If your photographs seem to often be too bright or the subject matter far lighter than it is supposed to be then you are taking shots that are overexposed.
Get into the habit of checking your camera settings, although most modern cameras do it all for you, it is better to be aware of what does what so to speak.
Cameras can get it wrong and for a once in a lifetime photo opportunity you need to be confident that you are going to get the perfect picture.
A good approach is to practice with your camera in different lighting conditions, including a varied time of day and even allow for seasonal adjustments. Try taking the same scene with different exposures both manually and by using the camera’s automatic procedures.
Examine the results and look for improvements, the ease of which photographs can be studied on the computer makes this a simple and quick process. A good image is the result of two factors which are shutter speed and the aperture.
The former is defined as the time it takes for the sensor to be exposed to light whilst the shot is taken and the latter is basically the lens hole, if it is big or small.
Using an automatic flash is not always the way to go, as sometimes they cannot do what they are expected to do under different light conditions. Often it is better to trust your instincts over the reliability of your equipment.