When shooting in a studio setting, the meter of preference is of course the domed incident meter, which gives a reading from the subject to the camera. The camera is then set to the given exposure, and the picture is taken. In most cases your exposure quite accurate. But what about shooting outside the studio on those days when you forgot to drop your incident meter into your camera bag?
Your camera has a reflected meter, that is, it gives a reading from the camera to the subject. Unfortunately, reflected meters are not as accurate as incident meters, and whether the meter in the camera is spot, centre weighted, or averaging, it is still calibrated as a reflected meter.

When the camera’s reflected meter looks at a given subject, what it sees is 18% gray image, not the lovely full–colour view you have in your viewfinder. This greying is because  a reflected meter records the average amount of light reflected back from the scene to the camera. Compare that to an incident meter, which records light as it actually falls on the scene to be photographed. The reflected meter’s averaging does not account for the exposure needs of a scene that may have both shadows and highlights—instead, it takes an average that may represent a very uniform scene well, but more often than not is inadequate to account for the light variations in a given possible exposure. To put it a different way, think of a scene with light and shadow as akin to a man with a size 10 left foot and a size 8 right foot. While the incident meter would sell him a size 10 shoe for his left foot and a size 8 for his right, the reflected meter would try to shoehorn both feet into size 9 shoes. You can guess which would be more comfortable, and you can also see which meter makes for a better, more accurate exposure.

As I stated earlier, incident measurement is from the subject to the camera, and as I explained above, incident measurement also gives the best exposure. But if you are away from your incident meter, there are ways to get your reflected camera meter to read like an external incident meter. You can purchase a lens filter for approximately $100, though some of the larger lens filters cost a lot more than the $100. But if you don’t want to part with a hundred dollars, there is an easier and much more inexpensive way to convert your reflected meter into an incident meter. And all you need is a clean white Styrofoam coffee cup.

You might be chuckling right now, but I have measured the coffee cup’s success against my Minolta 4 meter, and I can assure you that it works, despite its almost embarrassing simplicity. Follow the instructions below, and you’ll never again need to panic when you leave your incident meter at home or in the studio.

Position the camera at the subject location and remove the lens hood (if applicable).
Place the white Styrofoam coffee cup over the camera lens.
Set the exposure by reading the light through the coffee cup.
Set the camera to the exposure corresponding to that reading.
Remove the coffee cup from the lens, place it carefully in your camera bag for next time,  and replace the lens hood.
Reposition the camera if necessary to be ready to take the photograph.
Do not change the exposure, and take the picture.

You may test the same lighting by using an incident meter if you have one with you. You will see that the exposures are the same, as your coffee cup meter reads the light falling on the subject, not the light bouncing off the subject. The coffee cup acts as a diffuser, just like the one found in the incident meter.