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Hi,
I would love an answer to this question. Both a short one that I can understand and a long one so that I can confuse a friend with science.
I have a picture from my camera and it is a JPG of about 2MB. As soon as I make any ajustments to it (colour, perspective etc) the file size jumps right up to 25MB. Why?
Cheers
Lenny
Before I attempt an answer let me ask two leading questions. What is the pixel size of the file? My guess would be about 3000 x 4000 pixels. Secondly, in which file format are you saving the 25mb file. PSD?
JPEG - Joint Photographic Experts Group
The JPEG image is compressed and depending on the quality setting on your camera the compression will be higher or lower. High compression = Low quality. http://www.faqs.org/faqs/jpeg-faq/part1/
The PSD format is non-compressed and also stores all your selections, paths, channels etc. So the PSD format isn't a good choice if you just want to store your image. If you don't want to compress your images you could choose BMP or TIFF.
If you want to enhance your images always use the PSD format as a working copy. This will ensure that all your selections etc is there for you in the future.
Moltas provided the answer. I'll add a slightly technical desccription which won't be perfectly accurate but a give you a good idea without being too dense.
When a JPEG is saved, the image is first analyzed in 8 x8 pixel blocks. The upper left pixel is written in code while the rest of the block is written in a simpler code in relation to that reference pixel. This shrinks the file size. The greater the compression, the closer to the 'same as the upper left pixel' is how the file is written and the image becomes cruder as more data is thrown away. In it's worst case, which is never used, all 64 pixels in each 8 x 8 pixel block become the same. As soon as you can see the blocks of pixels in an image they are called artifacts. The fact of throwing away pixel data in the JPEG compression scheme makes the file smaller but because data is lost the technique is called lossy vs. a lossless file compression scheme in which no data is lost.
When you save a PSD file each and every pixel is written in the file, location, transparency, color. Each layer you add increases the file size, as does each channel and everything else needed to be written into the file. On top of that, there is now saved a composite image which is a flattened image. That boosts your file size tremendously but is necessary, we are told for old versions of Photoshop compatibility.
Actually it is necessary because .psd used to be Photoshop's proprietary file type. They allowed other applications to use their file type but most of them only can read flattened files, not the full .psd. In order for those applications to be able to read your .psd files, the flattened (composite) must be part of it.
Recently we had a fascinating (well to me it was ) discussion on saving your image files in .psd files vs. the .tif layered format which allows for one of two lossless compression schemes, LZW and ZIP. You might find it interesting.
I should add that you mentioned resaving your file as a JPEG. Generally that is a bad idea. Each time you resave a file as a JPEG it degrades as more pixel data is thrown away. You should save your original JPEGs separately but when you are working on a file save it in the .psd format or .tif with layers, channels, etc. A TIF with LZW or ZIP compression will save the same data as a .psd file but will be considerably smaller than the .psd.
So a file that has been lightned a little would save at 25mb (just an example) but a file that had been lightned and restored a little and maybe stretched but still only working on the original layer could then be 65mb? It all depends on how much it gets edited? Why can a single layered image be so different? isn't the image still using the same amount of pixels of the same size?
Cheers
Lenny
I've also noted that those file sizes in the bottom of the PS screen are kind of like Enron's books, inflated. I often start with a 10 meg raw file and open to 25 or so in PSD then by the time I've worked the image a while the funny number at the bottom of the screen will say 1.5 or more GIG. If I save it, Exploer seldom will show even 1 gig.
Hi Robt,
Your working on files that get to 1GB! That seems rather large, especially as it is only relatively recently that we have had HD's that size.
What did you do 5 years ago?
Lenny
Interesting question, maybe. 
Linear Mode
