xti JPEG Picture Size

My daughter just purchased an xti Rebel 10mp. We selected the "large" jpeg file size for taking pictures but the resuting pictures are approx 4mp. I thought the pictures would be closer to 10mp. Is the 4mp picture size correct? Is 4mp the largest jpeg picture size that can be taken with the xti?

When we swiched to "raw" the resulting pictures were close to 10mp.

Any assistance would be very much appreciated.

You're confusing megapixels with megabytes. One is the pixel resolution of your camera, the other is the amount of data in the picture file. Two entirely different animals.

Remember, JPEG is a compressed format. When you open the photo in PhotoShop or another program, it will tell you the actual size of the picture. That is one reason that a lot of the more experienced people on here will tell you that they use RAW most of the time. Every time you open and save a compressed format file, be it JPEG of another, you get some loss of quality. I am sure someone is going to respond with a really good link to an article that explaines it fully.

PhotoFranz wrote in post #2730301
Remember, JPEG is a compressed format..

Actually, both jpeg and raw are compressed, but raw uses a lossless compression technique. Secondly, jpeg is a 8 bit file format where as raw is 12. And here is the site with lots and lots of details. "click me"
ANd for those interested in more specifics about jpeg, "click here"

Thanks gil711, I knew someone would come up with links pretty quickly. I think I will read them and learn too. Thanks again.

This is a quote from dpreview.
Image type
Time, secs
(2 GB SanDisk)

Time, secs
(2 GB Lexar)

Time, secs
(8 GB Lexar)

Approx.
size
3888 x 2592 RAW + JPEG *1 3.2 3.2 4.3 15,260 KB
3888 x 2592 RAW 2.6 2.6 2.9 9,300 KB
3888 x 2592 JPEG Fine 1.4 1.5 1.9 3,100 KB
3888 x 2592 JPEG Standard 1.0 1.1 1.5 1,500 KB

You can goto the site and look at the relative size that the JPEG should be. I think it's about right though.
http://www.dpreview.co​m …s/canoneos400d/​page12.asp

gjl711 wrote in post #2730373
Actually, both jpeg and raw are compressed, but raw uses a lossless compression technique. Secondly, jpeg is a 8 bit file format where as raw is 12. And here is the site with lots and lots of details. "click me"
ANd for those interested in more specifics about jpeg, "click here"

Could you explain how RAW is a compressed format? My understanding is that a RAW file contains only the "raw" sensor data. As such, there is a red, green, or blue intensity level recorded for each pixel and you get, roughly, 1 pixel per byte. After the bayer conversion, each pixel now has a red, green, and blue value, or 3x as much data. So after conversion, an 8-bit TIFF is approx. 3x as large as the RAW file and a 16-bit TIFF is approx. 6x as large as the RAW file. So, it seems to me that a RAW file is not compressed at all, it just simply hasn't been converted to a RGB file format yet.

WGK wrote in post #2730274
My daughter just purchased an xti Rebel 10mp. We selected the "large" jpeg file size for taking pictures but the resuting pictures are approx 4mp. I thought the pictures would be closer to 10mp. Is the 4mp picture size correct? Is 4mp the largest jpeg picture size that can be taken with the xti?

crn3371 wrote in post #2730298
You're confusing megapixels with megabytes. One is the pixel resolution of your camera, the other is the amount of data in the picture file. Two entirely different animals.

gjl711 wrote in post #2730373
Actually, both jpeg and raw are compressed, but raw uses a lossless compression technique. Secondly, jpeg is a 8 bit file format where as raw is 12. And here is the site with lots and lots of details. "click me"
ANd for those interested in more specifics about jpeg, "click here"

Both answers are correct, but the 1st one is the answer to the OP's question.

Mark

BTBeilke wrote in post #2730687
Could you explain how RAW is a compressed format? My understanding is that a RAW file contains only the "raw" sensor data. As such, there is a red, green, or blue intensity level recorded for each pixel and you get, roughly, 1 pixel per byte. After the bayer conversion, each pixel now has a red, green, and blue value, or 3x as much data. So after conversion, an 8-bit TIFF is approx. 3x as large as the RAW file and a 16-bit TIFF is approx. 6x as large as the RAW file. So, it seems to me that a RAW file is not compressed at all, it just simply hasn't been converted to a RGB file format yet.

I do not know the specifics of the Canon Raw Compression Algorithm (it's a proprietary format), but the Canon website specifically lists the Raw Format as Raw Compression in their camera specifications.

Mark

Thanks to all, I was getting mexapixels and megabytes confused. Thanksfor the help.

Mark_Cohran wrote in post #2730749
I do not know the specifics of the Canon Raw Compression Algorithm (it's a proprietary format), but the Canon website specifically lists the Raw Format as Raw Compression in their camera specifications.

Thanks for the info, Mark. I did a little searching and found this on the OpenRaw.org site (http://www.openraw.org​/node/1482):

.CRW, .CR2 and .TIF Canon RAW photo formats
There are three main formats used by Canon for their RAW photo format. The oldest format is the .CRW format, which is known as the Canon CIFF format. This is used by the D30, D60 and so on. Canon then swapped to a new RAW format, with a .TIF extension, used by the 1Ds. Probably because of confusion between RAW TIFF files and normal TIF files, Canon then swapped to the .CR2 extension with the 1D MkII. Canon .CR2 and RAW .TIF files are essentially the same format. Canon’s RAW format is internally well structured and consistent between different cameras. The latest format draws heavily from the TIFF IFD format. Probably the purest and best of the various RAW formats in existence (albeit still undocumented by Canon). Uses a clever, lossless, JPEG encoding technique for RAW data.

In any event, it still seems to me that the biggest difference between the size of a RAW file and a TIFF file is the amount of information stored in the file and not any relatively small reductions due to lossless compression. But, every little bit helps.

If the raw file would be uncompressed, it would be 1.5 times larger than the megapixel count (12 bits = 1.5 bytes).
Thus, an 8Mp image from my 20D would be ~12MBytes. But my raw files are no larger than 8-9 Mbytes. So they must have been compressed.

PhotoFranz wrote in post #2730301
Every time you open and save a compressed format file, be it JPEG of another, you get some loss of quality.

Well open it once to work on it the save it when your done. Is there really a need to keep opening and resaving. How many times do you really resave an image?

BTBeilke wrote in post #2730687
Could you explain how RAW is a compressed format?

Canon's raw files are losslessly compressed, much like zip files -- you get the original data back while saving space.

JPEG uses lossy compression: the original data can only be approximated, never restored with bit-for-bit accuracy.

Some cameras don't compress their raw files at all. This results in 10+ MB files from 6 MP sensors, etc. They take longer to write to card and take up more space.

adas wrote in post #2731505
If the raw file would be uncompressed, it would be 1.5 times larger than the megapixel count (12 bits = 1.5 bytes).
Thus, an 8Mp image from my 20D would be ~12MBytes. But my raw files are no larger than 8-9 Mbytes. So they must have been compressed.

Hey, don't be bringing any math into this discussion! What's really embarrassing is that I'm an engineer and I got so caught up in the particulars of the file format that I didn't even think to just do a simple bits to bytes conversion.

And, as expected, the math is right on when calculating the size of a 16-bit TIFF file from my 5D:

16 bits / (8 bits / byte) = 2 bytes

12719616 pixels x (2 bytes / pixel) x (3 values / pixel) / 1024^2 = 72.28 MB

So, I stand corrected. It appears that the CR2 compression generally reduces the file size by about a third.