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Old Jan 6, 2004, 8:10 PM   #1
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Default Compression Retings

How can I tell how much J-peg compression a camera uses? Should I multiply the number of horizontal pixels by the number of vertical pixels and divide by the file size? I read a report about the Fuji S5000 and the report complained about the strong compression used. That it was 10:1. I've looked at other digital cameras, but can't find information about the level of the compression used. I have a Canon G-2 and want to know what compression Canon used in it. I know it has three settings, I would like to know what the best would be. Also, is there a way, like I suggested above to figure out compression settings used on digital camreras? When I use the method above, the G-2 would have a compression rating of 19-1 in its best setting. I hope it's not that bad.
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Old Jan 7, 2004, 6:42 AM   #2
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It's not as bad as it seems. The following link covers a lot of the issues to do with JPEG, and I'd particularly suggest that you have a read of section 4.

http://web.ask.com/redir?bpg=http%3a...q%2f&qte=0&o=0

In terms of the use of JPEG itself on your camera, It realy depends on the end use. The higher quality JPEG, the more subsequent flexibility you'll have for any post processing.

Hope this helps.

Regards,
Graham.
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Old Jan 7, 2004, 3:55 PM   #3
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This does not answer my question. How do I find out how much J-peg compression is being applied to my pictures in my camera?I know what J-peg compression is.
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Old Jan 7, 2004, 5:30 PM   #4
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What you do is look at your jpeg image file size straight off the camera in Explorer, then open it up in an editor and see from 'image propertie' how big the image file really is after decompressing it.

The best answer for buyers of new cameras is to download all the manuals of the ones which interest you, then for the same Mpix, see how many pics they say fit on a given size card at the various 'Quality' settings. This gives you the approximate compressed file size. If they give you a TIFF uncompressed file shooting option then that's your top limit. Obviously the bigger the compressed file size, the lower is the level of compression. But my take on this is that manufacturers are nervous about offering lower compression options than at present since the 'how many pics can I get on a card' gives a bad message (shame).

Saved compressed sizes are always approximate because many cameras will produce variations in files sizes according to amount of scene detail, but this variation isn't normally more than about 25%. That's why the number of pics you can expect on a card varies, and the estimate gets more accurate as you fill it up. Be careful when users say they can see no difference. It usually means they have little critical detail in a scene, can't recognise artefacts, camera shapening is hiding edge effects or they are printing small.

If you want to study artefacts, try subtracting a shot taken at the lowest compression TIFF or high quality, from the same shot using higher compression by using Photoshop, or do a similar thing on a clean uncompressed file doing saves at various jpeg 'quality' percentages in Windows on your pc. Doing this even on greyscale images is quite revealing. VOX
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