Calibrate Your Meter (More On Testing Your Meter)

 

 
Left Image: Tamron 70-210mm f-2.8 lens, 1/125 sec at f-4, multisegment metering. Bogen tripod and ball head.


Cameras, like any other piece of equipment need tune-up every once in a while. We take our cars for tune-up, but never even bother to check if our cameras are still functioning properly. Regardless of the type of camera you own, whether it is a $200 beginner or a $1500 pro model, you should check all of the camera functions once a month to make sure everything is working properly. This is especially important with exposure meters built into your camera.

Exposure meters need to be tuned-up occasionally. This does not mean there is something seriously wrong with the camera. You do not need to send you camera to the shop as you would with your car if it needed an engine tune-up. You can check your meter and calibrate it very easily with a few simple steps.

All camera meters regardless of the brand or model, are calibrated at the factory to give proper exposure for medium-tone subjects under most lighting conditions, especially under bright sun. Medium-tone means subject is not black nor white, not bright nor dark, but half way in between. Some examples of medium-tones are: green grass, blue sky, tree trunks, or your own faded blue jeans. Many of the subjects we photograph are medium-tone. This is why camera meters are calibrated for medium-tone subjects. It is important to know that the color of the subject does not really matter as long as it is neither black nor white, neither very light nor dark in color. The color can be medium red, medium blue, medium green etc. As long as the subject is of medium-tone, your camera should give proper exposure as long as it is calibrated correctly.

One way to check this is by pointing you camera at a medium-tone subject such as a tree trunk that is frontlit by the sun and fills most of your camera screen. If you stand in front of the subject, the sun must be coming behind you and over your shoulder illuminating the subject. This is called frontlit. To test your meter, I recommend purchasing an 18% gray card. They cost a few bucks and help you check your meter in only a few minutes. 18% gray cards are excellent tools for checking exposure, since all cameras are designed for this type of most subjects.

18% gray is as medium-tone as it can get. If you can calibrate your meter to get perfect exposure for 18% gray, you can feel confident with your exposure for any medium-tone subject you’ll be photographing.


Gray Cards

To test and calibrate your meter, choose a bright, sunny day, about two hours after sunrise and two hours before sunset. Place the 18% gray card or any other medium-tone object, such as your medium gray camera bag, outdoors. Make sure you place the card where it is frontlit by the sun. Use either a short telephoto lens, such as an 85mm or 100mm (zooms covering these focal lengths will work fine) or get close enough to fill the frame. Pick an ISO, such as 100 or 200. You do not need to load the camera with film. You are simply testing the meter. Set your exposure mode to manual and set the f-stop to f-16. Now, change shutter speed to the number closest to the ISO you have selected. For ISO 100 this would be 1/125 sec. or 1/250 sec. for ISO 200. Your meter should be null or show zero at this point. Look inside the viewfinder to see the display. Most cameras have some form of under/over exposure display (see your owner’s manual). If the display reads zero, you’re all done, otherwise you need to calibrate the meter.

First, you must know about an exposure rule called the Sunny f-16 rule. This rule states that correct exposure under bright sun for a frontlit subject of medium-tone is f-16 at the shutter speed closest to the ISO. This would be f-16 and 1/125 sec. with ISO 100, 1/250 sec. with ISO 200, 1/500 sec. with ISO 400 and so on. You can use any aperture/ shutter speed combination of similar values. For example,  with ISO 100, 1/125 sec. at f-16 gives identical exposure results as 1/250 sec. at f-11 or 1/500 sec. at f-8. Depth-of-field will be different, but exposure will be identical. Knowing the Sunny f-16 rule and how camera meters are calibrated for 18% gray subjects is a great help for checking and calibrating meters.


Left Image:
Tokina 300mm f-4 ATX PRO lens, 1/60 sec at f-4 in multisegment metering mode, Bogen tripod and ball head.

Let’s say your meter is off by one stop. Your camera, set to ISO 100, reads 1/60 sec at f-16 instead of 1/125 sec at f-16 as correct exposure. What you need to do is to set the ISO to ISO 200 to correct the meter. By setting the ISO number one stop faster or underexposed by one stop, the camera will give you correct exposure which, in this case, is 1/125 sec. at f-16. This test shows you that your camera is off by one full stop overexposure. Therefore, a full stop underexposure change of ISO is required whenever you use this camera. Whether you use ISO 100, 200, 400 or any other film speeds, set the ISO number manually to one stop faster than the film speed in use. This has nothing to do with pushing film or push processing (see Pushing film). You’ll still use normal processing and shooting, what you have done is simply calibrated your meter.


Your camera may be off by more or less stops. Perhaps only 1/3 stop is necessary to calibrate your meter or maybe two or more stops. It doesn’t matter how many stops the camera is off by. Just change the ISO dial until your meter reads zero. Once you have calibrated your meter, you can feel confident of getting proper exposure for any most subject. For non medium-tone subjects, you still need to make adjustments. This is true even if your camera is brand new and fully calibrated. Just remember, all cameras want to turn everything into medium-tone even if they are lighter or darker. If your meter is calibrated correctly, and your subject is medium-tone, just shoot at what the camera is suggesting. For lighter or darker than medium-tone, make adjustments accordingly.

I highly recommend calibrating your meter once a month. This is the only way to be sure of correct exposure every time. This way you will not be blaming the photo lab for bad prints or underdeveloped slides.