主题：1000d+sigma 17-70mm os 请指导 -- 半通

The biggest challenge when photographing snow lies in your camera’s metering system.

Camera meters measure the light reflecting off of subjects in the frame, and use those measurements to determine the appropriate exposure settings such as

Dark subjects such as this antique car may fool the camera's meter into overexposing the entire image, in an effort to render the subject as 'middle gray' (minus exposure compensation can be applied to correct this)

shutter speed, f-stop, or ISO. This system is universally designed to render subjects as ‘middle gray’ or ‘18% gray’.

Just to clarify what that means: Middle gray is roughly the midtone on a gray scale – appearing to fall exactly between pure black and pure white. Subjects of this tone reflect about 18% of light (comparitively, white objects reflect nearly 100% and black objects reflect nearly 0%).

Simply put, most cameras assume that everything they photograph reflects 18% of the light, and exposes accordingly. Put another way, your camera meters subjects assuming they should be photographically rendered as middle gray.

Freshly-fallen clean, white snow is obviously much more reflective than that, and the camera will automatically compensate by underexposing anywhere up to about two stops to correct for what it sees as a too-bright subject. This is exactly why many photographers will find their snow photos to be muddy and underexposed.

In-camera light metering works reasonably well with most subjects, in most lighting situations. However, there are tricky scenes that will baffle most meters – and snow is a classic example.