Photo Tip #1: Determining Exposure

Determining Exposure














PHOTO TIP #1: Turn off the Automatic Mode.

This is the first step in taking control of your image-making. Don’t let the camera make critical exposure decisions for you. In this easy lesson, you will gain an overview of light, the camera’s metering system, and accurately determining exposure. This is photography’s most basic precept and with a little practice it will quickly become second nature.

Let’s start with what is commonly referred to as the exposure triangle–which consists of your ISO (light sensitivity), aperture (lens opening), and shutter speed. There is an interrelationship between these three components and when you change one setting, it directly affects the other two. A camera’s ISO settings range from 50 (the least sensitive), 100, 200, 400, 800, etc…up to as high as 25,600, for extremely low-light situations. Changing your ISO setting from 100 to 200, increases the exposure by 1 stop. Reducing your ISO setting from, say, 400 to 200, decreases exposure by 1 stop. The general rule is to use as low an ISO setting as conditions allow to reduce unwanted noise in the image. Aperture refers to the size of the lens opening (f-stop). Your f-stop determines the amount of light reaching the sensor or film plane. Full stops range from f/1.8 (wide open), 2.8, 4, 5.6, 8, 11, 16, up to f/22 or f/32 (smallest opening). Stopping down from f/8 (larger) to f/16 (smaller) results in a 2-stop decrease in exposure, and so you must reduce your shutter speed and/or increase the ISO setting a total of 2 stops.  Shutter speed is the amount of time that the shutter is open and allowing light to hit the sensor. Settings on a digital SLR generally range from 30 seconds up to 1/8000 second. A slow shutter speed requires a smaller aperture to compensate for a longer exposure time. A fast shutter speed requires a larger lens opening to allow for a decreased exposure time. For example, if you are shooting a sporting event at an ISO of 100 with a lens opening of f/11 at 1/125 second and you want to increase your shutter speed to 1/500 second to freeze the action, you have reduced the amount of light you are letting in by 2 stops. Increasing your shutter speed from 1/125 to 1/250 second reduces your exposure 1 stop, and increasing it to 1/500 second reduces the exposure by another stop, for a total of 2 stops. To adjust for the decrease, it is necessary to open the lens and/or increase the ISO setting 2 stops. In this case, I would choose to open the lens 1 stop, from f/11 to f/8, and increase the ISO setting 1 stop, from ISO 100 to ISO 200. This allows me to maintain a relatively small aperture for increased depth-of-field and sharp focus with minimum noise.

A brief word on depth-of-field. The term depth-of-field refers to the area within the picture that is in sharp focus. In general, the smaller the lens opening (f/22 or f/32), the greater the depth-of-field. The larger the lens opening (f/1.4 or f/1.8), the shallower the depth-of-field. When you focus directly on your subject, the depth-of-field begins 1/3 in front of your subject and ends 2/3 behind it–that is, if you’ve determined your depth-of-field to be 9 feet, that area of sharp focus begins 3 feet in front of the subject and ends 6 feet behind it. I will talk more about depth-of-field in a future Photo Tips post.

The Gray Card and Determining Exposure

The average scene you encounter is calculated to have a light reflectivity of 18%, and so the camera’s light meter is calibrated to this 18% reflectivity. This is well and good, but the majority of scenes are not average, and if you rely on the Automatic setting, you will likely get average results, and most of the time. Have you ever taken a photo of a snow-covered landscape, to find that the snow is rendered a dingy gray tone? In the days of film, this was always a horrific discovery. Of course, with digital technology the results are instantaneous and so you can view the image and adjust accordingly. The camera’s meter is not able to determine when a particular scene deviates from this 18% reflectivity, such as sunlit snow, which may have an 80 or 90% reflectivity–or a dark object, which may have little-to-no reflection at all. The meter is always going to assign these things a tonal value of 18%.

An 18% Gray Card allows you to accurately determine exposure in any lighting condition. They are available at B & H PhotoVideo, or any camera supplier. To use the Gray Card, you simply point your camera in the direction of your intended subject. You then place the card in front of the camera lens until it just fills your viewfinder. It is imperative that your subject and gray card receive the same illumination. If your subject is in full sun, the gray card must also be reflecting full sun. Take your reading. You may choose to fine-tune your exposure by bracketing 1/2 to 1 stop.

A little trick I use while skiing is to meter directly off the snow. In bright light conditions, I will take my reading in the same direction and light as my intended subject, and then open the lens 2 stops and bracket. On overcast days, I will meter off the snow and open 1 to 1-1/2 stops, depending on the intensity of the light. You can also meter off the palm of your hand and open 1 stop. I have a lens cleaning cloth that doubles as a Gray Card. It’s very compact and extremely lightweight. For minimalists such as myself, it’s the perfect solution. Eventually, through using the gray card repeatedly, you will learn to recognize objects with an approximate 18% tonal value, allowing you to meter on the fly.

Please feel free to contact me with any comments, questions or suggestions.

Until next time, happy image-making…


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