Advanced Exposure and Metering
10156
Product Reviews > Digital Products
Mar 1, 2007
Jennifer Clarkson
Advanced Exposure and Metering
Most humans see the world in living colour, but did you know that your camera?s light meter does not? Then how does it capture photos? Does it see the world in black and white? No! When your camera left the factory, it was calibrated to see the world as neutral grey, no matter what the lighting conditions! Neutral grey is defined as an object that reflects 18% of the light rays that hit it.
What this really means is that your camera thinks
ALL light that hits ALL of your grey subjects will
reflect back approximately 18% of that light.
Even in the dark!
The 18% isn?t important ? just think of it as a mid-shade or midtone colour and slot the following concept into your wallet of knowledge:
The trick to ensuring your camera exposes the colour scene correctly is to meter off an image that emits 18% light, and use your camera's suggested settings from the grey scenario after recomposing your ultimate scene.
More specifically, point your camera at a midtone shade under the same lighting conditions, note the f-stop and shutter speed that your camera?s meter recommends with the given ISO, and use those values to snap your photo once you recompose your scene, despite your camera's tendency to use a different set. You will need to either lock your exposure or use manual settings to do this.
=> TIP: you can purchase an 18% grey card and use it to meter off of in tricky conditions. Grey cards are sold at most camera stores in the Photo Accessories section, but I got mine from the National Geographic Field Guide
Let?s recap:
Steps to meter a tricky scene correctly
1) Set your camera to spot metering and point it at an object that is neither dark nor light, but is instead a midtone.
2) Press your shutter release button half way and note what f-stop and shutter speed it recommends under the current ISO speed
3) Lock your exposure, or change your camera to use manual settings and set the recommended f-stop and shutter speed
4) Recompose your scene and snap that photo!
=> scenarios your camera should automatically expose correctly:
1) frontlit subjects
2) sidelit subjects
3) overcast skies
=> problem scenarios:
1) waterfalls or rapids
-- white water will look grey in auto-exposed photo, and the scenery will be under-exposed
2) snow
-- white snow will turn out grey, and the rest of the scenery will be under-exposed
3) bright yellow flowers
-- flower will turn out darker than you expect, as will the background
4) black animals
-- black animal will turn out dark grey, and the background will be over-exposed
5) dark-skinned people
-- skin will turn out light brown and the background will be over-exposed
6) backlit portraits
-- subject will turn out very dark and background will be less bright than you would expect
Again, the secret to properly exposed photos is to meter off something that uses midtones, but there are some caveats to that rule. Check out the list:
Frontlit Snowy Landscape
- take out your 18% grey card and place it in the sun, which is the same lighting as what your snowy landscape has
- point your camera at the grey card and fill your frame with it ? no need for the card to be in focus
- meter off the grey card (i.e. check the f-stop and shutter speed it recommends)
- put down your grey card, recompose your snowy landscape, and snap your photo with the f-stop and shutter speed determined in the previous step
Of course, if you don?t have a grey card, use the next scenario instead
Sunny Days
- meter off the blue sky
Coastal Scenes or Lake reflections at Sunrise or Sunset
- meter off the sky in the reflection
Backlit Sunrise or Sunset landscapes
- meter off the backlit sky, always without a sun in your frame
City or Country scenes at Dusk ? this is a tricky one!
1) set your aperture to f/4: don?t ask, just do!
2) meter off the dusky blue sky
3) do the math and figure out which exposure works based on the f-stop you actually want to use
When you increase your f-stop by one setting, you must double the length of time your shutter stays open. For example, suppose f/4 results in a recommended shutter speed of ? second
Then f/5.6 ~ 1 second
f/8 ~ 2 seconds
f/11 ~ 4 seconds
f/16 ~ 8 seconds
f/22 ~ 16 seconds
f/32 ~ 32 seconds
Do you see the pattern yet? Let?s look at another example: Suppose f/4 results in a recommended shutter speed of 1/15th second
Then f/5.6 ~ 1/8th second
f/8 ~ ? second
f/11 ~ ? seconds
f/16 ~ 1 seconds
f/22 ~ 2 seconds
f/32 ~ 4 seconds
TIP: for those of you who don?t want to carry an 18% grey card around in your camera bag, you can calibrate the palm of your hand against your grey card, and then leave your card at home. How?
1) set your aperture to f/8 and place your grey card in the sun
2) set your camera to spot metering and check the shutter speed it recommends
3) meter off your palm and check the difference ? approx 2/3 or one full stop over exposed?
