Part 2 - Image Resolution
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Digital photography has scared and confused many camera owners, but it’s resolution that seems to bewilder most. Peter Bargh aims to clear up the confusion.
Over the last few years camera users have been tempted to cross over to digital because they've been told about all those convenient must-have features. Things like live preview, editing facilities, no film or processing costs and ease of home printing and sharing are very large carrots to dangle. This digital world has a tendency to confuse those who’ve been taking photographs for years though, because the technology adds a few new variables that film users didn’t have to worry about. Sadly many haven’t got to grips with one of the most important of these - resolution or how pixels are related to a digital image size. It’s actually really simple if you go back to the basics.
This image shows how part of the picture becomes
pixelated when the area is enlarged.
A digital photograph is made up of tiny squares called pixels. A pixel is short for Picture Element and each pixel has a single colour value. Each pixel butts up to the next in a grid and these make up the photograph. The more pixels in the photograph the bigger the grid and the higher the resolution.
With those figures in mind consider a photo from a typical 6 million pixel digital camera. It has a grid measurement of around 2848 x 2136px. These two dimensions, when multiplied, give the total 6 million pixels. Easy so far, but confusion appears when we view or print the photograph.
This is because the methods of displaying a photo require different resolutions. The three main options you'll come across are the computer monitor, inkjet printer and magazine or book. A computer displays at 72 pixels per inch (ppi), a print made on an inkjet printer or processing lab requires a file of around 240ppi and a high quality magazine or book needs 300ppi.
If we view our 6Mp file on a computer monitor with a 72ppi resolution the image will be about 39inch when displayed at 100% magnification (2848 divided by 72). Most digital cameras capture photos with 72ppi as the default, so the photo will seem to be huge when you open it in a web browser or image editing program.
Enlarging a picture to more than it's actual size can
drastically reduce it's quality.
The same photo, when viewed in a magazine or printed on an inkjet printer, will appear much smaller. This is because the number of pixels displayed per inch increases. Our 2848px image example would be output with a length of roughly 12in (2848 divided by 240) and in a magazine it would be 9.5in wide (848 divided by 300).
There's one other measurement that confuses people - inkjet printer resolution - where a manufacturer may quote 1440 dots per inch (dpi) in the specifications. This is because the printing device lays down several dots of colour on the same point to get a more realistic colour representation of the pixel it’s trying to print.
To get an indication of the size your photo will print go to the image size option in your picture editing program, turn off resampling and change the setting from 72ppi to 240ppi or 300ppi and view the new measurements in the document size boxes.
Your camera usually has two types of quality setting – one to adjust total number of pixels recorded the other to set the compression level. Reducing either saves you space in the camera's memory allowing more photographs to be taken, but at a cost of quality.
Most cameras store photos on a removable memory card and additional ones are now inexpensive so it's always better to buy extra cards, especially if you are going to be away from civilisation for a number of days. Then you can always shoot at the highest pixel setting, unless you are certain the photos you take are only going to be viewed or printed at small sizes.
Whether you shoot in compressed JPEG format or RAW is dependent on how much work you want after taking the photo. JPEG mode is a compression format that reduces the file size, but also affects quality. If you compress too much, by setting the camera to standard rather than fine, you will be able to take many more photos on each card, but you may see irregular patterns in the colours, known as artefacts.
In JPEG mode the camera applies automatic processing, adjusting the exposure, sharpness, colour and noise to manufacturer programmed standards before the file is compressed. Enthusiast photographers prefer to shoot in RAW mode which captures the subject data, but doesn't apply any automated settings. The image is opened in a RAW converter and various sliding controls allow you to adjust technical aspects of the photograph on the computer.
If you have spare time this is the option to choose. If, for example, you photograph a backlit subject, you'll find the camera's exposure meter is fooled resulting in a silhouetted in any dense areas and bleached highlights in any see-through areas. RAW gives more experience users the opportunity to process two versions - one for detail in the see-through areas and one for detail in the dense areas. The two are then merged together to gain a better exposure. You can also increase sharpness and colour saturation or remove lens defects using various controls.
The way I'd suggest you handle digital photographs would be to shoot in RAW at the highest quality and save the images as master files on CDs or DVDs. Label the CDs with the particular holiday or event so they are easy to recall in the future. Then, using your image editing program, adjust size to suit the required media - reducing the image dimensions to email, upload to a web site or view on screen and leave at high res if you intend submitting your best shots for publication.
In next week's guide we will be looking at aperture and shutter speed in more detail, and their importance in creating the digital image.