Put a roll of 100 ISO slide film in your Pentax 645N (645n) camera and choose an outdoor subject. Activate the spotmetering option on your camera and set your camera’s mode to manual and to the shutter sped to 1/125 sec. To avoid confusion, in this book we will increase and decrease the exposure by opening-up and closing-down the aperture. The shutter speed will remain fixed.

Observe your subject and break it down into its simple subject (single tone) components. Choose a Reference Tone.  A Reference Tone is usually chosen from the most important part of the subject. Once you have selected the Reference Tone, point your spotmeter at this tone and find its normal exposure. If your subject does not have a distinct single tone and you are using slide film, pick out a tone that is medium gray or brighter. If you are using a negative film, pick out a tone that is medium gray or darker.

Let’s assume the "normal exposure" indicated by your camera for this Reference Tone is 125@f-8 (i.e., 1/125 sec. at an aperture setting of 8).
As you may know, the spotmeter’s "normal exposure" from any simple (single toned) surface will always provide the photographer with an 18% gray image tone.
The image tone that your normal exposure provides is independent of the original tone of your subject.   The following is an example:

In spotmetering, converting the "normal exposure" to the "correct exposure" is what a photographer must do. In other words, the "normal exposure" readings of the spotmeter must be interpreted by the photographer to determine the subject’s"correct exposure." The principle behind this simplified technique is that if one tone of a complex subject is exposed correctly, the rest of the tones follow and will also be correctly exposed . Now decide which one of the following tones would most closely matches your Reference Tone: Black, Dark Gray, Medium Gray, Light Gray or White. With this simplified technique you must choose one of these five tones. Once you have decided which one of these five tones best matches your Reference Tone, then adjust your camera settings accordingly:

If you chose Black, you need to decrease the "normal exposure" setting by two stops. Closing- down the aperture by two stops converts the 18% gray image tone to Black. The correct exposure is now 125@f-16.

If you chose Dark Gray, you need to decrease the "normal exposure" setting by one stop. Closing- down the aperture by one stop converts the 18% gray image tone to Dark gray. The correct exposure is now of 125@f-11.

If you chose Medium Gray, leave the aperture and shutter speed as they are since the meter is already creating an 18% gray (Medium Gray) image tone, i.e., the tone of the image matches the approximate tone of the Reference Tone. Also remember that when metering from a medium gray surface, the "normal exposure" and "correct exposure" settings are approximately the same.

If you chose Light Gray, you need to increase the "normal exposure" setting by one stop. Opening-up the aperture by one stop converts the 18% gray image tone to Light Gray. This results in a correct exposure of 125@f-5.6.

If you chose White, you need to increase the "normal exposure" setting by two stops. Opening-up the aperture by two stops converts the 18% gray image tone to White. The correct exposure is now 125@f-4.

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© confused photographer's guide books - excerpts from  Simplified zone system and on-camera spotmetering (spot metering)