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113 of 120 people found the following review helpful:
5.0 out of 5 stars
excellent price/value ratio for 4 megapixel cameras, January 18, 2003
Nobody would argue that the more memory you have in your digital camera, the better. The question is what the reasonable compromise between the price of a memory card and its size is, and how to predict how much memory you actually need before you paid for it. I looked up some literature on the image compression techniques, and made some experiments with printing out pictures with different resolution and different compression ratios, and came to the following conclusions.Most cameras offer a choice between several picture resolutions and several image compression modes. Low resolution is good for on-screen viewing, while high resolution is better suited for printouts. Additionally, one can choose between several compression modes. Higher resolution settings combined with the weakest compression result in better pictures at the expense of larger file sizes. The problem is that all digital cameras ship with very small memory cards - 8MB, 16 MB, sometimes 32 MB, and this is just not enough even for a day of shooting. I think most people would be happy to have a storage space for at least a couple of hundreds of puctures, equivalent to 8-9 rolls of film. In my experience, this is what you can expect to get from a 256 MB card if you use a 4 megapixel camera and medium compression. A few comments are in order to help you to understand better what resolution and compression mean in terms of the file size. 1. Resolution. The huge resolution numbers of ink-jet printers may be confusing, for instance, 1440 dots-per-inch (dpi) resolution may make you think that a 2000x2000 pixel image would print well only in the size of about 1.5x1.5 inches. This is absolutely wrong since resolution of the printer reflects primarily the minimum size of the ink dot which the printer can create, but not necessarily the size of a meaningful pixel of the image. In fact, printer manufacturers suggest that for the best results the resolution of the image should be 1/3 to 1/4 of the maximum printer resolution. Hence, expect that your final output will be printed with 250-300 dpi resolution. A four megapixel image will then print with a perfect quality up to the size of 8x10 inches. Smaller pictures, such as 4x6 inches, require a lot less pixels - 2 megapixels is sufficient! However, since you never know when you will need to print that one perfect picture in a larger size, I would suggest that you shoul always use the highest pixel resolution of your camera. Additionally, it gives you better flexibility in terms of cropping the image later on. 2. Compression. Unfortunately, high resolution images require lots of storage space: a 4-megapixel uncompressed image can be as large as 12 MB. JPEG compression, which is based on an algorithm specifically developed for image compressing helps to store large pictures in relatively small files. Specifically, JPEG compression algorithm divides an image into squares 8*8 pixels each, performes two-dimensional discrete cosine trasform of each of these squares, and compresses their frequency spectra by removing high-frequency components or by decreasing their accuracy through a "quantization" procedure. While this review is not appropriate for a discussion of technical details of the JPEG algorithm, it is worth saying that JPEG analyzes each small "brick" of the image individually and adapts the level of smoothing/compressing to the image to be compressed. In fact, when you choose the compression mode, you determine not the size of the compressed image, but its quality in the compressed form. Therefore, the final size of the compressed file may vary by as much as 2 to 3 times depending on each particular image. This is why you can never know for sure how many pictures will fit on your memory card, you can only have a rough estimate. The "adaptive" nature of JPEG makes it very efficient: it takes advantage of the smoothness of monotonically colored parts of the image by achieving great compression ratios in those areas without significant losses in quality, and it is also good at rendering sharp contrasts in the image. Since JPEG standard was optimized to take into account the specifics of our perception of visual information (like mp3 compression of music files takes advantage of the way how we hear music), JPEG compressed pictures are almost undistinguishable from the originals up to the compression ratios of 1:8 to 1:12. In my experience, medium compression works well almost always, with the exception of the case when you take a picture with lots of details, bright spots, contrast objects, etc., and want to print in 8x10 size. Then you should use the weakest compression ("fine"), or no compression at all. I have a Canon Powershot S40 and used it with highest resolution and medium compression. The size of the file varied from 500 kB to 1.8 MB from picture to picture. The average size (calculated using 800+ pictures) was 840 kB. Hence, I could fit 200-300 pictures on this 256 MB card. The last comment: note that there are three types of memory, flash, SmartMedia, and Sony memory stick. Check you camera manual which one you need, they are not interchangeable.
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