Copyright Michael Karbo, Denmark, Europe.


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    Chapter 7. Pixels and resolution

    All photographs are constructed by small coloured dots. The camera’s only task is, in fact, to produce a big collection of these dots. But how is an image actually created from all of these dots? How many pixels should there be in a good image and what do the words DPI and interpolation mean?

    These subjects can be a little theoretical but they are of great importance to the modern photographer – not only when it concerns understanding a camera but also when it concerns processing the images in a computer for printing good paper copies.

    Coloured image dots

    All printed images are made up of dots. In newspaper pictures we find thousands of small ink spots. Photographic film consists of microscopic silver particles and colour printing is built up by small dots of coloured printing ink.

    It’s the same with the images we see on a computer screen; they are also made up of coloured dots. The difference between a screen and paper is, that a screen’s colours send light out from “the inside”. Paper is, on the other hand, dependent on light from the outside, which falls on the paper.

    Basically every good image consists of – whether it is on a screen or a print – of millions of coloured dots. In digital images the coloured dots are called image dots or a little more technically pixels. What is special with digital images is that they are much more uniform in their construction than, for example, an oil painting. A digital image consists of pixels. All pixels are the same size, and they cannot be divided. Each and every pixel has, you see, one particular colour.

    In contrast to analog films, where you, in principle, can work with a never-ending number of different colours, the choice is restricted within the digital world. It’s not terribly important, however, there are still millions of colour nuances available.

    Figur 24. All digital photographs are made up of a certain number of pixels, which are of the same size and each with its own colour.

    Digital photography is always rectangular. It is made up of pixels in rows and columns. If there are 1024 rows, each with 768 pixels, then we say that the resolution 1024 X 768 pixels.

    Pixels and sharpness

    The more pixels a photograph consists of, the sharper the image. When there are millions of pixels, then you can’t see an individual pixel – it will disappear in the masses, which is what is intended; the coloured pixels sort of melt into to each, when they are small enough.

    If there are only a few pixels available in an image, then we can often see each one of them with the naked eye. It doesn’t look good; the phenomenon is called pixelling.

    The cat picture shows an example of pixelling; the photograph on the left consists of only 96 X 109 pixels. We can see what the picture is of but it is not very successful. We can see almost every one of the 10.464 pixels and the result is a rather hacked image.

    There are lots more pixels available in the version on the right – 1.046.400 pixels in fact, and the individual pixels can’t be told apart from each other. If this photograph with a high resolution is printed in the size of a poster, we might be able to see the individual pixels (if we get very close to the poster). A photograph printed in the size you see in this book doesn’t give any problems. There are lots of pixels and by this lots of image details. Even if you use a magnifying glass you will have a hard time finding individual pixels.

    The point being that an image’s resolution has to fit to the size of the paper it is to be printed on.

    Camera resolution

    Image size in pixels

    QVGA

    320 X 240 (videofilm, webcam, mobile telephone)

    VGA

    640 X 480 (video film, webcam, mobile telephone)

    1 MP

    1280 X 960

    2 MP

    1600 X 1200

    3 MP

    2048 X 1536

    4 MP

    2272 X 1704

    5 MP

    2592 X 1944

    6 MP

    3264 X 2176

    8 MP

    3264 X 2448

    12 MP

    4048 X 3040

    Figur 25. The table here shows image sizes with different sorts of cameras.

    Megapixels

    Digital cameras, then, produce images that are made up of lots of pixels.

    The more pixels there are in the photographs, the higher the resolution and the more details in the image. And when there are lots of image details, the image can be printed in large sizes. The first digital cameras had low resolutions, which were called kilopixels (thousands of pixels). After that cameras arrived with resolutions of millions of pixels, which are called megapixels and written as MP. If you don’t ever expect to print your photographs in a larger format than 10 x 15 cm (like normal holiday photographs), then a resolution of 3 MP is perfectly adequate.

    For many people, however, there are advantages with cameras in the 5 or 6 MP class or maybe even with higher resolutions. The images can be printed in larger formats. They can be trimmed, so that there are still lots of pixels and image details available. And when it comes to processing in image programs such as Photoshop Elements, then it is always a big advantage to have lots of pixels in the image material!

    Figur 26. The number of pixels has to fit to the size of the print! In the top image, the resolution is much to low for the size of the print, which means that the individual pixels can be seen.     

    Web or prints – calculate in DPI

    The same photograph can be used in several connections. Maybe you want to send a copy with e-mail, just for display on the screen. In which case, a resolution of 640 x 480 pixels will be fine. This is an image area of just 307.000 pixels or 0,3 MP, which fits in very well as a photograph on a homepage. If the photograph is to be printed, then more pixels will be necessary. And the bigger the paper copy and the better the paper quality, the more pixels are required. The unit of measurement used is traditionally DPI, when you state the image’s resolution for printing onto paper.

    The abbreviation DPI stands for Dots per inch. When working with digital images, dots are the same as pixels, so DPI means pixels per inch. It is easy to convert to pixels per cm, as one inch is the same as 2,54 cm.

    When a photo is displayed on a screen, then the resolution is about 75 DPI. A screen is, you see, not particularly fine grained. But with digital photographs to be printed on paper, there has to be more pixels. Here you typically operate with resolutions from 150 to 300 DPI. A resolution of 300 DPI is used for letterpress but 200-250 DPI will usually be quite adequate for normal photo prints.

    Required print

    quality:

    Pixels per photo by image size:

     

    DPI

    10 X
    15 cm

    13 X
    18 cm

    20 X 30 cm

    Good
    quality

    200

    930.000

    1,5 MP

    3,7 MP

    Optimal photo print

    250

    1,5 MP

    2,3 MP

    5,8 MP

    Book *
    printing

    300

    2 MP

    3,3 MP

    8,4 MP

    Figur 27. The amounts of pixels necessary for printing digital photos.


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