D200-->JPG question

[kam_bansal]

Hi all,

 

JPG's can be compressed in 3 ways, 1:4/Fine, 1:8/Normal and 1:16/Basic.

 

The camera takes a picture, compresses the image 1~3 times (1:4, 1:8

or 1:16).

 

Is there any information lost when it compresses? I mean, the more

compression, the smaller the file, but all the data is still there

(compressed). If thats the case, I would choise the 1:16 setting to

get the most pictures.

 

If this is not true, then there is lost data per compression cycle?

 

or did I totally miss the boat?!

 

~Kam (^8*

[wiseguyvisuals]

JPEG is by definition, not a file format, but an algorithm by which files are compressed. It is a "lossy" compression, meaning data is discarded according to an algorithm to yield a still-recognizable photo. Each compression, or once the files are in a computer, each save, causes another recompression and therefore a loss of some data.

[rainer_t]

Kam,

 

JPG compression is not a lossless compression! The bigger the gain

in space, the more information is lost.

 

Rainer

[pnance]

Which is why they have raw files, NEF's are supposed to be loss free. Even changes made to the files are reversible. JPGs lose data every time you save them.

[ellis_vener_photography]

<I>JPGs lose data every time you save them.

</I><P> Not quite. They lose data (through further compression) everytime you do editthe file (manipulate the iamge) and then save as a jpeg again.

[timberwulf]

Right Ellis. The idea if you're shooting JPG (and I always shoot JPG -> Fine, though I'm about to start experimenting with RAW) is to get the image off of the chip (drag it out into a folder, or what have you) and then once you've started working on it, save it as a PSD, or perhaps a TIF or other lossless format. This will enable you to save as many times as you like while working on the image while not losing further detail.

 

While I know there are great advantages to RAW, I've found JPG - Fine to really be acceptable as to image quality.

[marcofrancardi]

lose data when edit file or push the compression ratio further.

[jon_noble]

Wrong Ellis.

 

Someone has proven this before here by using the difference blending mode of an original jpg image and a(unedited)resaved jpg image

[jon_noble]

Proof<div></div>

[jon_noble]

The result of original and copied image (unedited and saved at default compression i.e. the one it was saved at originally) and amplified to show the artifacts.

 

If no quality is lost between the 2 it should be completely black<div></div>

[jim_gifford]

My understanding -- which may be imperfect! -- is that the data in a JPEG compressed image is subject to change and therefore subject to loss every time the file is saved.

 

Openeing a JPEG file requires the use of software to create the full-resolution image from the compressed data so that the image can be viewed. Saving the file as a JPEG compressed image requires the use of data compression software to create the instructions for the NEXT time the file is opened. The software does not necessarily make the same pixel-by-pixel choices every time a file is opened or closed, and that is why data is subject to loss at each retrieval.

 

The losses in JPEG file use are not as awful as the sort of degradation we all remember from the bad old days of analog audio copying (ever make a copy of a friend's copy of a buddy's audio cassette?). Nonetheless, saving a JPEG file is a new data manipulation every time, so it does involve some expectation of (very small) loss of data.

 

Be well,

[timberwulf]

I'm sorry, I misread Ellis' post. ;)

 

Yes, no matter what, every time you SAVE a JPG as a JPG it will lose data. I was thinking someone had said every time you VIEW a JPG it will lose data. Doh!

 

Was having a bad day Friday. :)