Correct Exposure

1. Capturing Light
The Correct Exposure

2. Exposure
• How a camera’s image receptor (film or digital
sensor) receives the right amount of light -
Exposure.

3. The human eye is supported by one of the most sophisticated
“automatic exposure meters”. As we look at a subject, no
matter how light or dark it is, the iris in our eye will adjust to it
very quickly.
Occasionally an image is just too light or too dark for us to see it
properly so we wear dark glasses or use a torchlight. Generally
though, our eyes cope pretty well with the lightness of an
image so we see it clearly and without any strain on our eyes.

4. Aperture
Most modern cameras work in a similar way. They have an automatic
exposure meter which reads the amount of light entering the camera and this
information is used to make an adjustment to it’s aperture - a device which is
very similar to the iris in our eye.
The aperture is just behind the camera’s lens and it opens to the correct
size to allow just the right amount of light to pass through to the image
receptor. It’s a bit like a water pipe - a large pipe would let more water
through that a small pipe.

6. The SLR Camera

8. Shutter Speed
At this point, there is a key difference between
our eyes and a camera. No matter how long we
look at something, it stays looking the same - no
lighter, no darker. If a camera receptor is
continually exposed to something, it will
eventually receive so much light that it will just
burn out - not literally, but it will record an image
as completely white.
To overcome this, the camera’s image receptor
needs to receive light through the aperture for a
certain length of time. Any longer is too much,
any shorter is not enough.

9. Using the water pipe analogy - if we are trying to fill a bath with water, we need to turn
the water off once the bath is full to avoid a flood, but not earlier because it won’t be
full enough!
The camera controls this time using its shutter. Depending upon the type of camera,
shutters take various forms but they all achieve the same result - control of the time for
which light is received by the image receptor.

10. Correct Exposure

11. Under-exposed

12. Over-exposed

13. Correct Exposure

14. Under-exposed

15. Over-exposed

16. Mid-tone exposure

17. 1 stop over-exposed

18. 1 stop under-exposed

19. The reciprocal relationship
A Golden Rule - For any given exposure, if we halve one of the three variables
(aperture / shutter speed / image receptor sensitivity) and double another, we
will still achieve the same exposure.

20. There is just one more simple factor - the sensitivity of the image receptor. In a
film camera, each film has a particular sensitivity, which is the speed at which it
responds to light. This is helpful because it enables us to take shots in less light
if we use a faster film and brighter light if we use a slower film.
With a digital camera we can decide upon the best sensitivity of the sensor and
set it for each individual photo. This is more flexible than with a film camera
where we are committed to the film’s sensitivity for the whole roll of photographs.
Sensitivity = ISO
International Standards Organization

21. To Summarize - The camera’s automatic exposure meter controls the
camera’s aperture and shutter combined with the film or digital sensor’s
speed setting, to ensure that the image receptor receives the right amount of
light.

22. Camera Exposure Meters
Most cameras have these options
Evaluative - Covering the whole area, as seen in the viewfinder or on the screen.
Centre Weighted - Just that. Covering the whole area with emphasis on the
central section.
Spot - Only a small area in the centre.
Partial - This mode meters a slightly larger area than spot metering.

24. Spot metering
Spot metering is very accurate and is not
influenced by other areas in the frame. It is
commonly used to shoot very high contrast
scenes.

25. Spot Metering

26. Partial Metering
This mode meters a larger area than spot
metering and is generally used when very bright
or very dark areas on the edges of the frame
would otherwise influence the metering unduly.

27. Partial Metering

28. Centre Weighted Metering
In this system, the meter concentrates between 60
to 80 percent of the sensitivity towards the central
part of the viewfinder. The balance is then
"feathered" out towards the edges.

29. Centre Weighted Metering

30. Evaluative Metering
the camera measures the light intensity in several
points in the scene, and then combines the results
to find the settings for the best exposure.

31. Evaluative Metering

32. Show your understanding of exposure
Take 3 images changing the shutter speed:
1 correct exposure / 1 underexposed / 1 overexposed
Take 3 images changing the Aperture (f stop):
1 correct exposure / 1 underexposed / 1 overexposed
Present these on your website on the
‘correct exposure’ page.