Manual Metering Doesn't Have to Mean Using a Gray Card
If left to their own devices, modern cameras generally do a reasonable job of metering automatically — so much so that many photographers never learn to meter manually for situations where they want to take full control of exposure. Those who do try often stop at simplistic techniques requiring the use of a gray card that may work well in theory but rarely do in practice, at least not for shooting landscapes. Here are some tips on making manual metering work in the outdoors.
The whole purpose of metering is that cameras aren't perfect. There's a gigantic range of brightness in the real world and any given press of the shutter release can capture only a small slice of it. To get an optimal or even usable shot from pressing that shutter release, it is necessary to align the range of brightness the camera is capable of recording with the range covered by the subject or subjects being photographed. It is commonly quoted that a camera has a five stop range, from two stops under what the meter points to, to two stops above the meter reading. This was reasonably true for the color slide film popular in the pre-digital era. Even then, the exact capabilities of various emulsions varied, and today most all digital cameras exceed this five stop range somewhat. Regardless of just what your camera can do though, the human eye can do far more. As such, the basic problem remains the same even if digital has mitigated it to a minor extent. For the sake of simplicity here, I'm going to stick with the -2, -1, 0, +1, +2 five-stop scale photographers have used as a rule of thumb all along. Feel free to extend things if your camera's capabilities warrant doing so.
A camera's meter is tuned to eighteen percent reflectance, what is commonly called "medium tone." The quirk of why eighteen percent is the middle of the scale rather than fifty percent has to do with the way human vision works. Rather than seeing brightness changes in a linear fashion our eyes see doubling and halving of brightness as being equal steps. You can double eighteen twice over before you start to get dangerously close to full one hundred percent brightness just as you can cut it in half twice over before you become lost in the shadows near zero brightness. Cameras that are capable of recording more than five stops basically do so by adding more stops into the shadows where it is at least theoretically possible to continue cutting in half forever.
Given all this background, the obvious purpose of a gray card is that it reflects the same amount of light as what is needed to achieve "medium tone" on a camera's meter. The gray card reflects eighteen percent gray and the camera meter renders whatever you point it at as eighteen percent gray. Thus, the resulting photograph shows the gray card looking just as it does in real life, and we say that the photo is correctly exposed.
But if that gray card is sitting in the shadows when you photograph it, should it really look medium gray? If you really want a literal rendition that is "correctly exposed," shouldn't the photo of the gray card in the shadows actually look dark gray rather than medium? Or if you photograph it in bright light shouldn't a "correctly exposed" photo come out brighter than medium gray? What if you photograph it under average lighting but want it to look like it was in the shadows, or vice versa?
The truth is you can make a photo come out looking any way you want. It's your choice. If you point your camera at something and set the meter reading dead even on the center zero mark, that object will come out rendered as medium, regardless of what it was or what you saw it as when you were standing behind the camera you shot it with. This works against you when your winter scene comes out with gray snow and you don't understand why, but it can also work for you as a tool once you accept the mechanism at work that makes it so. If you are shooting a mountain scene it's hard to hold a gray card at point miles away in the same light as the mountain. Instead, you might think it appropriate to isolate a granite cliff face to meter on, judging it to be medium toned. But if the bright snow above it at the top of the mountain still comes out burned out white, is the resulting image correctly exposed, or would it be better if that granite came out a tad darker in order to preserve detail in the snowy peak? The choice is yours, but I'd suggest that burned out highlights are rarely a good idea. In the end, "correct exposure" really means the image came out the way you wanted it to and it looks good — nothing more.
Once you let go of any sort of absolutist idea of what exposure should be, a gray card has little real meaning. Find something you want to be medium, point your camera at it, center the meter reading to zero, and shoot away. The object you metered on will end up medium in the resulting image.
But what if there's nothing in the scene that should come out medium? So long as you set the meter to match how you want what you point it at to look, all is well. If you point the meter at a patch of dark trees you want to come out darker than medium, you can set the meter to perhaps one stop below medium (minus one) and safely fire away. Had there been perhaps a rocky outcropping one stop brighter than those trees in the scene it would have come out one stop brighter than what you rendered the trees as. And since you metered to make the trees come out -1 the rocky outcropping would be rendered as medium since -1 plus 1 equals zero. And the rocks would be medium tone in your shot.
This concept also works to help check the metering choices you make. Once you've decide how you want to render the trees or rocks, you can check other points of interest to see what your meter thinks of them. Swing your camera around the frame without changing the meter to see what it says. Keep the focus and zoom constant too to avoid changing the lens's effective aperture and thus the exposure. If you point the camera meter towards the snowy peak you can see if your decision about the rocks was a good one and adjust accordingly. You can also point it toward the shadows in the valley below to find out if it will still show detail there or not. If you work with the basic -2, -1, 0, +1, +2 scale, anything outside that range will lack detail. Shadows that are lost in blackness are generally quite workable so long as they don't fill more of the frame than you want them to, but highlights lost in burned out white are generally best avoided.
Sometimes your final choice has to be a tradeoff based on the range of brightness within the scene. After checking, if it's not possible to keep detail both in the snowy peak and valley shadows you'll have to sacrifice one or the other. If you judge such a compromise to be unacceptable for what you want your image to look like, you'll need to make changes in your technique. One possibility would be to reframe the shot to crop out the problem area. You could instead use a graduated neutral density (GND) filter when shooting to compress the brightness range across the scene or shoot multiple frames at different exposures and combine them together with digital blending or HDR techniques.
Regardless of your choice, the point is, the choice is yours. It doesn't have to be made by your camera alone. And it doesn't have to be dictated by the actual brightness of a gray card or anything else.