JPG Files Sizes

[shortie]

HI All

A quick question for you experts

I have a G5 and it is set to Large and Superfine JPG - so how comes when I download files from the camera to harddisk the file sizes are all different

Just asking but is a 5MP picture is a 5MP picture YES or NO unless the compression does something depending on the number of colour in the shot

Shortie

[robertwgross]

If you shoot an image with a 5 megapixel camera and store it in JPEG, the compression takes over and crunches it down to a size of 1-3 megabytes. That varies a bit depending on whether it is the camera that does the JPEG or an editing program that does the JPEG and allows the user to set a compression quality.

If you have an image with lots of fine detail, then JPEG will crunch it down somewhat, but the image file will be somewhat larger than if you have no detail. If the image was very plain, like a white wall, then JPEG will crunch it down very tightly, and the file size will be somewhat smaller.

---Bob Gross---

[aam1234]

I think ISO setting can affect the size. The higher the iso the bigger the file is, due to noise.

[Jesper]

That is because images in JPEG format are compressed, which means the image data is converted, via a mathematical algorithm, to make the file smaller (so that it doesn't take up a lot of space on your memory card).

An uncompressed image from a 5 megapixel camera takes up 5 million pixels times 1 byte per color channel times 3 color channels (red, green and blue) = 15 MB. The camera compresses this to a much smaller size (1 to 3 MB, as Bob wrote).

The JPEG compression algorithm is lossy. That means that it throws away some of the information in the image to make the file smaller. The more fine detail there is in the image, the larger the file size will be. If you set the camera to a higher ISO setting, there will be more noise in the image. The compression algorithm can't see the difference between noise and fine detail, so if you choose a higher ISO setting, files will generally be larger.

Here are a few pointers to articles with more information:

http://www.photo.net/learn/jpeg/
http://www.normankoren.com/pixels_images.html
http://www.photo.net/learn/raw/
http://www.luminous-landscape.com/tutorials/understanding-series/u-raw-files.shtml