About formatting Compact Flash cards

[giovannis]

I noticed that every time I reformat a Compact flash card in my camera (canon

400d), the number of available photos decreases (2GB - from 202 photos to 190

as of yet). This is something I haven't previously noticed. Do I have to

expect a constant reduction of photos I can take over time? Thanx to all

[joe_allebaugh]

Giovanni,

 

Was the ISO set to the same value each time you formatted? Higher ISO exposures use more card memory resulting in less exposures on the card, up to 15% fewer exposures per card between ISO 100 and ISO 3200 on my Canon model. Otherwise, you may have failing memory components in the card which formatting removes from available space. If still under warranty, return it.

[giovannis]

Hi, thanx for your prompt reply. In fact, come to think of it, I did many shots at 1600 ISO and the selector has remained there. Now, if I set to 100 ISO, the number jumps up! - Didn't know that, thanks a lot :-)

[beeman458]

Anybody able to answer the why as to when you up the ISO, the image uses more memory. TIA.

[joe_allebaugh]

Thomas,

 

The best explanation I've seen is additional color information from noise at higher ISO settings. More noise/color information results in less compressibility results in larger files.

 

See this link for the discussion - http://photography-on-the.net/forum/showthread.php?t=19791.

[beeman458]

Thanks for the link. Never considered noise a "recordable" event. :O

[jimstrutz]

Noise itself isn't the real issue. The problem is JPG compression considers the noise to be detail, and won't squeeze it as tight. More noisy detail results in less compression. JPG does the same thing with other highly detailed images -- they're always larger than images with lots of smooth areas.

[philg]

Giovanni: You can do a simple thought experiment. Imagine an image that is 100 megapixels and all one color of blue. You can compress this image down to the following information: "10,000 pixels wide; 10,000 pixels high; all one color represented by the following three RGB values". You've stored a 100-megapixel image using maybe 100 bytes. Now imagine that you introduce a lot of noise into the image so that every pixel is a slightly different color. There is no predictable pattern to this noise, which is what makes it noise. The camera can't tell the difference between subject detail from the world and noise from the sensor. Now we need to record RGB values at every pixel. This takes six bytes per pixel (16 bits per color) times 100 megapixels = 600 megabytes. An entire CD-ROM for one image that is the same size as the image we stored with 100 bytes.