JPEG query - file sizes after editing

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sonic

Guest
I wonder if anyone can answer something which has been bugging me.

I'm using Adobe Photodeluxe to edit my photos. When saving my edited photos, I can choose the compression setting (ie. maximum, high, meduim, low). If I choose maximum or high, the resulting file size is often larger than the original! This even seems to happen when all I've done is cropped the picture. Logic would dictate that when compressing a picture, the resulting file size should be smaller, especially when you've cropped the picture.

Can anyone explain this?
 

ABC1

Novice Member
File size would depend on what the compression settings of the original file were, so if they were medium to start with if you go to maximum then the file will be larger.
With JPEG's the more you compress it the more you degrade the quality of the image, so if you value your images, then don't compress to much.
 

Liquid101

Distinguished Member
Another thing to remember is that every time you compress an image using JPEG, you are creating more 'noise' on the image. If you zoom into a heavily compressed image you will see the structure of the compression.

When you re-compress the image, the JPEG algorithm just sees this as image detail, and tries to record it - giving you lower rates of compression, and higher file sizes.

HTH
 
S

sonic

Guest
Thanks for the swift replies!

I think my confusion is because I've always thought compression to mean "reducing the file size by losing detail". But it seems there's much more to it than that.

So what is happening? When I take a picture with my digital camera, the image is saved to the memory card in JPEG. I assume some sort of compression has happened here. Then when I put the picture through Adobe PD (let's assume I don't do any editing) and save at maximum compression, the file size is bigger. What has happened to the picture, what's been added to make the file size bigger? (Surely not just noise?) And how does the quality of this image (the one compressed at maxiumum) compare with the quality of the image on the memory card?
 

Liquid101

Distinguished Member
sonic said:
So what is happening? When I take a picture with my digital camera, the image is saved to the memory card in JPEG. I assume some sort of compression has happened here. Then when I put the picture through Adobe PD (let's assume I don't do any editing) and save at maximum compression, the file size is bigger. What has happened to the picture, what's been added to make the file size bigger? (Surely not just noise?) And how does the quality of this image (the one compressed at maxiumum) compare with the quality of the image on the memory card?
When you make your digital photograph, the image is recorded as a giant grid of pixels. Performing the jpeg compression makes the grid slightly larger by grouping pixels of similar values in colour and tone together. The more compression you use, the larger the grid is, the wider the gamut of colours/tones that are grouped. This is why even in lightly compressed images large patches of single colours, seem to have more more visible compression artifacts.

When you bring this image into Photoshop, the pixels are 'uncompressed' and displayed as single pixels. i.e. your 400k .jpg now shows its full resolution of 12MB. The artifacts 'like a noisy grid pattern' are obviously still visible, but now exist as just part of the image. When you try to re-compress, these artifacts can make it more difficult to group larger areas, and therefore make slightly larger files.

Obviously, each times you open and re-compress your images you are going to lose detail and quality.

If you are planning to do any editing of your images, I would save them as TIFF files until you are happy with the result.

By using something loss-less like a TIFF file, you lose nothing. You can open and re save as many times as you like. you'll be recording each pixel individually. You can compress these files as well, using a loss-less compression called LZW, but thats another story.

I'm no expert on the matter, and this is a very basic description of what's going on.

HTH
 
S

sonic

Guest
Yes, that does help, thanks very much!

I get it now. Noise and artefacts notwithstanding, the image recorded on the memory card is compressed, it is uncompressed when put in Photoshop, and when saving, you can choose a setting which re-compresses at a lower or higher setting than the compression that happened when the camera saved the image to memory card, hence the change in file size.

That cliche about learning something new everyday is so true... Thanks again!
 

Paul Shirley

Novice Member
It sounds like your software describes compression by the output image quality rather than actual compression rate. Higher quality = lower compression and larger file sizes.

If all you're actually doing is cropping try to use a tool with lossless JPEG processing. You'll only be able to crop to 8 pixel boundaries but there will be zero quality loss, it does no recompression, just dumps the compressed blocks out unchanged. jpegcrop on http://jpegclub.org/ is one simple tool.
 

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