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The dilemma: "Should I take a lower quality, larger image, or a higher quality, smaller image?" That is the question facing photographers who want to fit more images into their camera's memory card. This dilemma is shared by those who want to minimize image storage, and need to choose between saving a smaller image and saving a compressed one. First some terminology. The term compression, as used here, refers to JPEG compression, a mathematical process that reduces the size of an image by sacrificing image detail. More compression means a smaller file, but less quality. Most digital camera manufacturers designate the amount of compression with terms such as "Basic", "Normal", and "Fine". Image editing programs generally use a numeric designation, with higher numbers indicating more quality, and less reduction in final file size. I will use the rather long-winded phrase JPEG compression quality setting to mean the level of compression applied to an image. The process of making an image larger or smaller is called re-sizing. Software uses a mathematical process called re-sampling to calculate new pixel values whose spacing is larger or smaller than the original pixel values, very much like changing the size of a drawing on a balloon by inflating or deflating the balloon. The end result is a larger or smaller image, and so a larger or smaller file. ----- First, let's look at the dilemma of the storage-starved image editor who wants to decide whether to reduce the pixel dimensions of an image, or save a larger version of the image at a lower JPEG compression quality setting. The following images represent two different ways to get a smaller image: reducing the Compress or Resize-and resizing. These are sub-sections of images, with the resized image magnified to cover the same relative area of the image. ----- |
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image to the right is a close-up of a butterfly. It was
saved using an image editor at the lowest possible JPEG compression
quality setting.
Click on the image and be stunned by the amount of detail possible in a relatively small, 192 Kbyte, image. Note: Explorer has a pesky "zoom to fit" option. If after clicking on the image to the right you see anything less than a very large image, move the mouse over the image. In a second you'll see an orange "zoom" icon in the bottom right corner of the image. Click on it for a major experience in large-image pixel-mania. Although the sharp details look good, jpeg does some slight of hand for areas of subtle gradation: can you spot the jpeg artrifacting in the center of the butterfly's body?
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| These are cropped images. Click on either image to view or download the entire image. | |
This is an enlarged section of an image created by resizing the original 3 megapixel image from 2048x1536 to 278x209pixels, for a final image size of 192 Kbytes. This is slightly larger than the compressed file above. This is a small section of that image, magnified horribly for comparison with the same section of image from the compressed file above. Click on the image to view it at its actual, and rather tiny, size. You may also download TIFF or PNG versions of this image. Though these are lossless formats, each comes with a price: you will need a TIFF compatible viewer to see this image directly (Irfanview is one of the best freeware image viewers for Windows). PNG is not supported by all browsers, though it works fine with Internet Explorer and Netscape.
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The Compression Theorum: if file sizes are equal, the more highly compressed image has superior image quality. So it is better to compress more than it is to settle for a smaller image. If you are editing an image, you are in a position to specify the image quality over a very large range. Likewise, a mixed strategy of resizing and compressing is a probably less effective use of storage space than simply compressing the original image at a lower JPEG compression quality setting. "Well and good", you may say, "for image editing software situations, where I have control over image size and JPEG compression quality, but a camera is different". And right you would be indeed. ----- A digital camera, sadly, offers only a few image size and JPEG compression quality settings, so it is not possible to finely adjust the JPEG quality. This means the file sizes of two different compression and resolution settings will probably never be exactly equal. So the photographer who wants to fit more images on a card is faced with a trade-off, juggling image size versus JPEG compression quality but lacking fine control of either. As we will see, the manufacturer can sometimes fumble the ball as well. Here is a "compression test" done with a Nikon 990, a 3 megapixel camera.
Looking at the chart, a full-size fine image is just under a megabyte, and is compressed at a ten to one ratio. Knowing what we know about compression versus file size, how should a photographer "hop scotch" his or her way to the optimal image size? Note that the most significant image change occurs between rows, as resolution decreases, even though the relative file size is changing less. A drop in resolution degrades the appearance of an image more, in proportion to file size, than a change in JPEG compression quality. If we could drop the jpeg quality compression setting even more, we could get the benefits of smaller file size without having to drop resolution and sacrifice image clarity. Conclusion: decrease JPEG compression quality without a qualm, but kick and scream a little before you drop the resolution. Ignoring the bottom row for the moment, the camera manufacturer has carefully set things up so that stepping down the chart from the left to right and down results in progressively smaller file sizes, and similarly lower quality. There is one exception to this pattern, and I have highlighted it in green. A VGA fine image is larger than a 1024 x 768 basic image, yet the image quality, as can be seen from the sample is worse. I would not recommend using the VGA setting for this reason, and consider it a mistake on the part of the manufacturer to provide this option at all. Conclusion: Manufacturers make mistakes. A larger file will usually capture a better image than a smaller one, but don't count on it. BTW - The bottom row of this chart, which appears in italics, was created using the camera's 3:2 ratio mode. This setting is designed to exactly fit the aspect ratio of a 4x6 inch snapshot, avoiding waste and providing accurate in-camera control over the framing of images destined to be printed at that size ratio. If you are going to print at this size, and don't want the option of cropping, it's a no-brainer to use this image size setting, since the extra pixels will not be printed, and you will be able to fit 10 percent more images on each memory card. ----- As always, I encourage you to try the experiments outlined here for yourself - if you find any interesting results, such as glitches in the compression scheme of other cameras, or if you have any questions or suggestions for improving this web page, your email is appreciated. Thanks in particular to Bernard Hill (http://www.braeburn.co.uk/jp.htm), whose suggestions for this article were invaluable. Mike Russell, June 12, 2002 |
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