Photographs pay attention The Sweet 16 Exposure levels

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Understanding Sky Conditions and Light levels

We are going to use the sky condition and the camera’s ISO setting to determine the correct exposure without the meter. Before the days of the ever-present light meter, photographers had to use exposure tables to determine a good baseline for taking a picture.

They still work. We are going to use the “Sweet 16” rule to explore exposure without a meter. Then learning to properly use a meter will be a snap. Exposure tables all work the same. The ISO speed is paired with a standard lighting condition to determine a suggested exposure.

Bright, noon-day sunlight on a cloudless day has a set intensity level that is constant enough to use as a reference. Add some white sand or snow to the scene, and the reflected light and the light level just about doubles. Add a few clouds, and that light level drops in half. If the clouds cover more of the sky and darken it, the light level is halved again.

We can tell how much the light has changed by looking at the shadows. If the sunlight in the scene is muted due to clouds, fog, or shade, then we have to increase the amount of exposure to get the same amount of light to the sensor.

Consider the broken bicycles assembled on the Amsterdam street shown in this image:

This was an overcast day and I took the picture in late morning. Notice the faint shadows cast by the bike wheels, and how much softer the light is? That is due to the shade of the buildings, which prevents most of the sunlight from directly reaching the bikes.

Let’s review. The average human eye can distinguish when the light intensity in a scene doubles or is cut in half. This amount of change is equal to moving the lens aperture or shutter speed one full stop for example, changing the lens aperture from f/11 to f/16, or the shutter setting from 1/25th of a second to 1/50th.

With a little practice the Sweet-16 system can be customized for your camera and shooting style. This skill frees you from relying on the meter and your camera’s automatic settings. You’ll know how to tweak the meter reading, and be able to tell when it is not accurate. You can also deliberately manipulate the exposure for artistic effects or to compensate for special situations (more on that later).

Let’s take some pictures. The first thing is to set the camera metering system to manual mode. There is often a dedicated button or dial that controls this. Be sure that any exposure compensation (plus or minus from the actual exposure setting) is set to a zero value, or turned off. Set your camera to ISO 400 and the lens aperture (f/stop) to f/16. That’s the 16 in Sweet-16.

Go outside at least two hours after sunrise, and at least two hours before sunset, and take some pictures of subjects in open lighting (not shaded by a tree or building) using the settings described in the following list, manually adjusting the exposure. Then take another copy of the same picture using the camera’s auto-exposure mode. The early-morning and late-day sunlight do not have the full intensity the system requires for reasonable exposures.

Next, set the shutter speed to 1/400th of a second. The basic exposure using this method is set by matching the shutter speed to the ISO number at f/16 (here, 400 and 400), when taking a picture in full sunlight on a cloudless day.

The Sweet-16 Exposure levels

• Bright sun with no clouds in the sky and distinct shadows: 1/ISO number at f/16 (basic Sweet 16)
• Bright sun on light sand, snow, or reflecting off a bright surface: 1/ISO at f/22. (Decrease one Exposure Value from f/16). This is a very contrasty type of lighting that produces very dark shadows and bright whites.
• Hazy sun, a few clouds in the sky causing soft shadows, or very light fog: 1/ISO number at f/8 (increase one f/stop from f/11)
• Cloudy bright with no shadows: 1/ISO number at f/5.6 (increase two f/stops from f/16). This is when the sky is mostly cloudy over the scene, but with areas of bight sun adding to the overall illumination.
• Cloudy dull: 1/ISO number at f/4 (increase three f/stops from f/16). The sky has a complete cloud cover, but sunlight filters through the clouds.
• Cloudy dark: 1/ISO number at f/2.8 (increase four f/stops from f/16). This is what we see when a thunderstorm approaches or during a fall rainstorm.

When setting exposures using the Sweet-16 reference points, changes are made in full f/stop increments (either doubling the amount of light reaching the sensor or cutting it in half).

Experienced photographers also often use the term full stop when referring to adjusting the shutter speed or ISO by that amount. The list below gives the range of full stops for all three exposure variables (aperture, shutter speed, and ISO) for you to use. Almost all DSLRs offer the ability to make smaller adjustments. Please use only full stops matching the Sweet-16 rule for right now.

Here is the range of traditional full f/stops: f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22. Remember that smaller numbers let in more light, so increasing two stops from f/8 would mean using f/4.

The range of shutter speeds in full stops (arranged from letting in the most light to the least) is 1/25, 1/50, 1/100, 1/200, 1/400, 1/800, 1/1600. Your camera will probably only show the number to the right of the slash mark. Full-stop ISO numbers also double or halve: 25, 50, 100, 200, 400, 800, 1600.

You may want to adjust the ISO or shutter speed to compensate for a lens that does not offer the proper f/stop. Many lenses don’t offer a setting of f/22 or f/2.8. Let’s use an example to make the options easier to understand. If your subject is near or on a large reflective surface, you can adjust the exposure by either increasing the shutter speed to 1/800th of a second or making the f/stop one stop smaller to f/22. If your lens doesn’t close that much, cut the ISO number in half.

One “full stop” lower adjustment of any one setting reduces the total light energy recorded the exposure in half. Many lenses don’t have a setting smaller than f/16. (As the numbers get bigger, the opening gets smaller. I’ll explain why in the next chapter, along with what those arcane numbers mean.)

Perhaps you want a faster or slower shutter speed than the one dictated by the ISO number, or a larger or smaller lens aperture (f/stop), but want the same amount of light to reach the sensor. In that case, you adjust two variables to record the same intensity. For example, f/16 at 1/200th of a second at ISO 100 is the same as f/8 at 1/100th of a second at ISO 100, and as 1/400th of a second at f/16 at ISO 200.

To really gain an understanding of both exposure and how different lighting conditions affect the way a scene appears, experiment with the same scene under different sky conditions over several days and compare the results. Notice how well the standard settings work with your camera. Do the results look over- or underexposed? Are the highlights too white, or the shadows too black? If so, some fine-tuning is in order.