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You are hereHome > Tutorials > What Is... Correct Exposure?

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What Is... Correct Exposure?

When you take a picture with your camera set on Auto mode, you are delegating responsibility for determining the correct exposure to the camera. Depending on the 'brain' (or programmed chip) inside your camera, the result may be pleasing or not to your satisfaction. But before you blame the camera for your lousy pictures, it pays to understand a bit what goes on behind the scenes when you press the shutter release button. In this article, we are going to look at what 'correct exposure' means.

Aperture and Shutter Speed

A correctly exposed image means that the right amount of light has exposed the image sensor. There are basically two ways your camera can ensure that: 1) open or close the aperture (by making the hole of the iris larger or smaller; or, as is becoming more and more common in point-and-shoot digicams, by using a neutral density filter to restrict the amount of light reaching the image sensor); 2) by deciding how long to leave the shutter open. A third way that we will also briefly look at is 3) by adjusting the ISO (basically boosting the light signal).

F2.8 F4 F5.6 F8

When you (or your camera) uses a larger aperture (e.g. F2.8), the hole of the iris is larger and more light reaches the image sensor. Conversely, using a smaller aperture (e.g. F8) means that the hole of the iris is smaller and less light reaches the image sensor.

When a fast shutter speed is used (e.g. 1/1,000 sec.), the image sensor is exposed for only that small amount of time (i.e. 1/1,000 sec.). Conversely, when a slow shutter speed is used (e.g. 1/30 sec.; some cameras allow slow shutter speeds up to 30 sec. or more), the image sensor is exposed for that longer amount of time.

F8
1/30

Obtaining correct exposure is a setting combination of aperture and shutter speed. For example, your camera's light meter may have measured a need for an aperture of F8 at a shutter speed of 1/30 sec. If you press the shutter release button now, your camera will close up the iris to an aperture of F8 and open the shutter for only 1/30 sec. to obtain a correctly exposed picture. The amount of light that squeezes through that opening for that amount of time goes to expose the image sensor.

Complications

However, here is where the complications show up. If you are taking a picture of a serene and calm landscape, a setting of F8 at 1/30 sec. may be perfect! The small aperture gives you good depth of field ensuring that objects near and far are in focus. The slow shutter speed may be a problem if you are not using a tripod. A slow shutter speed means that any camera shake (even ever so slightly) will result in some blurring of the final image. A perfectly exposed blurred image!

Two things you've learned right here: 1) a small aperture increases depth of field; 2) a slow shutter speed requires a tripod, or other ways to hold the camera steady (e.g. by bracing yourself against something).

But, what if you are trying to take a picture of your son flying a kite in the park? A slow shutter speed will not only result in a blurred image because of camera shake, but your son is moving fast across the camera and the resulting image is a perfectly exposed blurred image of your son. To 'freeze' fast action, you need to use a fast shutter speed. So, move the scene mode dial to 'Sports/Action' mode or, if your camera allows Shutter Priority mode, select that mode and set a shutter speed of 1/250 sec. To still obtain correct exposure, you also need to open up the aperture now.

Remember, you selected a faster shutter speed which means that the image sensor will now be exposed for a shorter time. If you don't open up the aperture but keep F8, then the image sensor receives less light (you maintained the same hole opening, but closed the shutter sooner), and the resulting image will be underexposed. In this case, you would have 'frozen' the action, but unexposed the image.

Shifting Aperture and Shutter Speed

F2.8 F4 F5.6 F8
1/250 1/125 1/60 1/30

'Program Shift' is the term used to shift the aperture/shutter speed combination in tandem and still obtain correct exposure. As is illustrated above, if 1/30 sec. at F8 gives a correct exposure, then the three other aperture/shutter speed combinations shown also give the same correct exposure.

It is easy to understand when you consider that each aperture shift from F2.8 to F8 halves the amount of light reaching the image sensor (or, conversely, each aperture shift from F8 to F2.8 doubles the amount of light reaching the image sensor). Each shutter speed shift from 1/250s to 1/30s doubles the amount of light reaching the image sensor (or, conversely, each shutter speed shift from 1/30s to 1/250s halves the amount of light reaching the image sensor).

So, to maintain correct exposure, if you halve the one, you need to double the other. For example, if you halve the shutter speed from 1/30s to 1/60s, then you need to double the aperture from F8 to F5.6. Another way to look at this (depending on whether your main purpose is to shift aperture or shutter speed): double the aperture from F8 to F5.6, and you need to halve the shutter speed from 1/30s to 1/60s.

