How many times, when evaluating an image on the screen, have you found it to be sharp and well-defined, only to discover that it is blurred and of poor quality on paper? It's all a matter of resolution, you've been told. Or maybe it's all a matter of the megapixels of your SLR. So let's try to put things in order and proceed step by step. Let's get to know the Megapixel.
Pixels are the smallest components of a sensor in digital cameras. Digital images come from thousands of tiny pixels that capture light and colour. A megapixel (MP) is about one million of these pixels, so a modern 30 megapixel camera has 30 million pixels on its sensor.
An image resolution is the number of pixels in an image. It is generally indicated in PPI, (Pixels Per Inch), a unit that indicates the number of pixels in one inch, (1 inch = 25.4 mm). The higher the number of pixels per inch, the more information there is and the more detailed and sharp the image. Conversely, when there are only a few pixels per inch, the image appears as a grid in which pixels are clearly visible.
At this point you might think that more pixels means better photos but if that is the case, the latest smartphones should outperform semi-professional DSLRs.
The sharpness, definition and detail of an image increase along with its resolution, a factor that determines image quality and relates to print size.
To find out the resolution of an image on your computer, simply hover your mouse cursor over the preview.
By right-clicking on the image and selecting 'Properties' on Windows or 'Get Info' on Mac, you can see all the EXIF (Exchangeable image file format) data of the shot.
For retouching you need the highest possible resolution, which you can get in the camera by shooting in RAW. RAW files are better for editing than JPEGs because they contain a lot of information.
But there is a difference between the screen resolution and the resolution of photos for printing. Your picture might look good on the computer screen. But when it comes to printing it or print quality, it's a different matter.
So far we have talked about PPI, which means the number of pixels per inch on a screen or sensor. For printing we need to talk about DPI, which means dots per inch, the number of ink dots on a print. The higher the number, the higher the quality.
According to the standards, 300PPI is the optimal resolution for a print, but it is an indicative value for a photo observed at about 20/30 cm. distance. For a poster, where the viewing distance is considerably greater, the value can be much lower, up to 72PPI for a 70x100 print, which assumes a viewing distance of at least 1/2 metres.
By standard, 300PPI is the optimal resolution for a print, but it is an indicative value for a photo viewed at about 20/30 cm.
In summary, to evaluate the printing resolution of an image, the formula should be applied:
number of horizontal (or vertical) pixels /horizontal (or vertical) size of the print X 2.54.
The result is the resolution in PPI of your print.
Below are approximate values with recommended print settings depending on the camera sensor.