4) move your grey card to the shade and repeat the exercise; you should observe that your palm is consistently 2/3 to a full stop over exposed, regardless of the lighting conditions
What this really means is that your camera thinks
ALL light that hits ALL of your grey subjects will
reflect back approximately 18% of that light.
Even in the dark!
The 18% isn?t important ? just think of it as a mid-shade or midtone colour and slot the following concept into your wallet of knowledge:
The trick to ensuring your camera exposes the colour scene correctly is to meter off an image that emits 18% light, and use your camera's suggested settings from the grey scenario after recomposing your ultimate scene.
More specifically, point your camera at a midtone shade under the same lighting conditions, note the f-stop and shutter speed that your camera?s meter recommends with the given ISO, and use those values to snap your photo once you recompose your scene, despite your camera's tendency to use a different set. You will need to either lock your exposure or use manual settings to do this.
=> TIP: you can purchase an 18% grey card and use it to meter off of in tricky conditions. Grey cards are sold at most camera stores in the Photo Accessories section, but I got mine from the National Geographic Field Guide
Let?s recap:
Steps to meter a tricky scene correctly
1) Set your camera to spot metering and point it at an object that is neither dark nor light, but is instead a midtone.
2) Press your shutter release button half way and note what f-stop and shutter speed it recommends under the current ISO speed
3) Lock your exposure, or change your camera to use manual settings and set the recommended f-stop and shutter speed
4) Recompose your scene and snap that photo!
=> scenarios your camera should automatically expose correctly:
1) frontlit subjects
2) sidelit subjects
3) overcast skies
=> problem scenarios:
1) waterfalls or rapids
-- white water will look grey in auto-exposed photo, and the scenery will be under-exposed
2) snow
-- white snow will turn out grey, and the rest of the scenery will be under-exposed
3) bright yellow flowers
-- flower will turn out darker than you expect, as will the background
4) black animals
-- black animal will turn out dark grey, and the background will be over-exposed
5) dark-skinned people
-- skin will turn out light brown and the background will be over-exposed
6) backlit portraits
-- subject will turn out very dark and background will be less bright than you would expect
Again, the secret to properly exposed photos is to meter off something that uses midtones, but there are some caveats to that rule. Check out the list:
Frontlit Snowy Landscape
- take out your 18% grey card and place it in the sun, which is the same lighting as what your snowy landscape has
- point your camera at the grey card and fill your frame with it ? no need for the card to be in focus
- meter off the grey card (i.e. check the f-stop and shutter speed it recommends)
- put down your grey card, recompose your snowy landscape, and snap your photo with the f-stop and shutter speed determined in the previous step
Of course, if you don?t have a grey card, use the next scenario instead
Sunny Days
- meter off the blue sky
Coastal Scenes or Lake reflections at Sunrise or Sunset
- meter off the sky in the reflection
Backlit Sunrise or Sunset landscapes
- meter off the backlit sky, always without a sun in your frame
City or Country scenes at Dusk ? this is a tricky one!
1) set your aperture to f/4: don?t ask, just do!
2) meter off the dusky blue sky
3) do the math and figure out which exposure works based on the f-stop you actually want to use
When you increase your f-stop by one setting, you must double the length of time your shutter stays open. For example, suppose f/4 results in a recommended shutter speed of ? second
Then f/5.6 ~ 1 second
f/8 ~ 2 seconds
f/11 ~ 4 seconds
f/16 ~ 8 seconds
f/22 ~ 16 seconds
f/32 ~ 32 seconds
Do you see the pattern yet? Let?s look at another example: Suppose f/4 results in a recommended shutter speed of 1/15th second
Then f/5.6 ~ 1/8th second
f/8 ~ ? second
f/11 ~ ? seconds
f/16 ~ 1 seconds
f/22 ~ 2 seconds
f/32 ~ 4 seconds
TIP: for those of you who don?t want to carry an 18% grey card around in your camera bag, you can calibrate the palm of your hand against your grey card, and then leave your card at home. How?
1) set your aperture to f/8 and place your grey card in the sun
2) set your camera to spot metering and check the shutter speed it recommends
3) meter off your palm and check the difference ? approx 2/3 or one full stop over exposed?
4) move your grey card to the shade and repeat the exercise; you should observe that your palm is consistently 2/3 to a full stop over exposed, regardless of the lighting conditions
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Jennifer Clarkson is a Canadian photographer living in Ottawa. To read more of her articles and see her photos, visit her website at www.jclarksonphotography.com Article Submission made possible by: www.articles-submit.com Courtesy of:ArticlesRus.Net
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