Thinking in “stops of light” for manual exposure

Technique
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Thinking in “stops of light” for manual exposure

thinking in stops of light

When I started writing this blog my intention was to help people better understand photography and photographic lighting by taking complex ideas and simplifying them as much as possible. One of the most important lessons you need to learn in order to be able to understand how your camera works is to appreciate the relationship between aperture, shutter speed and ISO and how each of those settings is measured in ‘stops of light’ which are relative to the others.

This is a fairly complicated concept and I don’t profess to understand it myself. Nevertheless, the technical and mathematical aspects of why this stuff works is not really important; instead, you simply need to appreciate what a ‘stop’ is so that you can set your camera up effectively for the given situation that you are in.

[box_yellow]What are ‘stops’?[/box_yellow]

A photograph is created when light hits your camera’s sensor. You can control how much light that is by varying the settings on your camera, including the amount of light that it sees (the aperture) and the duration for which that light is seen (the shutter speed), and the sensitivity of your camera to these two parameters (the ISO level).

All three of these components can each be referred to in ‘stops of light’.

1. Aperture

Aperture controls the size of the opening in your lens. The lower the number, the more light gets into your camera.

The most commonly used stops when referring to aperture are: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16 and 22.

[box_grey]The difference between each of these values is 1 stop.

Each time you go ‘up’ one stop (e.g. from 2 to 1.4) you double the light entering the lens.

Each time you go ‘down’ one stop (e.g. from 2 to 2.8) you half the light entering the lens.

So if I say I want to ‘open my aperture by one stop’ and I am at 2.8, I will change my aperture setting to 2. If I want to ‘close my aperture down two stops’ and I am at 2.8, I will change my aperture setting to 5.6.[/box_grey]

(It is common for a camera to be setup so that you can actually set the aperture in thirds of stops, rather than full increments. This gives you a little more control, but it’s not really important for the purpose of this article.)

2. Shutter speed

This is how long your shutter is open for. This is measured in seconds and fractions of seconds. The longer your shutter speed is open, the more light gets into your camera. The shorter your shutter speed, the less light gets in.

The most commonly used stops when referring to a fast shutter speed are: 1 second, 1/2 a second, 1/4th of a second, 1/8th of a second, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000 and 1/8000.

[box_grey]The difference between each of these values is 1 stop.

Each time you go ‘up’ one stop (e.g. from 1/250 to 1/125) you double the light entering the lens.

Each time you go ‘down’ one stop (e.g. from 1/250 to 1/500) you halve the light entering the lens.

If I want to ‘get an extra stop of speed’ and I’m at 1/60, I would change my shutter speed to 1/125. If I want to ‘slow down by 3 stops’ and I’m at 1/60, I would change my shutter speed to 1/8.[/box_grey]

If you want a slower shutter speed, you would go the other way: 1 second, 2 seconds, 4 seconds, 8 seconds, 15 seconds, 30 seconds, 60 seconds, etc.

3. ISO level

This setting decides how sensitive your camera will be to the chosen aperture and shutter speed. The higher the number, the brighter the final image will be. The trade off is that by making the image brighter, it becomes less clear and more noisy.

ISO levels are commonly measured in the following stops: 100, 200, 400, 800, 1600, 3200, 6400.

[box_grey]The difference between each of these values is 1 stop.

Each time you go ‘up’ one stop (e.g. from 400 to 200) you double the light entering the lens.

Each time you go ‘down’ one stop (e.g. from 400 to 800) you halve the light entering the lens.

If I want to ‘go down one ISO stop’ and I’m at 800, I would go to 400. If I want to ‘go up 3 ISO stops’ and I’m at 800, I will go to 6400. [/box_grey]

While generally speaking you should aim to shoot at the lowest ISO level available on your camera, many cameras contain excellent sensors which are noise free even until ISO 1600 and readily usable up to ISO 6400.

[box_yellow]It matters most in manual exposure mode[/box_yellow]

I’m assuming that you have a camera that is capable of more than just Auto mode, otherwise you probably won’t be too interested in anything I have written so far. So running with that assumption, you will have the option to shoot in aperture priority (A or Av), shutter priority (S or Tv) or manual exposure mode (M). All cameras have an auto ISO setting, but only a few allow auto ISO in manual exposure mode.

In aperture priority mode with auto ISO, you set the aperture. If you choose an fstop of 1.4, you will let loads of light into your camera. If you choose an fstop of 22 you will let far less light in.  Your camera will choose an appropriate shutter speed and ISO to get you a perfectly exposed photograph.

In shutter priority mode with auto ISO, you set the shutter speed. If you choose 1/2 a second your shutter will be open for a relatively long time and let in a lot of light. If you choose 1/1000th of a second it will be open for a very short time and not let in very much light. Your camera will choose an appropriate aperture setting and ISO to get you a perfectly exposed photograph.

In manual exposure mode, you set the aperture and the shutter speed and ISO and this is where it’s important to understand the relationship between stops of light so that you can obtain your perfectly exposed photograph.

[box_grey]If you alter one component (e.g. aperture), you must alter one of the other two components correspondingly (shutter speed or ISO) to maintain your chosen exposure.[/box_grey]

[box_yellow]Putting it together[/box_yellow]

Your camera has an exposure meter built into it, which comes alive when you switch over to manual exposure mode. The meter will jump around from minus to plus depending on the lighting conditions that it sees and the metering mode that the camera is in. When that meter is dead center (usually indicated by the number 0), you have ‘perfect exposure’ (according to the camera – more on that another day).

Here’s an example of a ‘perfectly exposed’ photograph:

perfect exposre

1/60s at f/5.6 at ISO 1600

This photo was taken at ISO 1600. On many cameras, ISO 1600 is where noise starts to become unacceptable. If I reduce the ISO level to 200 (3 stops), I get this:

1/60s at f/5.6 at ISO 200

This photo is underexposed by 3 stops. I know this because it’s really dark, and also because my in-camera exposure meter is at -3. I got this photo by reducing my ISO from 1600 to 200.

[box_grey]I would have got the same photo if, instead of reducing my ISO, I had closed my aperture by 3 stops (from 5.6 to 16) or if I had increased my shutter speed by 3 stops (from 1/60 to 1/500).[/box_grey]

If I wanted to bring that exposure meter back up to 0, I would need to either increase my aperture by 3 stops (to 1.4):

1/60s at f/1.4 at ISO 200

Alternatively, I could reduce my shutter speed by 3 stops (to 1/8s):

1/8s at f/5.6 at ISO 200

[box_yellow]Who cares? Why is it important to think in stops?[/box_yellow]

Because if you want to take good photos you need to know what your camera is doing. I always shoot in manual exposure mode when I am using flash, whether off-camera or bounced. This allows me total control of how my photos will look, and more importantly, consistency between images of the same party/event/person/whatever.

Here’s an example of an event situation where the above information might help me:

Since I’m bouncing flash in TTL mode, I don’t need to worry about how many stops of light my flash is chucking out – that’s for part 2!