Compression settings

[diyrus]

Hi, This is my first post here. I have just got a A20 and after going through the manual I am confident I know the camera well except for the compression settings.

What noticable difference does normal, fine & superfine make?

Apart from taking up more space on the CF is it best to use SF.

Thanks

[Conk]

My manual tells me to set the camera to superfine for better quality images.

[Rustle]

I think the compression ratio for SF is 5:1, whereas S is 10:1 and N is 20:1, but don't quote me on that for an A20. You can find out for sure by taking three exactly same picures at the different levels, then comparing the file sizes.

Russ

[Mariner]

I have essentially the same question about “What noticeable difference does normal, fine & superfine make?”

If you compress more are the pixels further apart? So the image is not as saturated or…detailed?

[Rustle]

JPEG compression is 'lossy', meaning that it compresses by losing data. When the picture is uncompressed for viewing, the viewer program fills in the blanks by combining data from surrounding pixels.

So, yeah, in a way the pixels are farther apart. The spaces in-between get filled in, but may not match the original data. That's why you'll get more artifacts and colour loss with higher compression. The Superfine setting has the least compression and, thus, keeps the most original data from the camera's sensor.

Canon's RAW image format is lossless--it compresses without deleting data. So, when you decompress a RAW file you can get the exact image you captured and save it as a standard TIFF file that will preserve the data. Unfortunately, the A30, A40, and ELPHs don't have the RAW setting.

So, your options are to get bigger pictures, better quality pictures, or more pictures. Or just buy a larger CF card or Microdrive if you can afford it.

Once you've thrown out the data via lossy compression, you can never get it back. With that in mind, I'll drop the image size if I'm running out of room, but I don't like to drop the quality setting.

Russ

[diyrus]

Rustle,

Thanks very much for your detailed reply. I will take some shots with the three compression settings and try to see if I can detect any noticable difference.

Tony

[john_houghton]

It's worth mentioning that while artifacts introduced by jpeg compression might not be immediately noticeable, subsequent processing of the image may make them much more visible. The screen shot at the link below shows the effect on jpeg and tiff images when the image is darkened by dragging out the tonal range using the curves feature in Photoshop. It shows wispy clouds in the sky, over-darkened deliberately to show the banding effect in the jpeg file. (Image displayed at 100% size).

artifacts (http://homepage.dtn.ntl.com/j.houghton/compress.jpg)


John