Exposure Rules Never Change
It’s important to get our heads round this. The RULES never change but exposure does. Thus if we follow the RULES then our exposure should be correct. Remember that there are two types of light that we are interested in: Incident and Reflected. So let’s deal with the Incident first.
Incident light is that which falls on our subject and in the days of film before cameras had in-built light meters it was what we relied on and used an external light meter to measure: you placed your light meter against the subject you were photographing and measured the light that was falling on the subject. The light meter indicated the camera settings to be used and thus everything was exposed correctly. You couldn’t change the ISO (the sensitivity of the capture medium i.e. film) on your camera so you changed the shutter speed and the aperture. As far as ISO was concerned you had already bought your film rated to the ISO you wanted for the conditions you were shooting in e.g. bright sun, shade, indoors etc. When you bought your roll of film something very important was either printed on a slip of paper in the film box or printed on the outside of the box: The Sunny Sixteen Rule. A rule that applied relative to the shooting conditions and the ISO of the film. Here’s the rule:
The following table applies for getting correct exposure PROVIDED it’s a bright sunny day and the light falls directly on the subject you’re photographing with the sun directly above and behind you (say between 10.00am and 2.00pm in the UK).
At ISO 200 the following Shutter Speeds and Apertures are set:
1/100 @f22 // 1/200 @f16 // 1/800 @f8 // 1/1600 @f5.6 // 1/3200 @f4
(‘Sunny Sixteen’ gets its name from using ISO 200 film and setting the camera to 1/200 of a second shutter speed at an aperture of f16)
Given the conditions I’ve just outlined (sunny day with the light above and behind you) and you took a photograph of every car in a car park I guarantee that they would all be perfectly exposed. Of course, there will be specular hightlights (the bright spot of light that appears on shiny objects when illuminated) but there’s nothing we can do about that.
The Sunny Sixteen Rule gives us a good starting point for ensuring correct exposure in the ‘field’. But it seems a little bit too much to remember. So here’s a rule of thumb: just remember ’88’ : SET YOUR ISO TO 200, YOUR SHUTTER SPEED TO 1/800 AND YOUR APERTURE TO f8. You’ll have the perfect exposure for every shot given the same conditions for each. ’88’ is your starting point.
So what happens if you need a faster shutter speed for moving subjects (say birds in flight)? Simply refer to the rule and change you settings accordingly. If you wanted a shutter speed of 1/1600 of a second then according to the rule you need an aperture of f5.6. Likewise, if you need a different depth of field then adjust as necessary.