One question appears time and time again when producing artwork for use on a popup exhibition stand, bannerstand and other large format printing application - "What d.p.i. should I save the images?" Bounce house rentals in Madison Wisconsin
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This is impossible to answer without knowing more information about how big is the initial, what size it is usually to be enlarged to, the subject matter, how it is usually to be used within the design and how it has been produced and several other factors.
It can be viewed that probably the question isn't the right choice to be asked but we shall return to that later.
The very first thing to be looked at is the origination. Most images available from image libraries as well as those sourced from professional photographers designed for the project can have originated on an electronic digital camera. At the time of writing the greatest resolutions available are around 15 megapixels. So it can be deduced that for a photographic image the maximum pixel size that may be achieved for an individual image is:
4492 pixels x 3328 pixels (which gives a 42.8M image). This, therefore gives us a maximum achievable image file size of 42.8M.
Just to obtain a handle on why this is significant let's now consider that image when it comes to d.p.i.
At screen resolution - 72 dpi this can translate to a physical dimension of approximately 158cm x 117cm
At a print resolution of 600 dpi which most large format printers can handle this equates to a physical size of 38cm x 28cm
Both images are displaying the exact same number of information but one is 5 times how big is one other! So you start to observe that considering dpi is meaningless without also knowing the physical size that you wish to achieve.
This concept is likely to be further complicated by the fact that the large format printer will print a certain resolution regardless of the image information that's sent to it because it will interpolate (add or remove pixel information) to accommodate the particular pitch of the dot pattern. If, for a certain size image, it's given more information than it could print at 600 dpi, it'll remove pixels at regular intervals and conversely if it has not enough information it'll add dots by considering the dots around usually the one to be printed and calculating along with that may produce the smoothest transition.
You will find limitations in both directions, enlarging and reducing. A graphic that's over-enlarged should go very soft and blurry at the transitions in one colour to the next and a picture that's printed too small can have not enough space, at a fixed dot size and pitch, to reproduce the graduation between one colour and the next. Imagine a typical page of text reduced so small that there surely is only 1 dot available for each letter - it would obviously be illegible.
A further complication is added once you factor in whether any jpeg compression has been put into the image. At high compression ratios the image can be damaged and show ugly artifacts especially at the edges of image elements and in regions of high contrast. Graduated areas such as for instance sky also can lose the subtlety of the gradient and become stepped and posterised.
On the plus side we ought to consider that we are dealing with large format graphics here and as a result they are generally viewed from some distance away. Despite quite low resolution origination, with the judicious utilization of interpolation to disguise pixellation, it's possible to produce results which are acceptable when viewed from a few feet away.
So, to go back to the initial question of what dpi a picture should be saved at, for a particular purpose, it's perhaps more effective to thing when it comes to simply how much information there's - its megabyte size, and its suitability for enlargement (i.e. its image quality and complexity of detail)
As a difficult guide it could be helpful to think when it comes to one image covering the entire of a popup stand. To offer a good balance between the caliber of the final print and how big is file, I would recommend a document size of around 150Mb - 200Mb. No I haven't forgotten that the initial image would have been 42Mb or less but it's worth increasing the pixel size of the image using interpolation (or 'resing up') to be able to minimise the enlargement of the pixel shape thus avoiding any blocky shapes or pixellisation.
If the image covers about half that area it need be only half the file size and so on. Another method to calculate this is always to save the image at around 25Mb for every square meter of space covered.
Matt has been in the exhibition industry for over 20 years and has run a successfull exhibition graphics business going back 10 Bounce house rentals in Madison Wisconsin.
