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|Category:||Digital Camera Operation|
Understanding your digital camera's image recording mode. - Pixels, resolution and compression expained.
Most digital cameras now offer megapixel quality and there's usually a feature to adjust how many pixels are recorded when you take a photo and another that allows various quality levels. These settings can be confusing for the newcomer and misused by existing digital camera owners. This article will guide you through the options to help you choose the best for your photography.
Let's first go back a step for those new to digital photography and explain how a picture is
Instead of film a digital camera records the photo on a CCD, short for Charge-Coupled Device. This is a light sensitive chip with a matrix of colour filtered picture elements, known as Pixels. This grid comprises a surprisingly large number of Pixels, considering the CCD is smaller than a standard 35mm film frame, and its these pixels that make up the digital picture. The amount of pixels the CCD records determines the megapixel rating of the camera. So, for example, one of the latest 5 megapixel cameras has a CCD with over 5 million pixels. The advantage of having more pixels is that you can enlarge the picture more without the individual pixels being seen. The bigger you enlarge the photo the more you'll see the jagged edges of the pixels, until eventually you end up with an unrecognisable image, displayed as a grid of individual colour squares.
To illustrate pixelation I've included a photo of a tortoiseshell butterfly at various pixel settings to show how the image breaks up the fewer pixels you have. To show this on screen I've had to reduce the number used considerably, but it gives you an idea. Notice when the number of pixels used is small (left) you see the jagged edges of the squares, and as the pixel numbers increase (right) the image becomes more defined and pixel unnoticeable.
The way we display or view the photo also needs to be considered when talking pixel values. There are basically three main types of viewing: a magazine/book reproduction, print from a desktop inkjet printer or on a computer monitor. Each displays the picture using a different number of pixels. A magazine or book, with its high reproduction values, needs around 300 pixel per inch (ppi) to display without any pixelation, an inkjet printer needs about 240ppi and a monitor needs only 72ppi. With these figures in mind you can easily work out how big your picture will appear in each format. For example a typical 5 megapixel camera can record a picture 2592x1944. All you need to do is divide these figures by the format you use to see how big the photo will be displayed without any sign of pixelation.
Taking the longer 2592 pixel length of our example and you see the following:
|/||Output ppi||=||Display size
So now we understand what a pixel is, let's look at how you can set your camera to avoid seeing the pixels.
There are two adjustments you can make on most cameras to the image setting. One adjusts the number of pixels recorded and is often wrongly described as the resolution. This is the setting that determines image size, the other adjusts the compression level and is often referred to as the quality level. This affects how many pictures you can take - a higher compression means more pictures can be taken, but the quality is reduced. We'll come back to that later.
Before taking a photo you should decide how big you are going to want the photo to be displayed. Using the above table it's clear to see that if you were just using the camera for taking onscreen photos, say for emailing to a friend, to view on the computer or to put on your web site or online gallery, you wouldn't need to shoot at the 2592 x 1944 setting. Likewise a 6x4 print from an inkjet printer doesn't need such a high setting.
For this reason most cameras have a selection of pixel recording modes. And the table below gives you a rough idea of the size you could print out or view.
|72ppi Web||240ppi Inkjet||300ppi Magazine|
|2592 x 1944||36in||10.8in||8.6in|
|2048 x 1536||28in||8.5in||6.8in|
|1600 x 1200||22in||6.7in||5.3in|
|1024 x 768||14in||4.3in||3.4in|
|640 x 480||8.8in||2.7in||2.1in|
Using this table it shows that shooting at 640x480 setting is fine if viewing an 8in wide print on screen is all you need but send this photo to a magazine or book publisher and the file would only create a 2 inch wide photo which would usually be no good.
Pictures with higher pixel counts take up much more space in the camera's memory or memory card. They also need more processing power and then, when you download to your computer, take up more space on the hard drive. To help most cameras have a series of quality settings that allow you to compress the photograph making it much smaller when stored. This does have an affect on the quality...the more you compress a photo the more you degrade the image. The picture is compressed using a file format known as JPEG which every piece of software an picture viewer can read. If the picture isn't compressed it's often recorded in either TIFF or RAW format and only a few pictures can be taken at this setting before your card is filled.
Below is a close up of our butterfly photo showing JPEG compression - left; low compression (often described as Best or Fine) - middle; medium compression (often listed as Normal or Better) and right; high compression (often listed as Basic or Good). Notice how the pixels start to clump together in larger squares at the high compression setting on the right. Avoid this setting where possible.
So with all this knowledge in mind we now come to the final table which most camera instruction manuals include. This gives you an indication of how many pictures you can shoot at the various levels. Choose your recorded pixel value from the left and the quality setting from above and where the two meet is how many pictures you can store on a card (in this case 32Mb)
Image storage capacity based on a 32Mb card
|2592 x 1944||2||8||16||28|
|2048 x 1536||14||24||36|
|1600 x 1200||22||40||54|
|1024 x 768||50||90||112|
|640 x 480||53||164||200|
My advice would be to shoot at the higher settings, even if you don't currently want to send pictures to magazines. Then when you've downloaded your photos to the computer save them in a folder until you have enough to fill a CD and write them onto a CD, make a back up for safety and then delete the ones on your computer to free up space. To allow this buy the largest memory card you can afford and use the above table to indicate how many photos you'll get by increasing the memory card capacity. A 128Mb card, for example, will allow you to multiply the above numbers by four (128/32).