One of the first basic concepts taught in a photography class is the “exposure triangle “. Exposure just means the lightness or darkness of your photo. They use the three-sided triangle shape to show the interaction of aperture, shutter speed, and ISO.
Aperture is the size of the hole inside your lens that lets light get thru to the camera’s sensor. Shutter speed is how long this hole is open after you press the shutter button. The ISO number shows how sensitive your camera’s sensor is to light. (ISO stands for the International Organization of Standardization, the main governing body that standardizes sensitivity ratings for camera sensors)
We designate Aperture by an F number; officially the F-number is the ratio of focal length to the effective aperture diameter; I don’t find that definition the least bit useful when I am out in the field shooting pictures. So here is a practical tip on how to remember how the F number and aperture size works; think of F as “fraction”. An F4 aperture opening is much larger than an F 22 opening, just as the fraction 1/4 is much larger than 1/22.
Shutter speeds are also in fractions; it is indicative, in seconds, of how long the shutter is open. A shutter speed of 1/250 is open longer, is slower, than a shutter speed of 1/2000 seconds, a fast shutter.
ISO numbers are just that, numbers. The lowest are usually 50 or 100; these are numbers you would set it to when you don’t need the sensor to be sensitive, such as in bright sunlight. The numbers double from there, 100>200>400 etc up to numbers such as 12,800. The higher numbers are for when you need the sensor to be more sensitive, such as in sunset photography, or even night photography, where you have low light.
The perfect exposure is a balance of these three elements. We measure this exposure as an Exposure value (Ev) or a “stop”. A wide open aperture and a fast shutter will give an Ev that a smaller aperture and a slow shutter speed can match. The “stop” helps determine how big an effect you have when you change one element. A change of one stop is a doubling or halving of light, depending on which direction you change that element. For example, changing the shutter speed from 1/500th second to 1/250th second is a one stop change to the positive since it allows in twice the light. Adjusting the aperture from F-22 to F-16 (making the aperture opening bigger) is a one stop change since it doubles the light available for the sensor. Changing the ISO from 100 to 200, or 200 to 400, results in a one stop positive change. More on stops in the exposure compensation discussion below.
You can set these elements manually, and hope you have the exposure close to right, but most of the cameras now have automatic modes. For example, you can set your camera to Av, the aperture mode. Select the aperture size you want for a particular scene or effect. Set the ISO at a fixed value, say 100, or a small automatic range such as 100-400 for normal photo shots. The camera will measure the scene’s available lighting (via an exposure meter, more on that next) and set the shutter speed it thinks will give you the best exposure for that shot.
Conversely, you can set your camera to Tv (shutter speed), select the shutter speed you want and the camera will decide which aperture size to set to get you a correct exposure. Many cameras also have a P (auto program) mode where the camera chooses both the aperture and shutter speed.
Most experienced photographers shoot in the Av aperture mode most of the time primarily to control the depth of field or sharpness in the photo.
And how does the camera decide this? Inside your camera’s operating program is an exposure meter; you usually have a choice of 4 meter settings. They are evaluative metering, central weighted metering, spot metering, and partial metering. Only thing to know here is that most of the time you should have your camera menu set to evaluative metering. (means you might have to dive into your camera user’s manual to figure out how to set this.) In this mode, the camera assesses the entire, or at least most, of the scene and adjusts the three triangle elements as described above.
Now this is where most discussions on the exposure triangle end, but since my triangle has 5 sides, I still have 2 sides to talk about. These are not just theoretical things you read about and never use; these are practical tidbits you will come to appreciate thru using them often, out in the field.
The fourth side in my “triangle “(properly called a pentagon) is Exposure Compensation (EC). You can only use this in the “auto “shooting modes; Tv, Av, or P. But since most people shoot in one of these modes, especially Av, most of the time, it can prove very helpful. Sometimes when your camera has gone through its determinations and gives you a photo, you might not like the exposure. Maybe some shadows are a little too dark, or the sky is a little too bright.
Using the exposure compensation function allows you to “tweak “the exposure lighter or darker. Instead of adjustments of a full “stop “, you can make 1/3 or 1/2 stop changes. On my Canon T6i camera, I can adjust the lightness/darkness up to 5 full stops, in 1/3 stop increments. You can see there is a lot of adjust-ability in the Exposure Compensation function.
You access this function thru a button that is usually a square; divided diagonally with half being black with a + sign, the other half is white with a - sign. (see photo) Push the button while turning the main adjustment “wheel “while watching the exposure gauge on your LCD. Please see attached photos to make this clearer. You can also see your photo in the Live View LCD get lighter and darker in real time as you turn the wheel. (see my blog post, Beginners’ Guide To Histograms to learn more about adjusting your exposure)
You can also see your photo in the Live View LCD get lighter and darker in real time as you turn the wheel.
The fifth element of my exposure pentagon is the Automatic Exposure Bracketing (AEB) function. This works with the Exposure Compensation (EC) function. It allows you to set an exposure on either side (the bracket) of where you have set the Exposure Compensation setting. To use this AEB function, you set where you want the EC setting, +1 stop, -2 stops, whatever you think you need to improve the exposure of your photo. Then you set how much lighter and darker you want to take an additional 2 photos. For example, you may want these to be 1/3 stop lighter and 1/3 stop darker than where you set the EC. So the three photos you will end up with are a “standard” EC exposure, an 1/3 stop lighter AEB exposure and a 1/3 darker AEB exposure. You can do this with 3 separate shutter button pushes, or you can set your shutter to “continuous shooting” mode. Then, when you push the shutter, your camera will take the 3 photos automatically, then stop.
Why would you want to take this kind of bracketed, 3 photo set? Perhaps you are in a situation when the lighting is very tricky, such as a street shoot using a variety of lighting sources for illumination (street lamps, neon signs, car lights,). Perhaps you are in a situation with a high dynamic range of lighting; with areas of dark shadows and areas of bright highlights. You could then merge these 3 photos into an HDR image with correct exposure for the shadowed areas and also correct exposure for the highlighted areas. Perhaps you are taking photos at a once in a lifetime event, such as a wedding or sporting event. where you can’t afford to miss the shot.
So once again I have taken what is usually a short blog subject and expanded it into a near manuscript. And I didn’t even touch on how you can adjust your exposure in your processing software, such as Adobe Lightroom. I hope you have been able to follow and understand my explanations; if not, break it down into smaller bites; exposure triangle, exposure compensation, automatic exposure bracketing, and use your camera’s user manual; if you don’t have one, google it and download it to your computer. And familiarize yourself with your camera so you can make the adjustments to Exposure Compensation and AEB quickly and proficiently.