Traveller, Photographer, Mountaineer, Human

Exposure Triangle

The Exposure Triangle is one of the oldest explanation for determining how to properly expose a picture.  Basically, the aperture, shutter speed and ISO value (ASA for film) all contribute to whether a picture is properly exposed, overexposed or underexposed.

Aperture is basically how big the opening of the lens is.  A bigger opening allows more light to enter and strike the sensor or film.  Aperture is measured in f stops wherein each f stop allows double or half the amount of light to enter the lens.  This is accomplished by opening or closing the aperture blades a little, thereby doubling or halving the area of the circle of the aperture.

Shutter speed is how fast the shutter (or covering) of the sensor/film is open.  The longer the shutter speed, the more light it allows to enter and strike the film.  Shutter speed is also measured in stops which doubles or halves the amount of time that the shutter is open.

ISO (or film ISO) determines how fast the sensor (or film) “absorbs” the light.  The higher the value of the ISO, the quicker light is absorbed by the sensor.  ISO is also measured in stops which doubles or halves as “light absorption” is increased or decreased.

I’ve read many articles about the exposure triangle but I decided to present a more graphical form of how each contribute to the final exposure and its effects.

Exposure Contribution Graph

I’ve prepared this simple graph showing the most common aperture, shutter speed and ISO stops (common for DSLRs) and their contribution to exposure (click on the graph for a bigger picture).

The vertical axis determines the unit of contribution of each stop is to the final exposure.  Aperture, shutter speed and ISO are presented in full stops (in the bars) with 1/3 stops presented beside it.  The effects of increasing or decreasing each of this is also presented.

On the right side, we can see that more exposure units are needed as available light (including both ambient light and light provided by flash guns) falls.

Aperture

For aperture, a larger aperture (smaller number) contributes more to exposure than a smaller aperture (larger number).  The effect of increasing or decreasing aperture value is also presented.  As aperture becomes larger, its contribution to exposure increases.  However, this also results in less area in the picture being in focus (shallower depth-of-field or DOF).  On the other hand, a decrease in aperture increases the area in focus (deeper DOF) but contributes less to exposure.  In addition, a smaller aperture (generally f11 onwards) results in less sharp picture due to diffraction.

The aperture of a lens is basically computed by dividing the focal length of the lens by the diameter of the lens opening (hence, an aperture of f2.0 means an opening with a diameter of 25mm on a 50mm lens, 100mm on a 200mm lens).  Because of this, it is easier to make large aperture lenses for wide standard lenses to medium telephoto lenses (35mm to 100mm) than it is for telephoto lenses (150 onwards) as the lens opening will have to be bigger making the lenses bigger and heavier.

The following pictures show the effect of aperture changes.  All of these were shot using the same lens (100mm macro).  The picture is focused on the number 11 (manually focused using live view).

Shot at f2.8 (top left) * Shot at f5.6 (top right)

Shot at f11 (bottom left) * Shot at f32 (bottom right)

As we can see, as the aperture is increased (smaller hole), the area in focus becomes bigger.  At f2.8, only the number 11 is in focus.  By f5.6, 10 and 12 are already readable but still blurred by a small amount.  At f11, 10 and 12 are in focus and 9 and 13 are readable but still blurred.  At f32 (the smallest aperture of the lens), everything is in focus.

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However, as discussed above, at a smaller aperture, diffraction kicks in.  This is dependent on the lens but usually starts at f8 (for large aperture lenses) or f11 (for medium aperture lenses).

Here are crops of the images shot at f5.6 (left) and at f32 (right).  Both shots are in focus but at f32, the number 11 is less sharp due to diffraction.

The aperture is determined by the lens being used.  Almost always, a larger aperture makes a lens much more expensive (for instance, the Canon 70-200 f2.8 is twice the price of the Canon 70-200 f4, with only 1 stop difference in aperture), bigger and heavier (just compare Canon 85mm 1.2L vs 85mm 1.8 or Canon 1.4 vs Canon 1.8). Lenses generally have minimum apertures ranging from f22 to f32 and maximum apertures ranging from 1.4 to 5.6.  f1.0 lenses are very very rare and expensive and f0.7 are almost non-existent (except for CCTV camera lenses).

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Shutter Speed

On shutter speed, a longer shutter speed contributes more to exposure than a shorter shutter speed.  Modern DSLRs usually have a fastest shutter speed of 1/4000s and a shortest shutter speed of 30 seconds.  Longer shutter speed is also possible using the bulb mode.  A shorter shutter speed freezes action, such as waves crashing on the sea.  A longer shutter speed on the hand results on a more blurred action (such as misty pictures of the sea) resulting from camera blur (if handheld) or motion blur (if the subject is moving).  Usually a slower shutter speed (1 second or longer) requires the use of a tripod).

Below are samples of pictures of a rotating electric fan taken at different shutter speeds.

Shot at 1/250 seconds.

Shot at 1/60 seconds (above)

Shot at 2 seconds (below)

As the shutter speed is lengthened, there is more blur on the rotating part of the fan.  Generally 1/250s or faster stops action.  This is usually useful for moving objects (such as players).  Longer shutter speeds are made to intentionally blur a moving object (such as a waterfall).

