JPEG Compression Factors

I've been trying to understand this business of 'JPEG compression

factors' a little more clearly. The Digimax 340 (3.2MP) offers data

compression rates of Normal, Fine & Superfine and I notice that most

other manufacturers offer roughly the same. Are they all produced to

the same 'Standard'. The above camera produces file sizes of

somewhere between 1.4MB and 1.9MB approx when set to 'Superfine' at

ISO 100. Image-size 2048 x 1536. Do all digicams produce roughly the

same size JPEGS at these settings. Including the DSLRs.

 

Perhaps one or two of you would be kind enough to tell me what size

files you get at these settings and your camera model.

 

Many thanks.

The Canon A70 (3.1 mp) is 1.6m, SF; 893k, F and 446 regular. ISO used could effect file size, because more noise will make for bigger files.

Hi John, I'm not sure if you've ever used a program like PKZIP to compress files, but I personally find it helpful to look at JPEGs the same way. If I have some files that I'd like to fit on a floppy disk, but they're a little too big, I can "compress" them with a program like PKZIP; they then take up less disk space. JPEGs are similar in a way. Image files typically take up quite a bit of space. Just one photograph from a 3mp camera won't fit on a floppy disk. Different manufacturers use this method of "scrunching" files to buy you more space, and while the format is standard, each manufacturer might "scrunch" there's in a slightly different way. One important thing to remember about JPEGs - each time they are compressed, they lose a little detail. That means it's a good plan to save your JPEGs just once, and from then on, as you work with your images, work from copies of the original files. Each time you open a JPEG and re-save it, it will experience a little degradation in image quality. If your camera supports RAW or TIFF, then these formats do not degrade, but the files are much bigger. So, it's a trade-off: JPEGs are smaller and more portable, but don't stand up to repeated saves. And I think it's safe to say that, no, not all digicams produce roughly the same size JPEGs. An inexpensive 2mp digital camera will have smaller JPEGs than a more expensive 5mp camera. Best wishes . . .

John,

 

Your post prompted me to do a quick little test to see what the numbers were. I have a Canon Powershot G3 (slightly under 4MP). It will save in Canon's "RAW" format and three levels of compression ratio for JPEGs. Keeping in mind that I always shoot in RAW mode (by default, the highest resolution, too) and convert to TIFF for editing, here are the numbers (all for 2272 x 1704 images):

 

RAW - 3.7MB

JPEG, Superfine - 1.4MB

JPEG, Fine - 794.7KB

JPEG, Regular - 312.95KB

TIFF from RAW file - 11.68MB

 

I can see the image degredation when going from RAW to even Superfine JPEG (though the difference between these two in a 5x7 print is very hard to discern). Image quality gets progressively worse as the JPEG compression in increased.

 

With that said, I believe most digital users who are focused on image quality shoot in their camera's native RAW format and convert to TIFF (or other non-lossy file format) for post-exposure work. They just have to fork out for larger storage media (only 8 RAW files will fit on the 32MB CF card that came with my camera - compared to 130 on my 512MB card). Also, previous comments about ISO should be expanded. Image quality suffers significantly on P&S digitals as you increase the ISO value. The very best image quality from a compact digital is produced by shooting at the lowest ISO for the camera, saving in RAW format and converting to TIFF (or other non-lossy) at the camera's native bit depth for color. Following this process will produce amazing 8x10 prints from even a 3.2MP camera.

Thanks Guys.

 

Beau, I realise that JPEG files are lossy, but thats all that my camera produces, so I keep the originals untouched and just open a new file and work on that if I want to print. Plus I also convert JPEGs to Tiff in some cases.

 

Rob, your largest image size is slightly larger than mine, yet on Superfine you mention 1.4MB JPEG files. On average mine come out a bit bigger than that, which suggests that your camera is compessing more than mine. Thats the point I was getting at, there doesn't seem to be a standard that they all work to.

 

Regards

Right, there isn't a part of the jpeg spec that defines the compression that is called fine, superfine, etc. I haven't looked but I'd expect that the various manufacturers keep in camera processing details pretty closely held as it's an important part of getting the results they get.

There is no set standard on JPEG compression modes.

 

If you look at major editing software, they actually have about 10 levels of compression, plus the capability of progressive scanning.

 

My advice: CD's are cheap. HD space is relatively cheap. I shoot at the maximimum size the camera will provide (least compression). If I need to "photoshop" the result, either for cropping or color balance, then I have more raw information to work with. I use the full size file for printing. I shrink the finished product for email.

 

Think of it another way: You paid a premium for a 3.2MP camera vs a 2.0 MP camera. If you use "normal", then you are taking images at the quality of the 2.0 MP camera.