Saturday, April 18, 2009

Kodak Z1485 IS Picture Size

This camera offers 14 Megapixels, which is a selling point. (More is better). It stores them JPEG compressed with a typical snapshot size of 6 megabytes. This is a compression ratio of about 7, quite modest as JPEG goes. Where does 7 come from? An crude approximation, assume each raw pixel is stored as Red, Blue, and Green (RGB), 8 bits (one byte) for each color. That makes 14 Megapixels into 43 megabytes. Divide by the JPEG compressed file size and you get to 7. Actually the camera probably uses YUV color encoding, which permits each color pixel to be stored in merely 16 bits, but let's stick to the crude estimate based on 24 bit per pixel RGB color encoding.
Of course, 6 megabyte snapshots are slow to upload, and suck up disk space. Plus having more pixels in the image than you have on the display device doesn't improve image quality. For instance my laptop LCD screen is 1024 * 768 or 786,432 pixels. Putting more than 786,432 pixels into the image won't improve the image seen on the screen. That's a lot less than 14 Megapixels.
So I set the picture size down to 5 megapixels and took some close up shots. This reduced the JPEG compressed image size to about 1 megabyte, with superb quality as viewed on my laptop screen. I plan to leave the camera set that way, cause the camera memory will hold more pictures, they upload faster, and it could be that the camera is smart enough to average adjacent pixels together which would reduce image noise and improve the light sensitivity of the image sensor. The Kodak documentation hints (but doesn't come flat out and say) that averaging does take place.
With a 2 Gig plugin memory card, the camera could hold 300 snap shots at 14 megapixels per shot. With 5 megabyte shots, the capacity jumps to 1800 snap shots. Which is like infinity on the smallest memory card Staples sells.

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