What’s exposure, after all?

By Moni Caron

Despite the hundreds of articles, books and tips about exposure, it seems there’s a gap of understanding so that the photography lover can produce images exposed with appropriate light to strike anybody’s attention.

So, what’s exposure, after all?

Exposure is the amount of light that strikes the film in that fraction of a second that you press the shutter button. It is a partnership between the diaphragm (that controls the size of aperture of the blades through which some amount of light enters the camera) and the shutter (the curtain that controls the length of light that will reach the film in that fraction of a second).

Think of aperture as being the iris that regulates the amount of light entering your eyes, and shutter as being the blinking eyelid that regulates the length of light entering your eyes. When there’s too much light, you blink faster and your iris gets small to filter the light and make you see. When it is too dark, you hardly blink, and you iris gets big to make more light enter you eyes and make you see.

Summing up, exposure is the result of certain amount of light reaching the film for some length of time.

That said, why people get so confused to understand exposure? It happens that light metering is given in combinations of numbers that need to be understood as well as the way metering systems works. Metering systems are calibrated to reflect light like 18% gray. This is because a typical scene reflects light as 18% gray, hence a guarantee of real, true colors. Usually, this calibration works just fine, but it presents problems in situations where you have predominance of too bright subjects or too dark subjects.

Traditionally, subjects as cloudy sky, beach, snow and white walls are risks for underexposure and subjects as lakes, mountains, forests, dusk, down and black walls are risks for overexposure. By the end of this article, you will know how to “fix” these problems. Now, let’s come back to the aperture and shutter.

The apertures of the diaphragm are given in numbers called f/stops. The most known are, f2.8, f4, f5.6, f8, f11, f16, f22. Each one of these f/stops allow a certain amount of light to enter the film, the smaller f/stop number being the largest aperture and the highest number being the smallest aperture.

The shutter numbers are given in fraction numbers, 1 second divided by those numbers written in the dial or LCD panel. The most known are: 1, 2, 4, 8, 15, 30 60, 125, 250, 500, 1000, 2000. Hence, the numbers you see in your camera concerning shutter speed are actually 1 second divided by 2, 1 second divided by 4, 1 second divided by 8 and so on. You can as well have longer shutter speed for some seconds, minutes or hours.

When you press the shutter button with one combination of aperture and shutter speed, the blades of the diaphragm will close to form a hole of the size of the chosen aperture and the curtain will move in the chosen shutter speed to let the light go in.

The secret for a good exposure is choosing the right combination of aperture and shutter. Your metering system will help you to interpret the reading of the light, so that you can make the decision of what you want.

Depending on what camera you have, you might have one of these metering systems (or 2 or all of them):

Spot/partial – the sport metering covers about 1% to 3.5% of the image area. Partial metering usually covers about 9.5% of the image area.

Center-weighted – this is the most common type of metering system. It makes an average of the entire scene, emphasizing the center area. The weight of this “average” light reading is 75% center and 25%outside.

Evaluative/multi-zone/matrix – the metering system is calculated based on data stored in a chip linked to the AF sensors. It compares the aimed scene you want to shoot with internal scenes of same pattern and weight of light.

When you aim the camera to the subject, the metering mode will read how much light is available and will suggest you an exposure. There are many possibilities, but you have to start with some idea of what you want, and then adjust the exposure.

If you use the sport /partial metering, the camera will read the light of the specific point/area where you are focusing and tell you how much light you need to expose that subject.

If you use the center-weighted metering, you have to assure the subject is in the 75% of the area to have an appropriate exposure, or you are under risk of under or over exposure.

If you use the evaluative/multi-zone/matrix system, your chances of getting wrong metering on your subject are smaller, due to the complex data system the cameras have, but you still have to be critic on evaluating the scene when you are composing your picture under the circumstances of too much white or too much black.

If you see a big contrast of shadows and bright light lighting a scene, you have to compensate the exposure, that is, add 1 f/stop of light to lit some too bright area (if you want the bright area well exposed) or subtracting 1 f/stop of light if you want to lit for a darker subject. Remember that all metering systems are calibrated for the 18% gray, which would make a total white wall or a total black wall look like gray.

When I say “compensate the exposure”, I mean you control the camera and add or subtract light reaching the film, even when the camera “say” it is too much or not enough light. That’s because of the metering systems calibrated for the 18% gray, as mentioned above: in a scene with a person in the snow, for instance, the camera would understand that the amount of snow around the person is too much light, hence, suggesting you to stop down the aperture and decreasing the light that would reach the film. However, in this kind of situation, if you do that, you will have a too dark picture, and the snow will look like blue/grayish instead of white. In order to “compensate” properly in this situation, all you need to do is to meter the light (let’s say you have a metering that says the adequate exposure is f11 with 1/250) and open up 1.5 f/stops (f11 with 1/90), 2 f/stops (f11 with 1/60) or even more. You are in charge of the exposure compensation. That’s what your brain is for J.

When you select shutter priority, you have in mind some specific purpose, whether to freeze a basketball player, to show the motion of a classic ballet, to capture the waterfall as a smoke or a racing car. The shutter will be chosen first if you have these kinds of worries. A fast shutter speed as 1/500 of a second can freeze drops of water in the air. A slow shutter speed as ¼ of a second can record the waterfall as an ethereal smoke.

When you select the aperture priority, you have in mind some purpose, whether to call attention to your family member and blur the background, or to have everything in precise sharpness to show the depth of field (the acceptable zone of sharp focus in front and behind your main subject) of a majestic landscape from the foreground to the infinity. A small aperture such as f22 will give you a big depth of field from very close to you to the infinity, while a big aperture such as f2 emphasizes only your main subject, with a small depth of field, focusing all the attention on your subject and blurring everything in front or behind it

Once you choose which is your priority, you half-press the button, set the shutter or the aperture, and the metering system will tell you if you have too much light or not enough light, so that you can make the adjustments until the right combination is set.

The relationship between the aperture and shutter walks in opposite sites, that is: given a certain combination, when you want to stop down the aperture to have more depth of field with the same proportion of light, you have to choose a slow shutter speed to equal the same amount of light.

Watch this:

Given a metering of f8 with shutter speed 1/30:

If you want more depth of field and want to keep the same amount of light reaching the film, you stop down the aperture to f11 and on the other side, open up to a slower shutter speed of 1/15.

Making the adjustments, given the above metering of f8 with shutter speed 1/30 , would give you all the possibilities below. They have the same amount of light reaching the film, but will show different results:

F2.8 with 1/250

F4 with 1/125

F5.6 with 1/60

F8 with 1/30

F11 with 1/15

F16 with 1/8

F22 with ¼

If the subject of this metering were a waterfall, f22 with ¼ (on a tripod) would make a smoke effect on the water. If the subject were a person, f5.6 with 1/60 would make a nice portrait, with some blurred background behind the person. If the subject were a child jumping, f2.8 with 1/250 would freeze the child in the air, and blur completely the background.

Are you still with me? Do you see how important it is to understand the combinations of aperture and shutter??

There’s one more important thing you should know: when you shoot with slow shutter speed of 1/30 or slower (1/15, 1/8, ¼, ½, 1 second or longer exposure), use a tripod. The slow shutter will make the curtain be open for a length of time you can’t hold the camera steady without shaking the camera.

That’s all the basics about exposure. I encourage you to try some new pictures and discuss them with me. Have fun!!