Not "negative" in the sense of film days. The film developed to a "negative" for purpoes of producing "positive" prints. To pick on black & white photography, the paper is normally "white" but turns grey and ultimately black as more light hits the paper. So the negative is "transparent" where the area is supposed to be black (or very dark) and it's black and non-transparent where the image is supposed to have a very light tone or white. Hence the image on the film is actually the opposite of what we want on the print and so it's called a "negative". Color film does the same thing... it blocks the light we do NOT want on the photographic paper.
But some film is color positive. Slide film is "positive". It develops just the way we'd expect. Green grass is actually green. Blue skies are actually blue on the slide.
Digital cameras technically record the image as a positive (like color slide slide film) and not the inverse. But once we take the picture, the camera converts the data from the sensor and combines the red, blue, and green values of light recorded at each photo-site on the sensor surface to create color "pixels".
Most cameras will, by default, record images in JPEG format (.jpeg or .jpg files). JPEG was created to allow for image compression to save space by eliminating unnecessary data that the human eye would almost certainly not be able to notice. When you go the point store, you can find paint samples in shades which are so nearly identical that you almost cannot tell them apart. The camera plays upon this weakness and takes color hues which are so nearly identical that your eye would not be able to tell they are not technically the same -- and it flattens them to just one single hue so that they just are the same. Your eye cannot tell.
The problem is that while your eye would not have noticed the difference at the current brightness value of those hues, at a different brightness level the difference might have been obvious. If a photograph of a white wedding dress with a lot of detail in the white lace pattern is a bit overexposed, a JPEG might just flatten the whole area to simply "white". When you adjust the image by decreasing the brightness, you expect the detail to return... but it doesn't. It's gone forever because the JPEG storage algorithm threw out the original data in favor of data that was all the same and did this because your eye would not have been able to tell the difference at that particularly brightness level.
RAW format is a concept where the camera is not allowed to change the image in any way that would result in a loss of original data. In this sense, you can think of it as being like a "negative" (except it's really a positive -- it's just a data-preserving format) This provides the best opportunity for adjustment and detail recovery in post processing but there are two downsides. The first downside is that the files will be larger -- in a very obvious way. A "RAW" file from a camera might be 25 megabytes. But JPEG of the same image might be a few hundred kilobytes (less than 1 MB). How much compression it gets depends on the complexity of the source image.
The 2nd downside is that there is a lot of work your camera does with an image before it converts it to a JPEG and since you used a storage option that specifically tells the camera not to make any changes that would result in a loss of original data, the camera will NOT perform some of these time-saving tasks.
So for example... a camera will normally apply a "white balance" adjustment, might apply some de-noising. It might apply sharpening to some areas of an image ... the list goes on. But when you shoot in RAW format, these tasks are NOT performed because if it changed the source data then it wouldn't really be a "RAW" image anymore.
Many RAW workflow programs (like Aperture or Lightroom) will automatically apply the kinds of adjustments that a camera would have done had you shot in JPEG and it will happen automatically as you import the images into the software. The change is non-destructive, however (it applies the changes as an adjustment on top of the original source data which it still maintains.) In effect, you get all the conviences of shooting in JPEG, but with all the power and adjustability of RAW and without losing any detail so that you have maximum adjustment latitude available to you.
Disk spindles are pretty cheap these days... for the most part I only shoot in RAW.
There is one last note... it's subtle but important to remember. If you shoot in continuous burst mode -- where you just hold the shutter button down and the camera keeps clicking away -- then you'll find you can shoot MANY MORE frames in JPEG format than you can in RAW format. This is because the camera has an internal memory buffer and must write those images to the card as quickly as possible, the card has a maximum speed that at which data can be recorded (which is why it's a good idea to get a very good memory card.) Since JPEG images are tiny in comparison to their RAW counterparts, the camera can shoot MANY MANY more JPEGs in a row before the memory buffer in the camera fills up and the camera has to slow to a crawl. This only applies to continuos burst mode.
Tim Campbell
5D III, 5D IV, 60Da
