In digital photography exposure is the amount of light that hits a digital camera’s sensor (or film for an SLR camera) to create an image. So exposure time is the length of time the sensor is exposed to light entering the lens, and is controlled by shutter speed.
Even if you use the auto modes like auto or program, or the semi-auto modes like aperture priority or shutter priority, you still need to know about exposure time. It’s not just for photographers who shoot in manual mode. Also, while we’re on the subject, I strongly advise you not to use auto, rather use program mode. It’s still an automatic mode, but much better.
What is shutter speed?
Imagine you’re in a dark room with blackout blinds. If you flick open the blinds, light enters, but as soon as you snap the blinds shut again the light is blocked out. You’ve just performed the exact function of your camera’s shutter.
You could open the blinds for a split second or for longer. This is how shutter speed works. The amount of time that you have the blinds open is the same as shutter speed in photography.
- Fast shutter speed = short exposure time
- Slow shutter speed = long exposure time
How do you change shutter speed?
Shutter speed is measured in fractions of seconds (short time for most types of photography) and full seconds (long time for long exposure photography).
The reason you see the shutter speed value expressed as a fraction most of the time is because it takes just a split second to create an exposure (image). So when you see a shutter speed of 1/60th (or 1/60) it means that the shutter is open for 1/60th of a second.
You’ll see these numbers displayed in the LCD screens on the top and back of your camera when you turn the shutter speed dial. You can also see them when you look through the viewfinder.
Shutter speed can be increased by a full stop or by a third of a stop at a time. If you’ve set your camera settings to third stops, then every time you turn the shutter dial it’ll increase or decrease the shutter speed value by a third of a stop. So to increase a full stop, you’d turn the dial three notches.
Why is exposure time important?
The length of time the shutter stays open (aka shutter speed) shortens or lengthens the exposure time, so shutter speed affects two things:
- Correct exposure
- Sharp photos
1. Setting exposure time for the correct exposure
Our eyes are far better than a camera sensor, so we can see everything the instant the blinds are opened. Cameras, even modern cameras, can’t. They need time for the light to hit the the sensor and create an image. When there’s more light they need less time than when there’s less light.
So lighting conditions, in other words how much light is available, affect how much time is needed for a proper exposure.
To change the exposure time (how long or short the shutter remains open) you adjust the shutter speed setting. The trick is to know how long the shutter needs to be open and I’ll go into that detail in a moment.
Natural light shutter speed settings
Remember, just because you can see, doesn’t mean your camera can. Too little time and the photo will be underexposed (too dark) and too much time and the photo will be overexposed (too bright).
- Set a longer shutter speed when there’s not much light, like on an overcast day, at night or when indoors
- Set a slower shutter speed when there’s a lot of light, like photographing outside on a bright sunny day
Flash shutter speed settings
I won’t go into detail here on camera settings for flash, except to say that the sharp burst of light produced by flash means that shutter speed isn’t as relevant with flash as it is with any form of continuous light, including natural light.
2. Exposure settings for sharp photos
Using the wrong exposure time is one of the main reasons why new photographers struggle to get sharp images.
The amount of time the shutter is open is key to avoiding blurry photos. There are two ways that you could accidentally blur your photos with the wrong exposure time:
- Camera shake
- Motion blur (subject movement)
1. Exposure time to prevent camera shake
Camera shake happens when the exposure time is too long and the movement of your hand holding the camera makes the whole image blurry. In other words you’ve used a slow shutter speed. Even the tiniest movement, so small that you won’t notice your hand moving, will cause camera shake.
It helps if your lens or camera has image stabilization, because then you can photograph a couple of stops slower without the risk of camera shake.
Some people have a sturdier grip than others and can “hand hold” a camera at lower shutter speeds than others. In other words, when the camera isn’t supported on a tripod or something similar. I’m one of those people that can’t.
This is really important! Your lens focal length determines the minimum shutter speed you can use to safely avoid camera shake while hand holding your camera when photographing with continuous light.
You know how when you look at a small, far away object through a pair of binoculars you have to try and be really really still so that the object doesn’t waggle around?
It’s the same with focal length, because of the effect of magnification the smallest movement of the lens is exaggerated. The longer the focal length, the more obvious the movement will be. So you need a shorter shutter speed to reduce the chance of that movement being captured.
The rule of thumb is that your shutter speed must be at least equivalent to the camera lens focal length you’re using. If you’re using a zoom lens, that’s not just the focal length of the lens, but the actual focal length that you’ve zoomed to. So:
- 50mm focal length – set shutter speed to at least 1/50th
- 100mm focal length – set shutter speed to at least 1/100th
That’s not good enough for me and many other photographers. I need at least 1.5 times the lens focal length. So if I’m shooting at 200mm my shutter speed must be at least 1/300th. In fact, if at all possible, I prefer to use 2 times the focal length.
To use long exposure times of more than a split second mount your camera on a tripod, or anything solid, that won’t move while the photo is being taken.
Shutter speeds of a full second or longer are expressed with a “. So a 1 second shutter speed is written as 1”.
