Notes by Dr. Optoglass: Camera Exposure

Topics Covered:

  • Exposure and Camera Exposure
  • Exposure Value (EV) and Stops
  • F-number and Shutter speeds

Look and think before opening the shutter. The heart and mind are the true lens of the camera – Yousuf Karsh

This is how a camera works:

Camera Exposure

The sensor is a jar that can take in light till a certain limit. The width of the opening to the sensor is the aperture. On a camera you can reduce (stop down) the aperture or increase (open up) it. The tap (or faucet) is the shutter. It decides for how long the light will flow into the jar. It is measured in seconds.

The amount of light collected every time the tap is opened and shut is called the exposure.

Let’s say at a given aperture and shutter speed I get a certain exposure. If my aperture is doubled and my shutter speed halved, I’ll get the same exposure. If my aperture is reduced by one-fourth and my shutter speed is quadrupled, I’ll still get the same exposure.

The key is to get the right exposure, and there are many ways to get it. But what really is a right exposure?

The height of the jar decides the maximum amount of light it can hold. If you pour too much, you will overexpose. If you pour too less, you will underexpose. This capacity or useful exposure range is called the dynamic range of the sensor.

The idea is to fill that jar perfectly. In many ways, the sensor limits the dynamic range that one can capture. But within these extremes, a correct exposure lurks – and this can only be decided by the person using the camera. You like your jar half-full? There’s your correct exposure. You like it overflowing and overexposed? Go for it.

From now on, when I refer to exposure, I’m not referring to its subjective qualities, but rather how it relates to light levels and the units we’ve seen earlier.

Luminous/Camera Exposure

If illuminance is measured in lux, then the exposure is measured by lux seconds. This exposure is photometric and is called the luminous exposure. Here’s the formula:

Exposure = lux x time in seconds

Exposure Value

Exposure Value (EV) is a unit that tries to show all combinations of a camera’s shutter speed and relative aperture that give the same exposure. It has its own formula:

where

  • N is the f-number
  • t is the exposure time (shutter speed) in seconds

The advantage of EV is that it allows people to talk to each other in terms of apertures and shutter speeds corresponding to exposure values. The disadvantage is that camera sensors, meters and lenses have minds of their own – and two systems with the same EV values will not give you the same image. Even within a camera system, by varying aperture and shutter, you’ll get different photographs – even if they all have the same EV.

Camera apertures and shutter speeds are designed to move in ‘stops’ of light – one stop is one EV. Every stop doubles or halves the light.

F-number and Shutter

What does the f-number signify? Every step in the f-number doubles or halves the light entering the lens. It is based on ?2, as follows:

f/0.7 = 1/?2
f/1.0 = (?2)0
f/1.4 = (?2)1
f/2 = (?2)2
f/2.8 = (?2)3
f/4 = (?2)4
f/5.6 = (?2)5
…and so on.

To remember it, start with 1 and double as you go along. Then start with 1.4 and double as you go along. Simple?

Shutter speeds are based as follows:
30s, 15s, 8s, 4s, 2s, 1s, 1/2s, 1/4s, 1/8s, 1/15s, 1/30s, 1/60s, 1/125s, 1/250s, 1/500s, 1/1000s, 1/2000s and so on.

Shutter speeds are based on easy to remember numbers in multiples of 5 or 10. They started with 2, but decided it wasn’t going to help much as technology advanced! Imagine trying to remember 1/4096 over 1/4000! The difference is negligible.

Shutter speeds, too, double and halve as they go along, with each stop doubling or halving the light.

Now we know how camera exposure works, and the tools that govern this. If you put in the f-numbers and shutter speeds into the EV formula, you’ll get a range similar to this:

…-6, -5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5, 6…

Each number is one stop added or reduced (light doubled or halved). But what exactly do they mean? If I say I have an EV of 3, what do you do with it?

The big disadvantage of the EV system is that it does not consider the ambient light levels or the light hitting the sensor. The luminous exposure value takes into account the lux falling on the sensor, but it does not take into account the lux levels in the scene being photographed or recorded. And neither takes into account reciprocity failure.

To make matters worse, depending on the kind of meter used, the values can vary based on different meter calibration constants.

Unfortunately, there can’t be an easy direct relationship between Exposure and EV and the real world, but over many years a good rule of thumb has been found. This is what I’ll cover next.

Takeaways:

  • The Luminous or Camera Exposure is lux seconds, where lux is the illuminance hitting the sensor/film.
  • EV is a unit that tries to unify f-numbers and shutter speeds with stops of light.
  • A footcandle is lumens per square feet.

Links for further study:

Next: How to Read Light for Exposure
Previous: The Units of Photometry Part II