Where were the pixels gone?

[yusia]

Maybe my question is completely dumb, but I still will ask it.
I shoot with 8 megapixels camera in the large RAW format. The files are really big when I download them. I edit them in RAW format, save as jpeg, and edit them a little bit more. After that I save them again as jpegs at max quality. The final file size is about 3.2 megabyte without any cropping.
Where do I lose my pixels?
I don`t understand! Please, help!

 

[Alex_B]

The magic word is compression.

The data is compressed even at maximum quality ... as far as I understand.

 

[Garbz]

Yes JPEG like the mp3 audio format is a lossy compressor. It dumps pixels for a set of algorithms to re-construct the image. Even at max quality the original can not be reproduced bit-perfectly. Although I have edited the same file over and over again and always save as a Quality 11 (not even the highest) jpeg and it still looks fantastic.

The other place you loose data is in the bitdepth. JPEG is 24bit (3x8bits), whereas RAW (at least from Nikon cameras) is 36bit (3x12bits). Now the difference visually is nill. Even at 24bits you can reproduce over 16.7million colours, however when you edit images and use tools like curves or levels to adjust brightness colour or contrast, you are using those extra colours to fill in gaps which would otherwise lead to "posterisation" of an image.

/EDIT: Math mistake

 

[table1349]

You are also losing data and quality when you save the photo to JPG work it over and save it again. You are compressing a compression. It would be better if you did all your work in raw and save only the final product as a JPG.

 

[Ockie]

is 32bit (3x12bits)

Sorry to go off-topic but I just HAD to quote that.

 

[kevin_c]

.... is 32bit (3x12bits)..........

Er... when I went to school many (many) years ago, 3 x 12 equalled 36, but hey, maybe things have changed since then...

 

[Funky]

almost tempted to save it on my siggy....

 

[burtharrris]

It comes from computer language. Computers only know two commands, On and Off, 1 and 0. Ever hear of a bit? That's one command, either 0 or 1. So there are two possible choices, infinite times. 32 bit is 2 raised to the 5th power (2^5=32). 16 is 2 to the 4th power (2^4=16).

 

[table1349]

Er... when I went to school many (many) years ago, 3 x 12 equalled 36, but hey, maybe things have changed since then...

That's the New Math!

 

[droyz2000]

Your best bet is to save your files as either Tif or psd so that they retain as much information as possible.

 

[Garbz]

Let me just say i'm used to calculating in base 8.6 and 3*8.6+6 is infact 32 But yes after 5 years of maths at university (i'm an electrical engineer), I can't do anything anymore without a scientific calculator. lol

 

[EOS_JD]

To answer the OPs question he's confusing Megapixels with megabytes.

An 8Mp camera prodices an image that has 8million pixels (roughly)

Look at the size of the image in pixels

My 20D files are 3504px x 2336px

3504x2336 = 8,185,344 pixels (or 8Mp)

 

[yusia]

You are also losing data and quality when you save the photo to JPG work it over and save it again. You are compressing a compression. It would be better if you did all your work in raw and save only the final product as a JPG.

But if I want to use blending modes or channel mixer? Can I do it in RAW?

 

[EOS_JD]

But if I want to use blending modes or channel mixer? Can I do it in RAW?

Export from RAW as a tif. Edit your image as you normally would inphotoshop and finally save as a jpg.

I output my RAWs as Tif files with LZW compression (which is a lossless compression). then finally save as a jpg once I've edited.

 

[Digital Matt]

As JD said, do not confuse megapixels with megabytes. They are two totally different concepts.