2. Exposure time to prevent motion blur
Motion blur is another way of describing blur caused by subject movement. If your subject moves while the shutter is open they’ll be blurry, unless you use a fast shutter speed for a short exposure time. The faster your subject moves, the faster your shutter speed needs to be.
On the other hand, if you want to blur the subject to show a sense of movement in an image, your shutter needs to be open long enough to allow the subject’s movement to blur.
Either way, exposure time is the key to controlling motion blur in photos.
Other camera settings for correct exposure
However, exposure time isn’t the only factor that affects exposure and therefore the brightness of an image. We talk about the exposure triangle in photography as a way to think through adjusting exposure settings to achieve the look you want.
Aperture size, for example F2.8, also affects exposure. The wider the aperture of your lens during exposure time the more light will enter the lens and the shallower the depth of field will be.
Going back to the blinds example from earlier, the wider you open the blinds the more light flows into the dark room while the blinds are open.
So while aperture controls how much light enters the lens and hits the sensor, shutter speed controls the duration of the exposure time. Both are essential to create a photo.
Further reading: What does aperture do in the exposure triangle?
How wide the shutter opens (aperture) affects two things:
- Correct exposure
- Depth of field
1. Aperture settings for correct exposure
Shutter speed and aperture work together like filling a sink with water. If you’re in a rush you open the faucet wide to pour the water in fast and fill the sink quickly. When time isn’t the priority, you can take longer to fill the sink so don’t open the faucet so wide.
- To freeze movement you want the shutter open for a short time, so you need to get light in quickly with a wide aperture to ensure that there’s enough light for the right exposure.
- For long exposure shots you need to take longer to let the light in, so use a smaller aperture to reduce the amount of light entering the lens for a correct exposure.
- Or put another way… If for some reason not much water comes out of your tap, you need to keep it open for longer to fill the sink.
Either way, you need to fill that sink to a certain point and you can either blast the water in quickly or trickle it in and take longer.
In photography this is the exposure value of a scene, and is made up of exposure time and amount of available light for the perfect exposure.
2. Aperture settings to control depth of field
What if, instead of thinking about movement, it’s more important to you to get a blurry background, or the opposite effect, a sharp image from front to back? Well, that’s when aperture becomes important.
Blurry backgrounds are very popular in portrait photography, so many portrait photographers prioritise a wide aperture over shutter speed. Because you have to choose one or the other.
- For a deep depth of field, use a narrow aperture and therefore slower shutter speed (longer exposure times)
- For a shallow depth of field (blurry background), use a wide aperture and therefore faster shutter speed (shorter exposure time)
So shutter speed and aperture work together to achieve the look you want for the lighting conditions and subject.
Aperture can be confusing to new photographers, because a small F number is a wider aperture than a large F number. That’s because F numbers are actually fractions, although they’re not written like fractions. So, for example, F2.8 is a wide aperture (for a blurry background) and F16 is a narrow aperture (for a sharp background).
- Lower numbers = bigger apertures = blurry background
- Higher numbers = smaller apertures = sharper background
The last camera setting you can use to help with the brightness of an image is ISO. While ISO doesn’t have any impact on setting the exposure (how much or for how long light enters the lens), it helps to brighten or darken an image.
ISO is much easier to understand than the other two camera settings in the exposure triangle. A low number is a low ISO and a high number is a high ISO.
- If there’s not enough light for you to have a fast shutter speed (to freeze movement) even with a wide aperture (to let in light), increase the ISO setting to brighten the exposure so your image isn’t underexposed.
- If after setting your shutter speed and aperture for the desired effect the exposure indicator shows that the image will be overexposed, reduce ISO if possible to darken the exposure.
Pro tip: using a higher ISO on entry level cameras can introduce digital noise (grain) to an image, which can make it look less sharp. So for best results try to keep your ISO as low as possible.
Further reading: What is ISO for in the exposure triangle and how to control it
What’s the best shutter speed for portraits?
This question has one of those dreaded “it depends” answers, because the best shutter speed for natural light portrait photography depends on:
- How much light is available?
- Is your subject moving and if so how fast?
- Are you using a wide angle lens or a telephoto lens?
- Do you want to freeze subject movement or capture movement as a blur?
- Do you want a blurry background or not?
So, instead, here’s a chart of shutter speeds for portrait photography to use as a basic guideline.
Once you understand what is exposure time in photography and can comfortably control exposure for a perfectly exposed photo you’ll begin to use exposure settings creatively. And that’s when photography becomes really exciting!
How do you know the correct exposure setting?
Your camera’s internal light meter will read the scene to assess how bright it is.
In manual mode you can see the exposure reading of the light meter at the bottom of the screen as you look through the lens, or on the LCD screen. It’s a simple graph with a center line. As you increase and decrease exposure settings you’ll see a cursor moving from one side to the next.
If the cursor is:
- To the left of the middle line, the image is underexposed
- In the middle, the image is correctly exposed
- To the right of the middle line, the image is overexposed
You won’t see the exposure indicator if photographing in other modes, because the camera will automatically adjust exposure settings for you, so it’s not needed.
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If you have any questions about what is exposure time in photography, let us know in the comments and I’ll get back to you.