Math for Digital picture size?

Question

Explain me where am I going wrong?
24 bits per pixel =3bytes/pixel
8 Megapixel image (8x10E6)
Image size should be 8x10E6x3 ~ 24MB

However, my JPEG images are 3.3MB and RAW images are 10MB.

Why isn't my picture 24MB? Or where is my math wrong?

Thanks.

I don't understand the compression algorithm. However, I know that JPEGs can be compressed quite a lot with loss of quality.

My question is more related to the RAW format image. Why are RAW images not 24MB according to the Math above.

Say, if each pixel is represented by a single number minumum is 0 and maximum is 2^24 (~16 million), and some of them might take less bits per pixel, then the lower image size is possible.

So, if the above is true, would a perfectly Black (under exposed) image be 24MB, since it requires R/G/B values to be 255 each?

Answer 1

Your math is not wrong. If you open a pic in Photoshop it will be in the neighborhood of 24 MB after decompression. It's not exact because the sensor is not exactly 8 MP, and there is additional fixed overhead in the JPEG file.

Low compression JPEG (as it comes from the camera) is not inherently low quality. The real problem is in repeatedly saving and reopening the same pic. You lose a little each time.

I think RAW is a completely different animal in how it is coded.

Good Luck

Answer 2

I'm not sure about the bits per pixel math, but you are not taking into account that jpeg files are compressed to a minimum ratio of 1:4 at best. RAW images are not compressed. Image data includes MORE than just the picture itself, also, so you can't do the math without consideration for the exif file and so on. Flickr doesn't seem to be working right now, or I could show you. Try there sometime and click on any photo and then look to the right for a link to "More Properties" and you'll see what is connected to each photo - in the camera and on the memory card.

RAW images are generally about the same as the stated picture size. In other words, an 8 MP RAW image is going to be around 8 MB. Jpeg images are compressed. The file size will vary GREATLY, even at the same picture size and resolution (which equates to comression). On my 10 MP camera set to the biggest image at the highest resolution, I can get jpeg files from as low as about 3.5 MB to as high as about 7.0 MB. The highest I am finding at the moment is 7.43 MB.

```ADD'L```

Okay, Flickr is back up. Go to http://www.flickr.com/photos/samfeinstein/499092917/ for instance. Click on "More Properties" on the right. Most of the information in this table comes from the CAMERA and not from Flickr. This is cropped to 3,000 x 2,400 pixels, or 7.2 MP from a 10 MP image. The file size is still 6.12 MB, though, so ... "go figger!" I'm not sure which of my uneditted photos is the original, but I have 14 that are pretty much the same, except for exposure. They range from 5.32 MB to 6.70 in size. The smaller files are too dark, so there must not be as much image data in the dark spots. The lightest images - the ones with blown out highlights - are not the largest ones, though. It seems like the largest files are made from the best exposures, so I guess they have captured more details across the dynamic range of the image and so they have more data stored.

In other words, you can't compute the file size of a jpeg image by simple math.

Answer 3

Also the camera only stores the number of colors necessary to create the image. So file size is also affected by the number of colors in an image.