In order to have complete control over your photography you need to understand how to effectively use shutter speed and aperture together. Because the one impacts the other you need know when to use aperture priority, the Av setting, and shutter priority which is the Tv setting on you camera. How it’s done is very simple and will help you improve your photos dramatically.
There are basically three questions here that need to be answered:
1. When do I use aperture priority (Av)
In order to use aperture priority mode you need to change the f-stops or f numbers on your camera. Check out your manual to find out where this is. What aperture priority does is allow you to manually change the f-stops. By setting it to Av it allows you to choose the f-stop and then the camera will automatically set the right shutter speed. You tell it how much light you want to let in and the camera sets the length of time the light will be allowed to reach the sensor, which is the shutter speed. So when do I use this feature in my photography? If you’ve seen those beautiful portraits of someone with the background all blurry then you’ve seen the effective use of the aperture. This is called depth field or depth of focus. The amount of focus in front of the subject and behind it. This is what you see in the portrait image, a shallow depth of field. It’s controlled by using a small f-stop, e.g. f2.8. The opposite happens for a landscape photo which is in focus from the foreground all the way to the background, a wide depth of field. Here you would use a large f-stop of say f22 right up to f32.
2. When do I use shutter speed priority (TV)?
Using shutter priority is even easier. Setting the camera to Tv allows you to control the shutter speed while the camera automatically sets the correct aperture. You want to control the length of time the shutter remains open and then camera will set the correct amount of light that reaches the sensor. Too much light will overexpose and give a very bright white image and too little light will underexpose and give a dark image. The shutter speed is measured in seconds and fractions of a second. Mostly you’ll use the fractions such as 1/60th or 1/250th and so on. Shutter speed is used to freeze or blur action like a motor car at speed when you would use a shorter time such as 1/250th or higher. Blurring action would require a shutter speed of 1/60th or slower such 1/15th.
3. How does aperture (Av) affect shutter speed (Tv) and vice versa?
What you need to know is that aperture and shutter speed are siamese twins. What you do to the one affects the other. This is quite simple. When you set a fast shutter speed of say 1/100th of a second, you need sufficient light to create a clear image, not too light and not too dark. Let’s say under the lighting conditions the camera chooses f5.6. Now, if you want to take a shot of a speeding car you would need to increase the shutter speed 1/250th so that the photo isn’t blurred. Now you have halved the amount of time the shutter stays open. This will cause the picture to be underexposed or too dark. So you have to change the aperture by opening it by one f-stop to f4. Moving up through the f-stops halves the amount of light allowed through and therefore moving down doubles it. There is another setting on your camera called manual (M) which allows you complete control of both aperture and shutter speed but this needs a lot more understanding which I will explain in another article.
To summarise, if you want to change the depth of field you would use aperture priority and change the f-stops. If you want to blur or freeze action you would use the shutter priority. And remember, when you change the one the camera will always change other.
This is why using these two settings is so great. You can choose which one to use for the type of photo you want to shoot and the camera will automatically set the other to the correct setting. Now cameras will not always make a perfect choice for several reasons but most of the time it will be the right one. The key to using these modes is experimentation and practise. So take your time to practise and you will soon learn how to effectively use these two settings.