So you got a new DSLR, maybe for the holidays, and you’ve now been using it a couple of months. It’s got all sorts of knobs and buttons on it, and it looks a bit intimidating. Good for you, you’re using it! You have quickly learned that autofocus, auto-exposure and auto-ISO gives you very nice photos.
You will soon realize that DSLR can do so much more than just act as a bigger and heavier point and shoot camera. It is a machine that, in conjunction with your use of it, creates art. It can create beauty. But where do you start?
How do you go about learning to use your camera and understanding the concepts that are the foundation of photography? The answer is, as with everything, one step at a time
In this article we will discuss a way, by changing the exposure mode, which will make the camera less completely automatic. It will still do most things on its own, but this will begin to give you more control over how your images look, and send you on your way towards releasing your inner artist.
Before we can talk about how to change exposure controls, we need to talk a little about what exposure is. Exposure is the amount of light reaching your digital sensor. It is determined by:
Shutter speed, which is the amount of time that light is allowed to hit the sensor. It’s like your eyelid. You open and shut your eyelid and let in light. Cameras typically have shutter speed that can range from as short as 1/8000 of a second to many seconds long.
- Aperture, which is the size of the opening inside your lens that lets in light through to your sensor or film. It works like the pupil (or really, the iris) of your eye. The larger the opening, the more light is let in.
The combination of the shutter speed and aperture is called an “Exposure Value,” or EV. EVs work in conjunction with one other part of exposure, the ISO setting. The ISO setting is a measure of light sensitivity of your sensor. The higher the number, the more sensitive it is to light For any particular ISO setting, there will be an appropriate amount (aperture) and duration (shutter speed) of light for a certain exposure.
Another thing to remember is that there is not just one setting for any particular exposure, but an entire range of equivalent shutter speeds and apertures. All combinations that yield the same exposure amount have the same EV.
It works like this: If you open up your aperture bigger, you need less time for the light to hit the sensor. If you make the aperture smaller (also called stopping down), you need more time. While each of these combinations give an equivalent exposure and EV, they also can give a different appearance to your final image. We’ll explore these differences further on.
When a camera determines all or part of your exposure, it’s called auto-exposure. Cameras equipped with auto-exposure have the ability to measure the amount of light coming in through the lens and to compute an appropriate middle of the road aperture and shutter speed combination, based on your ISO setting. It doesn’t know what you are taking a picture of, or how you want it to look, so it will generally pick some average setting that will be acceptable for typical situations. Some cameras do this better than others, but this is what all auto-exposure features do. There are four main levels of auto-exposure mode:
Program mode. The camera chooses both the aperture and shutter speed for you.
Shutter Priority mode. You select the shutter speed; the camera chooses the aperture.
Aperture Priority mode. You select the aperture; the camera chooses the shutter speed.
- Manual mode. You choose both the shutter speed and aperture. In this mode, the camera may give you some feedback as to whether it thinks you are exposing the image correctly, but it takes no other action.
Note how the names match the modes in the mode dial in figure 1 (P,S,A, and M). In addition to the above, your camera likely also has an Auto-ISO function, which will adjust the sensor’s sensitivity to light for a proper exposure based on the shutter speed, aperture, and light level.
Using your camera in Program mode (or one of the various automatic modes it might have) allows it to make all these decisions. What we are going to do is take your camera out of Program mode, and put it in Aperture Priority mode and examine what this control can do for your photos. The camera will still be selecting a shutter speed and ISO value, but you’ll be controlling this one aspect of previously automatic function.
Aperture and f/Stops
As mentioned before, aperture refers to the size of the opening within the lens that lets light through to the sensor. The various levels of light that lenses allow through are referred to as f/stops.
An f/stop represents the ratio of the focal length of the lens, (for example 50mm or 200mm) and the opening required for a certain amount of light to be let into that lens. The light levels are represented by progressive powers of the square root of 2. By taking a power of the square root of 2, and dividing it by the focal length of the lens, you determine how large the internal opening must be to let in the appropriate light level.
The chart below shows approximate values for a typical range of f/stops and the opening required on a 50mm lens.
|√ 2 x or f/Stop
|Aperture Opening Required On 50mm Lens (50/f-stop)
You may recognize the numbers in the center column from the f/stops in figure 2. These are whole f/stops. The formula provides that each number in this column either doubles or halves the amount of light from the next or previous f/stop. For example, f/1.4 lets in double the light as f/2, while f/8 lets in half the light as f/5.6. Doubling and halving like this makes calculating exposure simpler. You may also note that shutter speeds and ISO values also do the same sort of doubling and halving.
Numbers that are not in the above table usually represent either 1/3 or 1/2 stops that a lens or camera is capable of using. For example, f/1.8 (which is often seen as the maximum aperture of some lenses) is about 1/3 a stop smaller than f/1.4. You will always see lenses rated by their maximum f/stop value. (Some lenses have a variable maximum f/stop value due to the way they were constructed. In that case you’ll see lenses rated as something like f/3.5-5.6). The other thing to note is that because you divide the focal length of the lens by the f/stop value, as the f/stop increases in value, the opening actually gets smaller.
Do you need to ever know the actual opening size of the lens? No. Do you need to know which power of the square root of 2 you are using? No, although you can use the relationship between those numbers when saying something like “two stops lighter.” Otherwise there is almost no reason to talk about either. What you do need is the f/stop number. All cameras and lenses use this system. This means that f/5.6 on any lens is going to allow through the same level of light to the sensor regardless of its focal length. This is how the light meter in your camera is able to work, and how it can compute proper exposure regardless of lens focal length or manufacturer.
Remember: The lower the f/stop, the bigger the opening, and the more light it lets in, which is called a faster lens. The higher the f/stop number on a lens, the smaller the opening, and the less light it can let it, which is called a slower lens. Faster lenses are harder to make well, and are generally more expensive to purchase than slower lenses.
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