Existing Light Exposure Metering

There are numerous tutorials on metering and exposure on the web, I just haven't seen one that I can refer the novice to and expect she'll come away with a clear understanding and a good working knowledge of exposure and metering. Hence this tutorial. Yes, it's long and it's actioned-packed with technical information. But that's because the breezy, here's-all-you-really-need-to-know tutorials just don't prepare you for the real world, as endless posts on photography forums will attest.

Since you probably use a digital camera and can instantly review your last exposure to check for problems, you may well wonder why you should bother learning this material. If you find your best shots being ruined by bad exposures, then read on – otherwise, by all means give it a miss.

Background: before proceeding, we need to get squared away on some basic concepts. If you feel at all hazy about f/stops, shutter speeds, or the basics of how cameras work, please read Camera Fundamentals, then return here.

Exposure – the basic concept

I can (with difficulty) read a book by the light of a single candle. The pupils of my eyes expand to their widest to let in as much light as possible, yet still the book is dim, making it hard to separate the letters from the off-white of the paper. I can read a book in direct sunlight at noon. The pupils of my eyes contract to their narrowest to let in as little light as possible, yet still the book glares making it hard again to read. The brightest daylight is some 10,000 foot-candles (10,000 times as bright as a single candle one foot distant from your eyes), so the fact that the human eye can do anything at all in both extremes is astounding.

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Fig 1 Too few photons vs. too many photons

We might want to generalize from this to say that the human eye can handle more than a 10,000x range of brightnesses, but it's not quite that simple. The human eye can cope with only a much smaller range of brightnesses for any given pupil diameter. That's why shadows appear black on a sunny day even though they actually contain much more illumination than a candle-lit room.

To take a photograph we use either film or a photo-sensitive silicon chip called an imager, or more generically, a sensor.

Film: Everything in this tutorial applies equally to digital and film photography except where otherwise indicated. Because the vast majority of readers use digital cameras I will talk in terms of digital, but film photographers should mentally substitute the word "film" for the word "sensor". Film is a sensor; it's just a chemical sensor instead of a photoelectric one.

Few cameras can handle anything like a 10,000x range of brightnesses in the same scene, any more than the eye can. To specify where on the scale from dimmest to brightest a given sensor can effectively operate, we use a numeric scale approved by the International Standards Organization, called the ISO number. The larger the ISO number, the dimmer the light the sensor can record in for a given time of exposure.

Time is of the Essence

Which introduces a second essential concept: time. If I look at a typical scene I will see some regions that are brighter than other regions. White objects intrinsically reflect more light than brown objects, for example. Also a white object in shade may reflect less light than a brown object in the full glare of the sun. Sensors work by changing with the number of photons (light particles) that strike a given point on their surface. But for this to make sense there has to be a starting time at which the change begins and an ending time at which the change ends. So in a typical camera there is a shutter, or curtain, over the sensor that stays closed except during the period of exposure.

To take a picture the shutter opens for a period of time then closes. The sensor records how many photons landed at each point of its surface (and what frequency or colour they were in colour photography). The more photons were recorded at a given point, the brighter that point will be in the resulting photograph. But, just as too much light hitting our eyes makes us squint and tear, so if too many photons strike a point on the sensor, that point will be either fully exposed or overly exposed.

Take a roll of unexposed film out of its light-proof container anywhere but in a totally black room and you've ruined the film. If you develop and print all you will get is pure white prints. Similarly, if film or sensor is rated for very bright light and you expose it to light too briefly, the image will be under-exposed – everything will be black or at least too dark.

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Fig 2. Black, middle grey, white

The trick is to open the shutter just long enough so that a pure white object will record as pure white, a middlin' grey object will record as middlin' grey, and a black cat will be midnight black. That's where the ISO number comes in to play. If the sensor is rated at an ISO of 100 it might take 1/10th of a second to achieve this ideal exposure in a certain lighting. If the sensor is rated at an ISO of 800 it would take 1/8th as long in the same light, or 1/80th of a second to achieve the same exposure.