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ISO

ISO is the current battleground of most modern cameras as each new model tries to outdo the preceding and the competition on how clean it can render a picture at higher and higher ISO.  A doubling of the ISO number increases its contribution to exposure by one stop (requiring a shorter shutter speed or a smaller aperture).  However, as ISO is increased, noise – in the form of chroma noise (colored specks) and luminance noise (black and white specks).

As an example, here is a picture of a miniature replica of Rodin’s The Thinker.

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I shot the same picture using different ISOs and made crops of each (near the face).

Notice that as ISO is increased, the level of noise (specks) also increases.  At ISO 200, there is hardly any specks.  At ISO 1600, the grain becomes noticeable and at ISO 6400, everything looks very grainy.

Shot at ISO 200 (left) * Shot at ISO 1600 (center) * Shot at ISO 6400 (right)

Each of these three factors contribute to how much exposure the sensor or film gets for the image.

Putting It Together and Sunny f16 Rule

The sunny f16 rule is an old photographers rule or guide that states that on a sunny day, a proper exposure on an aperture of f16 would require a shutter speed that is inverse of the ISO speed.  The first two bars shows this rule.  On the first bar, the aperture of f16.0 required an ISO of 200 and a shutter speed of 1/200second.  The second one follows the same example but uses ISO 800 and a faster shutter speed of 1/800 seconds.

Using the preceding table and by adding up the exposure contribution of each factor, we get an exposure value of 11 1/3.  For the first bar (f16, ISO 200 and shutter speed of 1/200s), the aperture contributed 3 stops, ISO contributed 3 stops and shutter speed contributed 5 1/3 stops (the chart of course can be adjusted).

Now let’s assume that I want to take a portrait on a sunny day and I want a shallower DOF:  I’d use a faster aperture of f2.8.  Since the aperture already provided 8 stops to exposure, shutter speed and ISO will have to provide less to get to the 11 1/3 exposure.  In this case, I will use an ISO of 100 (2 stops) and a very fast shutter speed of 1/3200 (1 1/3 stops).  All these three will provide the same total for a proper exposure as the sunny f16 rule with most of the exposure contributed by the aperture.

Assume further that I want an even shallower DOF:  In this case, I’d use an aperture of f2.0.  This already provided 9 stops to the total of 11 1/3 exposure stops needed.  The remaining 2 1/3 stops will have to be divided between shutter speed and ISO.  The lowest ISO for most camera is ISO 100 so this will provide exposure contribution of 2, leaving only a remaining 1/3 stop for shutter speed.  The fastest shutter speed for a DSLR is usually 1/4000 second, which will provide exposure contribution of 1 for a total of 12 exposure stops, instead of the proper exposure of only 11 1/3 stop.  With cameras these days, a 2/3 stop overexposure is ok as details and exposure can be adjusted during post-processing.

Real World Use and Exposure Compensation

As an example of a real world picture, here is a picture of a sunset I took in Dampalitan Beach.  The light is fading and this required a lot of camera exposure to expose properly.  Looking at the bar graph on the left, the first bar shows the exposure detail for this shot.  An aperture of f11 (to maximize depth of field), ISO 200 (to minimize noise) and 20 seconds of exposure to bring in as much light as possible to the sensor (shot was tripod mounted).

Majority of the need for exposure was provided by a slow shutter speed.  The other two bars show possible exposures for the same picture.  Bar 2 required only 1/6 seconds of shutter (possible to hand hold with lens image stabilization), a f5.6 aperture and ISO of 6400.  I did not choose this of course because I wanted to maximize depth of field so that both the mountains in the background and the sand in the foreground will be in focus.  Furthermore, I wanted a clean picture (little noise) and I wanted to blur the sea so that it will be one giant mirror (instead of seeing individual waves).

The third bar provides another possible exposure scenario.  In here, a faster aperture of f1.4 is used with a handholdable shutter speed of 1/50 seconds and an ISO of 3200.  This would have been ok but there will be very little in focus that it may no longer look like a landscape photo but a picture of a mountain with some smudgy foreground or a picture of sand with colorful background.

On the left of the bar chart, we can see five notes.  The one in bold indicates the camera metered exposure (the one used by the camera).  If I wanted a brighter picture, I could have used exposure compensation by adding 1 exposure unit (longer shutter speed or faster aperture or faster ISO).  On the other hand, if I wanted a darker picture, I could have used negative exposure compensation by reducing 1 exposure unit (shorter shutter speed, smaller aperture or slower ISO).

Generally, there is only 1 proper exposure for a picture (exposure compensation is used if you think that the camera failed in metering the image).  However, there are lots of ways to achieve that proper exposure by balancing aperture, shutter speed and ISO.  It all comes down to what we want to show in the picture – our artistic choice.

Importance of Knowing Exposure Triangle in the World of Auto-Everything

I think all modern cameras feature autoexposure and almost all does pretty good exposure metering.  So what’s the importance of knowing the exposure triangle?  By knowing this, we can make our choices on aperture or shutter speed or ISO (if a cleaner or grainier image is desired) to make our vision shown in the final image.  By knowing that adjusting aperture (if more or less area in focus is desired) will have a corresponding impact on shutter speed and ISO or that choosing a faster shutter speed would require faster aperture and higher ISO, we can choose our priority and make compromise on the other factors affecting exposure.

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One response

  1. Pingback: Exposure Triangle | Digital Photography

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