, the photo resolution will also be 5616 x 3744 pixels. This refers to the size of the photo, i.e. the number of pixels of which it is composed. The more pixels, the finer and more detailed the photo can be.
This says nothing about the dimensions of the digital photo, which depend on the medium for which the image is used:
- Monitor: resolution 72 DPI
- Print: resolution 150 DPI
- Professional printed matter: resolution 300 DPI.
The resolution is expressed here in the number of pixels per inch (dots per inch = DPI), a standard in digital photography and graphic design/layout. Since the resolution only needs to be 72 DPI for a monitor, while 300 DPI is needed for good-quality printed matter, it follows that the same photo can be used on a monitor in a much larger size than in printed matter.
An indication of the resolution (and thus the quality) of a digital photo is the size of the digital file in kilobytes (kB) or megabytes (MB). A JPG of 100 kB has a very low resolution; this would be a web image. A JPG of 1 or more MB has a decent resolution. Print files are often (much) larger than 5 MB.
High-resolution photos, which are necessarily large files, can be scaled down to a lower resolution using an image editing program such as Adobe Photoshop. The program shrinks the photo by removing pixels, but leaves you with a good-quality photo, as measured by the new resolution.
Low-resolution photos cannot be enlarged to a higher resolution. Enlarging such photos will reduce the image quality, as the larger photo has to make do with the image information of the small photo. This results in a pixelated or blurry image. For this reason, it is important to request the largest photo file with the highest resolution; you can always shrink it later.
Illustrations are often created in special programs such as Adobe Illustrator. Because these files are made up of vectors (mathematically defined lines and surfaces), they can in theory be made infinitely large. The resolution is defined by the output medium (printer, monitor, etc.) rather than limited by the file itself. These files must be saved as resolution-independent file types such as EPS, PDF or AI.
Handmade illustrations and drawings can be photographed or scanned. The resolution of the resulting file will depend on the resolution of the camera or scanner used to create it.
Graphs and tables
Here, too, the resolution depends on the software used to create the graphics. Graphs and tables from Word or Excel can be saved as PDF files; as such, they are in principle resolution independent. The same goes for graphics from Illustrator (EPS, PDF or AI).