So the converse is also true, i.e. if you double the one you need to halve the other. In fact, say it anyway you like and the fact remains that if you halve the one (aperture or shutter speed), you'll need to double the other (shutter speed or aperture), And, if you double the one (aperture or shutter speed), you'll need to halve the other (shutter speed or aperture).

The Garden Hose Metaphor

Let's take an aside here to explain a bit more about aperture/shutter speed combination. Think of your garden water hose that you are using to fill a bucket with water. The diameter of the hose can be thought of as the aperture: the larger the diameter, the more water flows through. The length of time you leave the tap open can be thought of as the shutter speed: the longer you leave the tap open, the more water flows through. The speed of water flow can be thought of as the ISO: the faster the water flows through the hose, the more water flows through. The amount of water that collects into the bucket is the exposure.

Let's also pretend that you have two water hoses, one with a small diameter (our F8 aperture) and the other one with a larger diameter (aperture of F2.8).

To fill our bucket, we experiment with the smaller of the two hoses and find that we need to leave the tap open for 10 min (our shutter speed of 1/30 sec.).

So, here we have the following 'exposure setting':

  • small hose & long time of 10 min.
  • small aperture (F8) & slow shutter speed of 1/30 sec.

OK, so what happens if you use the same small hose but close the tap after, let's say, 3 min? Of course, it's clear that the bucket won't be full.

That is exactly what you did when you kept the same aperture and used a faster shutter speed. Not enough light came in to properly expose the image sensor ('the bucket is not full').

What do you have to do to fill up the bucket? Use the bigger hose! Aha, now with more water gushing out of your bigger hose, you can now close the tap earlier and still obtain a full bucket.

Similarly, to use a faster shutter speed, you need to use a larger aperture. Dial in F2.8 at 1/250 sec. and presto! you've just taken a perfectly exposed picture of your son frozen in motion.

Here is your new 'exposure setting':

  • big hose & short time of 3 min
  • large aperture (F2.8) & fast shutter speed of 1/250 sec.

Two more things you've learned right here: 1) a fast shutter speed will 'freeze' action; 2) the combination of shutter speed/aperture walks in opposite direction to each other: for a particular shutter speed/aperture combination required for correct exposure, if you now increase the shutter speed, you also need to open up the aperture; and if you use an even slower shutter speed, you will need to use an even smaller aperture.

OK, the lingo we use can sometimes get confusing. Let's sort out 'increase', 'double', halve', 'open', 'close', etc.:

- A shutter speed of 1/60s is faster than one of 1/30s.
- So we speak of 'increasing' the shutter speed going from 1/30s to 1/60s. Technically we are 'decreasing' the time the shutter is left open, but the shutter speed has increased.
- In so doing, we have 'halved' the shutter speed: 1/60s is half of 1/30s. A smaller shutter speed value is a faster shutter speed.
- An easy way to think of shutter speed is that when the number below the '1' (i.e. the denominator) increases (say, from '1/2' to '1/1000'), we are using a faster shutter speed and letting less light in; conversely, when the denominator decreases (say, from '1/1000' to '1/2'), we are using a slower shutter speed and letting more light in.

- An aperture of F8 is smaller than one of F5.6.
- So we 'open' up the aperture when we go from F8 to F5.6.
- In so doing, we have 'doubled' the aperture [opening].
- An easy way to think of aperture is that as the number gets bigger (say, from '2.8' to '8'), the aperture [opening] gets smaller and we are letting less light in; and when the number gets smaller (say, from '8' to '2.8'), the aperture [opening] gets bigger and we are letting more light in.

Of course, depending on the specifications of your digital camera, you may have only a restricted set of aperture/shutter speed combinations to work with. For example, a common shutter speed range is 1 sec - 1/1,000 sec. and a common aperture range is F2.8 - F8.

That is the range of aperture/shutter speed you can play with to obtain correct exposure. If you can't select a combination for correct exposure using these ranges, then your image will be either over or underexposed.

Over and Underexposure

F1.8 F2.8 F4 F5.6 F8
1/500 1/250 1/125 1/60 1/30

Let's say, in our example above, you decide you want to use an even faster shutter speed of 1/500 sec. You now need to open your aperture up one more setting, to F1.8. But wait! Your camera may have a maximum aperture of F2.8, so you can't open it up more. If you go ahead and take the picture anyway, the result if an underexposed picture of your son frozen in motion.

Conversely, if you use a slower shutter speed, say 1/125 sec., but maintain the aperture of F2.8, now you have too much light and your picture is overexposed. For a correct exposure at 1/125 sec., you'll need to use an aperture of F4.

ISO

Where does ISO factor in all this? Remember, we used the garden hose metaphor to understand aperture/shutter speed combination required for correct exposure: small hose, longer time required to fill the bucket; to use a shorter time to fill the bucket, we needed to switch to a larger hose.

