In section "Night and Low-Light Phtography" that starts on page 128 of Bryan Peterson's "Understanding Exposure" book, he has several examples where he (spot) meters off the sky with a large aperture, then recomposes, reducing the aperture and increasing the shutter speed in order to maintain the same exposure. My question is: why does he not simply take the spot meter reading using the desired aperture or shutter speed? For example, on p128:
... I began with my aperture at f/2.8 and pointed the camera toward the sky... I then adjusted my shutter speed to 1/4 sec... I set the lens to f/22 and simply did the math to determine the new and correct exposure ... I needed to increase the exposure time to 30 seconds.
Would he have obtained the same result by taking a meter reading off the sky with the aperture set to f/22? Perhaps the 30s shutter speed in this example would make this a challenge for the camera's metering system; however, in other examples he does the same, but increasing the shutter speed to, say 4s.