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I remember when I first started shooting. The camera arrived at my house, and I had no idea where it would honestly take me! About 4-5 years has passed since that day, and I still feel there are a million more things for me to learn. Today, shooting in RAW format almost all the time is the most important lesson for my career. Almost every commercial client is looking for either a RAW file straight from the camera, or a TIF file after being edited in photoshop.
When you shoot in JPEG, your camera automatically makes all the editing calls. It adjusts the exposure, shadow brightness, highlight darkness, curves, sharpening, and many others. Then it compresses the image down by saving only a few details in similar areas.
This can be great for those shoots where the client doesn't require the highest quality images, and for shoots that involve repetition. If I'm shooting a bike race to sell images directly to the racers, I will shoot in a mode that records a RAW file and a JPEG. This way I can get the photos online for sale as soon as I am home. If the JPEG doesn't look right, I can still open the RAW file and do my own edit.
The rest of the time I am ALWAYS shooting in RAW mode. One day several years back, I received a call from Powder Magazine. They were interested in using one of the photos I had submitted! The file was originally a JPEG, that I had upscaled(never do this!!!) to a TIF file to meet their submittal requirements. The editor asked that I send him a RAW file. When he heard that the original was a JPEG, he almost instantly turned down the image. I sent him the actual JPEG, and he realized that I had done a lot of editing to make it look like what he was seeing.
It's extremely disappointing to lose a sale simply because you didn't know any better, although I'm sure this happens to everyone at some point in their early career. Think of a RAW file as an untouched image. The file is still full size, and has the capacity for a lot of editing. It's also far more forgiving if your original is over or under exposed. If you are becoming serious and still shooting JPEG all the time, switch the file setting in your camera to RAW now. Through all the control you gain and potential for MANY more sales, you will never look back again!
Remember: All RAW files require some degree of editing. They will generally lack contrast when you first open them, and will always need some degree of sharpening. They are also very large files, so get a large external hard drive. Once you learn to edit them however, you will never want to shoot any other way.