But what if we do not have a larger hose? We then increase the speed water flows thru the hose. We use the speed of water flow as a metaphor for ISO. Using the smaller hose, if we could increase the speed of water flow, we would then be able to close the tap earlier and still end up with a full bucket! Similarly, increase the ISO, and you've increased the sensitivity of the image sensor -- i.e. it now needs less light to register an image.

This is not a technically accurate metaphor since when we increase the ISO, we do not increase the speed of light!!! Instead, using a speaker metaphor, we 'crank up the sound volume'. In practical effect though, it looks like we increased the amount of light falling on the sensor, hence our 'speed water flows thru the hose' metaphor.

Suppose we meter a correct exposure at 1/30 sec., F8, at ISO 100. By increasing the ISO to 200, we can now use either a faster shutter speed at F8, or use a smaller aperture at 1/30 sec. and still obtain correct exposure. In our example above where we reached the limit of our camera's aperture range, we could have used a higher ISO to allow us to use 1/250 sec at F2.8.

Sounds great, isn't it? So, why don't we just use the highest ISO all the time? The answer is the dreaded 'noise'. The image sensor does not suddenly becomes more sensitive just because you tell it so by dialing in a higher ISO; it gains that increase in sensitivity at the cost of image quality [due to noise].

Think of an image sensor as comprised of a matrix of photosites, with a photo sensor at each photosite capturing light for one pixel of information. Say, each photosite captures one pixel of info. When you dial in a higher ISO, you are increasing the sensitivity of the image sensor. Unfortunately the image sensor now captures not only more light signal but also more noise (any signal not generated by the light from your subject). It is the ratio of light Signal to Noise (S/N ratio) that determines the noise in your image. The more light signal (as opposed to noise) your image sensor is able to capture, the less noise in your image. Generally, the smaller the image sensor the worse the noise is. Most consumer digital cameras suffer from noise problems at high ISOs. Some Digital SLR cameras have practically no noise at the higher ISOs; they, of course, use a rather large image sensor.

Cameras are often advertised with high ISO capability with the promise that this will allow you to capture low light images without using flash. But in cameras with tiny sensors, the use of high ISOs generates lots of noise in the pictures. To reduce the high amount of noise generated at high ISOs, cameras have noise reduction technology built-in. This is no panacea since the way noise reduction works is to smooth out the pixels, leading to loss of fine image detail.

The take away from this discussion is that you would want to always use a small ISO for the best image quality. In low light situations, you may have no choice but to use a higher ISO to allow you to capture a correctly exposed picture, but be aware that this comes at the price of increased noise (and hence decreased image quality). If you are not happy with the amount of noise in your pictures, consider moving up to a DSLR that has a low noise high ISO capability.

Common Complaints

Some of the most common complaints we hear from beginners are:

  • Underexposure
  • Overexposure
  • Blurred Images

We already covered these above, but let's recap.

If your pictures are consistently coming out dark, then you are underexposing them. Remember, that setting your camera on Auto mode does not guarantee a correctly exposed picture if you ignore the warning the camera gives you when there's just not enough light. You need to increase the light source by using the on-camera flash, or an alternate light source, or move your subject to someplace brighter. Otherwise, an underexposed shot will result.

When there's too much light, and you cannot close the aperture enough and/or use a faster shutter speed, you end up with an overexposed image.

A blurred image results when you are either using a shutter speed that is too slow to avoid camera shake or too slow to 'freeze' motion. Use a faster shutter speed and adjust the aperture accordingly. The rule of thumb to avoiding camera shake while hand holding your camera is to use the reciprocal of the 35mm equivalent focal length in use. Say you zoom to 125mm (35mm equiv.), then use a shutter speed of 1/125 sec. or faster to avoid camera shake. To 'freeze' motion, a shutter speed faster than 1/60 sec. is usually necessary depending on the type and speed of the motion.

Exposure Bracketing

Sometimes, on one picture, you might have some areas of underexposure (usually in the shadows) and some areas of overexposure (we call them 'highlights'). If you meter for the shadows, the highlights will be 'blown' -- i.e. way overexposed. Conversely, if you meter for the highlights, the shadows will be way underexposed and you'll lose all details there.

One solution is to use exposure bracketing. Take one picture metering for the shadows; without moving the camera (hance the use of a tripod is mandatory here), take a second picture metering for the highlights; then, take a third picture with an average metering. Load all three images into your favorite image editing software and overlay one on top of another. By carefully removing the portions of the image that are not properly exposed (or, if you prefer, by carefully combining the portions that are correctly exposed), you end up with one image with correct exposure throughout.

 

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