Photo Resolution
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When it comes to taking, viewing and printing digital photographs even a basic understanding of the technology goes a long way. Resolution is one of the most misunderstood concepts in the digital photography world. Many digital camera makers and sellers have pushed hard for the idea that - in terms of resolution - more is always better. There is certainly something to be said for having sharp, high resolution images. Unfortunately it isn't the case that having a large megapixel count will actually improve the photograph. The ultra-high megapixel cameras being sold in today's market do not necessarily guarantee a high quality image. The word "resolution" itself can be misleading. There are different meanings depending on the context. For example when you talk about the "image resolution" of a digital photograph you are referring to the actual pixel dimensions of the image. "Display resolution" refers to the size, in pixels, of the image after it has been scaled to fit on a certain screen - this can also refer to the resolution of a monitor. Finally, "print resolution" refers not to the number of pixels displayed on a screen but rather to the conversion from screen pixels to dots of ink per inch of print area.
PixelsThe first thing that must be understood about resolution is the pixel. Pixels are the most basic element of a digital photograph - the word pixel itself stands for "picture element". Each pixel contains information about intensity (brightness) and display color. An image file contains information defining each pixel individually and mapping how the pixels are laid out in a grid pattern. This in turn tells a screen how to display the image or a printer how to render it.There are different ways of rendering pixel information which is why you can have apparent smooth curves on a screen even though the curves are defined in a grid pattern. If, however, you zoom in beyond 100% display resolution the pixels quickly become apparent. Image and display resolutions are usually expressed as a ratio of pixels. For example the standard VGA screen size is 640x480 which means there are 640 pixels on the width dimension and 480 on the height. XGA is 1024x768, SXGA is 1280x1024 and so forth. The display resolution of a screen defines how images can be displayed on that screen. For example if you have a picture which is sized at 1280x1024 but you are using an XGA monitor the image will be larger then the total number of pixels available (1280 > 1024) and therefore must be shrunk to fit onto the screen.
MegapixelsDigital camera resolution is measured in megapixels. The key thing to understand is that megapixels are not big pixels. Megapixels are the actual area of the image expressed in the form of millions of pixels squared. The megapixel size of an image or a cameras shooting resolution can be found by multiplying its dimensional sizes and dividing by 1,000,000. Let's look at it in terms of the sizes described above:
Already at the SXGA size of 1280x1024 we are starting to get into the resolution range common for certain types of digital cameras. Many cellphone cameras as well as many integrated webcams for netbooks and laptops are 1.3 megapixels. At the same time this image resolution is bigger than would be needed to display on a XGA monitor with a resolution of 1024x768. Most modern digital cameras shoot at much higher resolutions. For example I have an Olympus μ780 which shoots up to 7.1 megapixels. When I go to the options menu and choose my image quality settings I am presented with a variety of different resolutions to shoot at. The highest resolution available is 3072x2304. I also have an Acer X223W 22in. flat screen monitor which displays at a maximum resolution of 1680x1050. Lets do a quick comparison.
Clearly my camera is capable of shooting at a much higher resolution than my monitor is capable of displaying. If my entire goal of taking a picture were to display it on a 22 or smaller screen I could shoot at a lower resolution supported by my camera - say, 2048x1536 - without ever being able to tell the difference onscreen. To make this clearer I took a series of identically composed photographs in nine different resolutions ranging from 640x480 all the way up to 3072x2304. I then cropped out a 300x200 rectangular portion of the image so they could easily be compared side by side. Keep in mind all of these image samples are being compared at 100% resolution - there is no resizing being done to fit the screen. This means that you can see the relative change in image quality as resolution progresses. The most remarkable thing about this comparison is how little of a difference there actually is between the resolutions. Once a large enough resolution is reached (in my opinion at around 1600x1200) it becomes very difficult to discern any noticeable differences.
Go small - Just not too smallThere are some big advantages to shooting at slightly lower resolutions. The most noticeable difference is reflected in the size of the actual file. Because every pixel is encoded into the image file the more pixels you have the bigger the file will be. Many people have been convinced that bigger is always better and shoot at incredibly high resolutions - far higher than they are able to display on most monitors - and end up storing files that are anywhere from 3-5 megabytes and up. Using the same sample batch of photographs from above, comparing the file sizes themselves provided some more interesting results:
While the actual quality of the image did not increase drastically after the three lowest resolutions the file size continued to grow by leaps and bounds. This should be a real consideration for many people. The 2048x1536 photograph was only 718 kb where the 3072x2304 image was 3,389 kb. On a 1GB memory card you can hold roughly 1,048,576 kilobytes worth of data. This equates to 1,460 images at 2048x1536 or just 309 images at 3072x2304. It is often worth it to shoot at a slightly lower resolution and have much smaller file sizes to manage.
But what about printing?Many people still like to print their digital images to have hardcopy backup, send to friends and family or simply display conveniently in their house. Printing is different than screen size but has certain things in common. Printers do not use pixels in the same way but they do have an equivalent idea which is the number of dots-per-inch (DPI). DPI is related to another similar acronym - PPI - pixels per inch. PPI is the number of screen pixels assigned to be rendered per inch. This number - for example 72 PPI - means there will be 72 pixels on both the x and y dimension of each square inch of print area.In order to better understand how the digital pixel layout for an image file compares let us see what information GiMP can tell us about the 1600x1200 sized image used in the example above. GiMP has an option to "Set Print Resolution". The secret to understanding print resolution understanding DPI - also known as print resolution. Some printers can print at very high resolutions with some high end laser printers printing up to 1800 dots of ink per square inch. Standard print sizes range anywhere from 72 dpi (low quality) to 300 or 400 dpi - resolutions which are considered suitable for use by professional photographers. The default setting for GiMPs print resolution was 72 dpi. Because 1600/72=22.2222 and 1200/72 =16.6667 and so this means that a 1600x1200 pixel image printed at 72 dots per inch^2 will be printed at 22.2x16.667 inches. I adjusted the x and y resolutions (pixels/in) to 200. Now the 1600x1200 image would be output as 8x6 inches. For the 3rd test I bumped it up to 300 pixels per inch resulting in a 5.333x4 inch print size.
Judging by the right-most column it is clear that there is no need to have more than a roughly 7.2 megapixel camera to get print quality images. The 17x22 image needing a 33 megapixel camera is sort of a fluke. Certainly that type of resolution would be needed in order to accommodate a print of that size at 300 ppi but prints that size are never intended to be viewed from as close a distance and large prints can often have a lower overall print resolution (PPI) because they are being seen at a distance. |
