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Will Enns · 11-19-2002, 05:37 PM

Lon,

By way of explanation...

A 4 megapixel digital image is comprised of (roughly) 4 million pixels.

The 4 meg (megabyte) uncompressed image file (aprox 4,800,000 bytes) you refer to is about 1280x1024 pixels in size, and equals roughly 1.3 megapixels.

The camera that took it is known as a "megapixel" camera, that is, its capacity is over 1 megapixel. In '98, this was considered pretty good; by today's standards 5 megapixels is considered good, but still not professional.

In other words, image (megapixel) size does not equal file size (megabyte,) although the two are directly related according to the file type and level of compression used.

As shown above, a 1.3 megapixel uncompressed image will generate an uncompressed tif file of 3.79 megabytes.

The same image saved as a jpg file at 100% quality generates a file of about 1.1 megabytes.

Saved at 75% quality (common web standard,) that same file shrinks to 240k, or about 1/4 of a megabyte. The price of this compression is loss of image quality, and that's why the jpg format is referred to as a "lossey" file.

As a matter of interest, the same file saved at 50% quality shrinks to 157k; at 25% quality, it shrinks to 97k; at 10%, 43k, all without compromising the image *dimensions.*

I've attached an image below that demonstrates the effects of compression. The one on left is from a jpg file compressed at 90% quality; the one at left is from the same file compressed at 10% quality (not recommended - I did this to show the effect of quality loss due to compression.)

Notice the extreme pixelation and loss of detail. Higher compression of a lossy file results in this effect, proportionate to the level of compression.

11-19-2002, 05:37 PM	#5
Will Enns Associate Member Joined: Sep 2002 Location: Summerland, BC, Canada Posts: 86	Lon, By way of explanation... A 4 megapixel digital image is comprised of (roughly) 4 million pixels. The 4 meg (megabyte) uncompressed image file (aprox 4,800,000 bytes) you refer to is about 1280x1024 pixels in size, and equals roughly 1.3 megapixels. The camera that took it is known as a "megapixel" camera, that is, its capacity is over 1 megapixel. In '98, this was considered pretty good; by today's standards 5 megapixels is considered good, but still not professional. In other words, image (megapixel) size does not equal file size (megabyte,) although the two are directly related according to the file type and level of compression used. As shown above, a 1.3 megapixel uncompressed image will generate an uncompressed tif file of 3.79 megabytes. The same image saved as a jpg file at 100% quality generates a file of about 1.1 megabytes. Saved at 75% quality (common web standard,) that same file shrinks to 240k, or about 1/4 of a megabyte. The price of this compression is loss of image quality, and that's why the jpg format is referred to as a "lossey" file. As a matter of interest, the same file saved at 50% quality shrinks to 157k; at 25% quality, it shrinks to 97k; at 10%, 43k, all without compromising the image dimensions. I've attached an image below that demonstrates the effects of compression. The one on left is from a jpg file compressed at 90% quality; the one at left is from the same file compressed at 10% quality (not recommended - I did this to show the effect of quality loss due to compression.) Notice the extreme pixelation and loss of detail. Higher compression of a lossy file results in this effect, proportionate to the level of compression. Attached Images __________________ Will Enns