31 December 2013
New Year’s Resolution: 300 ppi?
Did you know that image resolution has absolutely nothing to do with how an image looks on a screen? It is a fairly safe bet that more of our collections will be digitised in the next few years. As technology moves on with great pace there is often debate as to the “best resolution” that images should be captured at. But what does that actually mean? This post will try to explain what is meant by the terms pixel and image resolution, and will demonstrate the relationship between them.
Pixels and megapixels
Digital images are made up of thousands or even millions of pixels (picture elements). A pixel is the smallest addressable element in a display device with a specific assigned value that can be read by a computer and mapped onto a grid to recreate an image. Each pixel is a sample of an original image, so the more samples available result in a more accurate representation of the original. We can change the appearance of an image by manipulating the pixels or by getting rid of some of them to reduce the file size. Below we see a digital image of the Gospel of St John from the Lindisfarne Gospels (British Library, Cotton MS Nero D.IV). It is obvious that the image with more pixels is of a higher quality than that with less pixels.
Figure 1: Cropped portrait of St John wearing purple, gold and green robes.
Figure 2: Pixelated close-up image of the portrait of St John.
Figure 1 has more pixels and so produces a more accurate representation of the subject matter. Figure 2 looks “pixelated” due to the visibility of the pixel boundaries.
How pixels control resolution
Pixels control image resolution because the closer the pixels are placed (i.e. the more there are per inch), then the denser the image becomes with detail. Similarly, the fewer pixels an image has per inch, the further apart they are spaced, resulting in less detail and an image of poor quality.
Image resolution is therefore concerned with the number of pixels per inch (ppi) printed out on a piece of paper, and the size of those pixels. Since the software takes care of the pixel size, it’s really just the ppi that you need to think about.
Let’s try to understand that better by taking a look at an image captured with a DSLR camera. Below is a photograph of our new multispectral imaging system opened in the open source image processing software package ImageJ.
Figure 3: Full-size, uncompressed photograph opened in image processing software package ImageJ.
If we look at the title bar of the image we can see some details about the image file.
Figure 4: The title bar tells us the name of the image file, the percentage size in brackets, and the number of pixels.
The title bar (DSC_0074.JPG (16.7%)) tells us that this file is only opening up to 16.7% of full size. The image is just too large to open on the screen at 100%. Below the title bar we can also see that the size of the image is 6,000 x 4,000 pixels (i.e. there are 6,000 pixels running along the image from left to right and 4,000 pixels running from top to bottom). That sounds like a lot of pixels. If we now zoom in on any part of the image we can see these pixels as little squares of colour.
Figure 5: A cropped portion of the original photograph.
Figure 6: Zoomed in portion of the original image.
Figure 7: At maximum zoom it becomes apparent that the image is made up of pixels.
If there are 6,000 pixels along the top of the image, and 4,000 pixels along the side, then my incredible math skills suggest that there must be 24,000,000 (= 4,000 x 6,000) pixels in total, or 24 million pixels, or 24 megapixels (MP). A quick glance at the camera manual will show that this camera (Nikon D5200) has in fact got a 24 MP CMOS sensor, so our powers of deduction are correct.
Resolution doesn’t mean anything until you go to print
We now know that there are 6,000 x 4,000 pixels in our image. Great! But what does that mean if we want to print out this image on a piece of paper? How does a pixel correlate to the size of the page? Will the image fill the whole page or will it just appear as a tiny thumbnail? Take a look at the image resolution by opening the image up in another great open source image processing package called GIMP, and opening the Set Image Print Resolution window.
Figure 8: Set image print resolution page.
Here we can see that the X and Y resolution is 300 pixels/in which means that that for every inch of paper we have, there will be 300 pixels printed. So if we have 6,000 pixels along the top and 4,000 along the side that means we must have 6,000/300 = 20 inches along the top and 4,000/300 = 13.333 inches along the side… and if we look at the print size in the window above we can see that has already been calculated for us.
20 by 13+ inches is quite a large size. How can we print it out smaller to fit on our page? We need to fit more pixels into each inch, and since the size of an inch can’t change then the size of the pixels must change. That is done automatically for us by GIMP or Photoshop, or whatever image processing software package you are using. Let’s say we set our image resolution to be 600 pixels per inch. In that case we can see that the print size has adjusted to a much more manageable 10 x 6.67 inches. The resolution changes as the physical image size changes because the number of pixels that make up the image are being spread over a greater or lesser area.
Figure 9: By increasing the number of pixels per inch we can fit our image into a smaller area of the page.
PC monitors are generally considered to be low resolution devices meaning that images look good on screen even if they have a very small total number of pixels. This reduced number of pixels also allows images to load faster leading to an overall better user experience. But if you try to print it out, you may be disappointed at the tiny image that emerges from your printer. Printers are high resolution devices and require an image to have a resolution of about 300 pixels per inch to look sharp and to be of a good quality. 300 ppi is generally accepted as the resolution for professional quality printing, but that number is increasing all of the time. There are many great articles and tutorials about this and other aspects of digital objects found on the Digital Photo Essentials Tutorial for anyone new to the world of digital photography or photo-editing.
Best of luck with your New Year's Resolutions!
Christina Duffy (@DuffyChristina)