Understanding Image Resolution for Medical Illustration

We are often asked about the resolution of the digital files we supply as part of many projects. Resolution refers to the number of pixels in a given area of an image. Usually it is measured in Dots Per Inch, DPI or Pixels Per Inch, PPI. Each dot or pixel being an individual spot of colour which together forms the image. The resolution of a digital image file is important as it affects where and how the image can be used.

How resolution affects image use

The printing process requires images to have a resolution of around 300dpi (some print processes may be less such as newspapers and some increased, such as art books. Using an image on a website requires the image to only have a resolution of 72dpi (as this was the number of pixels on the early color computer screens). Having a correct dpi therefore, ensures that the person looking at the image perceives it as sharp, whether it is on a screen or a printed piece such as a brochure. You may have experienced this difference when printing an image from a website; on screen it looks sharp but when it comes off the printer it looks fuzzy.

Resolution is relative

It is important to remember that the resolution of an image is relative to its width and height dimensions. In the example below our image is 1800 pixels wide by 1341pixels high. The image (document) size is 6 inches by 4.47inches at 300dpi.

example image showing 1800 pixels wide at 300dpi giving a document size of 6 inches

You can change an image's resolution in a programme like Photoshop® by changing it's width / height. In this example we can make the image 72dpi and you can see that the total number of pixels has stayed the same but the image (document) size is now 25 by 18.625 inches. In effect we have not changed the image size, simply the way the image data is presented by changing the resolution.

changing an image's resolution to 72dpi while maintaining the same number of pixels

An image 25 inches wide is not going to fit well on the typical website so if we wanted to use this image online we would not only want to change the resolution but also the actual number of pixels the image is made up of. In the example below you can see that we have changed the total number of pixels to just 432 x 322 pixels so that our image is still 6 inches wide but at a 72dpi resolution.

keeping the physical, document, size of the image the same at 6 inches but with a reduction of the resolution to 72dpi shows the resultant change in the number of pixels that make up the image

The total number of pixels that the image is made up of is now only 139,104 (432x322). Compared to our original image which was made up of 2,413,800 pixels (1800x1341) you can see that there is much less information defining our image. This is also highlighted by the actual file size in the bottom left of the image window which has gone from 6.91Mb to only 407.5K.

Picking the right resolution for your project

So images can be reduced in resolution while keeping their dimensions the same, but
you cannot increase resolution of an image without either proportionally reducing its physical size or reducing its quality. The importance of this when commissioning medical illustrations relates to the image use now and in the future. An image that is good enough quality for print (300dpi) can also be used online / for digital applications later if you want. However if you commission artwork to be used online at 72dpi you will not be able to use these images for print in the future.

You may think that the answer is to create all of the medical illustrations at a high resolution so that their use is not restricted in the future. This issue with this is that the larger the illustration in terms of the number of pixels the longer it will take to create, therefore it will cost more. So one of the questions we ask when people commission us to create medical illustrations is "What are the potential future uses for the images?" If there is a chance that an image being created for an online use may need to be used in a printed situation we would recommend spending more in their initial creation to ensure you have that flexibility (subject to licensing) in the future.

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For further information on how this may affect you and your projects please contact us directly and we will be happy to help.

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