View Single Post
  #2  
Old August 18th 07, 08:29 PM posted to rec.photo.digital,uk.rec.photo.misc
harrogate3[_2_]
external usenet poster
 
Posts: 10
Default Compression in JPEG files in digital cameras


wrote in message
ups.com...
I need some help in explaining the JPEG compression feature in

digital
cameras. My camera (which likely is similar to most others) has the
feature to compress the photo JPEG files in the storage card. It

also
has the choice to have different pixel sizes (example: 3000x2250,
2000x1500, 1024x768, etc). What is the difference of the above two
features? If you store a 3000x2250 pixel data in compressed mode,
does it loose its quality? Can it be re-instated to full

uncompressed
size without loosing photo quality?. When I compressed the data, it
will fit more pictures in a single storage card. But, is it the same
if I choose 2000x1500 pixel and no compression instead?
Thanks for info.



Don't ask much, do you?

Jpg is a lossy form of saving the picture - Google on 'explanation of
jpg' and it will give you the detail - as distinct from compressed per
se. As you have lost picture info you cannot get it back, so there is
no way to regenerate the original intact. If you want to save in a
non-compressed format you need to use TIFF (OK it is slightly
compressed) or better still RAW which is just the raw data off the
sensor without adjustment.

The differences in resolution affect file size and quality of
reproduction. 3000x2250 is about 6.7Mp, 2000x1500 is 3Mp, etc. For
'normal' printing at full frame (i.e. without cropping) 3Mp will
produce an acceptable colour picture at A4 or thereabouts. For most
day-to-day use 5Mp is enough, 6Mp is really the limit for a compact.
If you go much higher than that you start to get digital noise in
saturated colours due mainly to thermal effects in the sensor. Another
point that is often missed is that the lenses on many compacts -
particularly at the cheaper end of the market - often do not have the
resolution in themselves to match the resolution of the sensor.

In a nutshell, use RAW or TIFF if your camera has it, otherwise go for
the best quality that the camera can provide - memory cards are dirt
cheap these days.


--
Woody

harrogate3 at ntlworld dot com