Without going into to many details, and to keep it simple, the camera sees everything in shades of grey. Not only that, it wants to turn everything into an average grey tone often referred to as 18% grey.
The problem is that not everything is middle grey, think Bride and Groom (White dress & Black suit). So what do you do?
Once you understand how your camera thinks you will, with lots of practice, start "seeing" like your camera and you will be able to adjust the exposure compensation on the fly and get pretty close. The general rule is if the background is dark or your subject is wearing dark clothes (Groom), under expose by .3 to 1.0 stops. If your background is bright or your subject is wearing bright clothes (Bride), over expose by .3 to 1.0 stops using your exposure compensation dial. If you have the time, you can also use a 18% grey card and set your exposure manually off of that card. Hand held exposure meters also work extremely well. I love my Sekonic L-358 light meter. The great thing about light meters is that they read the amount of light falling on your subject. Your camera reads the amount of light that reflects off of your subject and is therefore influenced by how reflective or absorbent the subject is and can be fooled quite easily.
Most novice photographers do not use grey cards or light meters. Both of these approaches are a bit more advanced and require a good understanding of light. Not to mention light meters can be quite expensive too. For a professional photographer however, getting the exposure perfect is not an option! Can you imagine how a bride would feel if her beautiful white wedding dress looked off white or grey in all her photos? Run for the hills if this happens and you were the photographer! Unfortunately I hear these types of horror stories all the time. That's why it's not worth saving a few dollars and taking a chance that your "once in a lifetime" photos don't turn out perfect!
The picture below is straight out of camera. Meaning it has zero edits or adjustments. A light meter was used to get a proper exposure reading off of her face. Had I used the cameras meter, it would have over exposed the image because of all that dark hair in the image. Remember, it wants to turn everything to middle grey, and this hair is a lot darker that that. The resulting image would have had washed-out skin tones and the details would not be accurate.
So next time you're out with your camera, start using your exposure compensation dial. I think you will be amazed at the results you can achieve. If you have any questions, you can always send me note.
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