Why is Raw better than jpeg

On 06/08/2010 14:02, ransley wrote:
I use the T1i, dpreview gives a higher rating to Jpeg over Raw. I
believe its because Jpegs settings are optimised by Canon very well.
For my jpegs they come out very good. Is Raw recomended because jpeg
looses quality every time you open and close it?

No... RAW is recommended because JPEG can only code 8 bits per "channel"
(*). In a camera with significantly more than 8 bits per channel (**)
going to JPEG requires to discard some information, which cannot be
recoverdd later. RAW allows these choices to be made later (and pick up
among them the best choice for a specific photo).

Is the difference
noticable by opening and closing it say for example 5 times printed at
5x7 or 8x11?

JPEG loss only happen when you save the file. If you only open the file
for printing nothing happens to the orginal file.

Does the loss on jpeg only occur if you completely close
the photo?

"Close", no. "Save", yes, to some extent. This is why applications that
re-save the picture behind your back should be taken out and shot (this
is what happens with Windows Picture and Fax viewer when you rotate the
photo). But for the "quality" setting of most photos, this is very minor
and you won't notice anything un,les you edit and save the image ober a
dozen times. But you can completely avoid this by saving the
intermediate versions in a lossless format (TIFF, for instance) or the
native format of you picture editor (this will save layers, selections
and whatever) and only export to JPEG the final result.

What are other benefits of Raw to make it worth the extra
hassle of complete editing.

Showing off :-)

I am happy shooting jpeg, I am working
with 5 shot Photomatrix hdr and have done both Raw and jpeg [I think
Photomatrix loaded the jpeg] I am fully happy with the results but
jpeg is so much easier. I think for special photos made and composed
Raw may be optimal , but its time very consuming.

Agreed. I usually shoot JPEG too. I use RAW only when I know I'm going
to do some extensive work on the picture (difficule shooting conditions,
etc...).

(*) without getting into goory details, JPEG actually encodes luminance
and chrominance separately, and puts less emphasis on chrominance bcause
we are less sensitive to it, so it doesn't really encodes the primmary
colors...

(**) moderns SLRs achieve more than 10 bits most of the time

--
Bertrand

"Ofnuts" wrote in message
...
[]
No... RAW is recommended because JPEG can only code 8 bits per "channel"
(*). In a camera with significantly more than 8 bits per channel (**)
going to JPEG requires to discard some information, which cannot be
recoverdd later. RAW allows these choices to be made later (and pick up
among them the best choice for a specific photo).
[]
(*) without getting into goory details, JPEG actually encodes luminance
and chrominance separately, and puts less emphasis on chrominance bcause
we are less sensitive to it, so it doesn't really encodes the primmary
colors...

(**) moderns SLRs achieve more than 10 bits most of the time

--
Bertrand

Bertrand,

Don't forget that the brightness range coding in RAW is linear, but the
coding in JPEG is "gamma-corrected", meaning that JPEG can actually handle
a /greater/ dynamic range than RAW, but at a lower precision for a given
brightness level.

Where JPEG codes colour differently brightness is in the spatial
resolution. The eye cannot perceive colours as finely (spatially) as it
can greyscale differences, so in JPEG the colour component may only be
encoded at half the resolution (for example, you could look at it as 2 x
2.5MP colour difference images with a 10MP greyscale image). It can
encode primary colours as well as RAW - but at a lower resolution.

Appreciate you are trying to simplify, though.

Cheers,
David