Why meter a night scene wide open, then stop down and recalculate the shutter speed?
Asked 9/28/2011
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In Bryan Peterson’s Understanding Exposure, some night examples show spot-metering the sky at a wide aperture (for example f/2.8), then recomposing, stopping down to something like f/22, and increasing shutter time to keep the same exposure. Why not simply set the desired aperture first and take the meter reading there? Would spot-metering the sky at f/22 give the same result, aside from very long exposures that may exceed the camera’s normal metering or timed-shutter limits?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
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With an old fully manual aperture lens you want to meter wide open so you give the metering sensor as much light to work with as possible. However with a fully electronic camera the aperture stays open 'till the point of exposure so all meter readings are taken wide open. With this in mind I can't think of any reason why you'd want to meter with the camera set to f/2.8 and then do the calculation yourself - instead of letting the camera do it! The only exception is if you're using bulb mode and need to calculate exposures of longer than 30 seconds as the camera wont do this.
My approach to metering for night shots is to simply shoot a bunch of exposures and look at the histogram - I find the camera's main sensor to be the most accurate meter :) If I want to save time I'll use a higher ISO to shorten the exposures and then multiply up the shutter time.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
14y ago
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In most cases, yes: if the camera meters normally at the chosen settings, metering first at f/22 should give essentially the same exposure result.
Why meter wide open first? A few likely reasons:
- Working method: Peterson often teaches exposure as a two-step process: first find a technically correct exposure, then shift aperture/shutter to get the artistic result you want (depth of field or motion blur).
- SLR/open-aperture metering: On many cameras, metering is effectively done with the lens wide open anyway, and the camera accounts for the selected f-stop. With older manual-aperture setups, metering wide open also gives the meter more light to work with.
- Very long exposures: If the final exposure goes beyond the camera’s timed limits (for example into bulb territory), you may need to meter a shorter equivalent exposure and calculate the longer one yourself.
So the answer is mostly: he probably could have metered at the final aperture, but he was either following a teaching workflow or dealing with practical limits of metering/timed exposures in low light.
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