Basics of exposure on dSLR cameras

 

Upgrading from a point-and-shoot camera to a digital SLR opens up a lot of creative avenues, but it also requires you to gain new technical skills, key among which is knowledge of exposure controls. You didn’t spend thousands of dollars on new equipment only to leave it in auto mode, so I hope you will walk away from this article with the confidence to always set exposure controls manually.

In the simplest sense, a camera is nothing more than a box that captures light at a certain moment in time, and this act of capturing light depends on three things:

- Time
- Aperture
- Sensor sensitivity

Time

Setting the exposure time depends on the amount of light available, and on the focal length that you’ve set your lens to. Without thinking of focal length from a physics perspective, a higher number means more zoom, unless you’re talking about macro lenses, but I’ll explain that in a separate article.

If you set the exposure time to, say, 1 second, you’re guaranteed to get a disappointingly blurred photo, unless you’re using a tripod. It’s simply not possible to hand-hold a camera for that long without causing natural tremors in your hands from being recorded on the sensor. To avoid this problem, the rule of thumb that you need to follow is:

Time < 1/(focal length) seconds

So, for example, if you’re using a 50mm lens, you can use time = 1/50s, or 1/100s, or 1/500s, etc. But if you set it to 1/40s, you’re likely to get a blurred photo. If you’re using a full-frame sensor camera such as the Canon 5D Mark II, this is the rule of thumb you need to keep in mind. However, most cameras on the market today use what’s called a cropped sensor, which is smaller than a full-frame sensor.

In these cropped-sensor cameras, effective focal length = crop factor x focal length reading on the lens.

For Canon cropped sensor cameras such as the Canon 7D, or T3i or the Rebel line, the crop factor is 1.6, so if your lens says 50mm, it’s actually 50 x 1.6 = 80mm. In this case, the rule of thumb requires you to choose a time faster than 1/80s, so 1/100 is ok, but 1/70 is not.

If you’re using a tripod, you can choose as long a time as required.

The above rule of thumb doesn’t apply to macro lenses because the higher magnification factor also accentuates your hand’s tremor. In this case, you need to ensure that

Time < 1/ [ (focal length) x (magnification factor + 1)^2 ] seconds

Again, if you’re using a cropped sensor camera, effective focal length = crop factor x focal length.

Macro lenses are usually prime lenses, in which the focal length is fixed. For example, Canon makes an excellent 100mm macro lens, among others. If you’re using this lens at 1:1 magnification factor, you must ensure that

Time < 1/ [100 x (1+1)^2] seconds

i.e., time < 1/400s

Notice that if this had been a regular 100mm lens, your limitation would have been time < 1/100s. That’s quite a large difference between a regular lens, and a high magnification macro lens!

With newer lenses, there’s one more thing to take into consideration: image stabilization, or vibration compensation. With this technology, tiny gyrometers in the lens detect the velocity of your hand’s tremor, causing tiny motors to move the lens in the opposite `direction to neutralize the camera shake. Pretty neat technology, if you ask me! The result is that you can hand-hold the camera for a longer amount of time than the above rule of thumb suggests. Once we have discussed the concept of stops of light below, I’ll tell you what your image stabilizer lens can allow you as far as exposure time is concerned.

Stops of light

The word “stop” refers to either the halving or the doubling of light, and we’ll tie the above points together using the concept of stops of light.

Aperture

You probably studied the physics of pinhole cameras back in elementary school; in that class you learnt that a tiny hole was made in front of a box, inside which was a sheet of light-sensitive paper. The image was sharp as long as light had only one precisely tiny hole to pass through. If this hole were any bigger, many rays of light would enter the camera, blurring the image. Modern cameras work on the same principle: the wider the lens opening, the more light flows in, and the smaller the opening, the less light you allow to hit the sensor. This opening is called the aperture of the lens, which is defined as focal length divided by the opening diameter.

Aperture is written out in f-stops, and you’ll hear people refer to f/4, f/5.6, f/11, and and so on. The slash / tells you that, for example, f/4 is actually 1/4; similarly, f/11 is 1/11. Since 1/11 is a smaller number than 1/4, setting your camera to f/11 makes the lens opening smaller than the case where you select f/4. As in the pinhole camera, the smaller the lens opening, the more in-focus all elements of the scene will be. So, for example, if you’re in the Rocky Mountains and you would like everything to be in focus, you would select f/11 or f/13, rather than f/4. On the other hand, if you wanted to selectively focus on a flower, throwing the background out of focus, you would make the lens opening larger. In other words, you would use f/8 or f/5.6 instead of f/13. A photo taken with a large aperture, such as f/4, is said to have a shallow depth of field; one taken with a small aperture, such as f/13, is said to have a large depth of field.

Setting the aperture, then, is equally about the creative look that you are aiming for as it is about allowing the necessary amount of light to enter the camera. In the photos below, the lens was manually focused on the chick’s eyes.

f/2.8

f/4

f/5.6

f/8

f/11

f/16

Modern cameras and lenses can be stopped down to f/32 or smaller, and it seems from the above photos that if you want everything to be in sharp focus, you should make the aperture as small as possible (which means, the number after f/ as large as possible), but because of an optical effect known as diffraction, the overall picture sharpness decreases once you’ve moved past what’s called the diffraction limited aperture, which is approximately f/10 on the full-frame Canon 5D Mark II, and f/7 on the cropped-sensor Canon 7D. Further discussion of this topic will have to be deferred to another article.

Changing the aperture from f/1 to f/1.4 halves the amount of light entering the camera. We just defined one stop as the halving or doubling of light, so going from f/1 to f/1.4, we have reduced the light entering the camera by one stop. Going from f/1.4 to f/2 is again one stop. In other words, going from f/1 to f/2 has resulted in two stops of light reduction. Likewise, going from f/2 to f/1 will result in two stops of light increase. The illustration below may help you understand this better.

Notice that the bigger the number after f/, the smaller the lens opening, and making the lens opening smaller must necessarily reduce the amount of light entering the camera.

Adjacent entries below differ from each other by one stop of light:

f/1 f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22 f/32

What the above line is telling you is that if, for example, you change your camera’s aperture setting from f/11 to f/8, you have doubled the amount of light hitting the sensor. Likewise, going from f/11 to f/5.6 is equivalent to 2×2 = 4 times increase in light, which is 2 stops of light. This is because between each adjacent aperture, the area of the circle has doubled or halved.

All modern dSLR cameras allow you to alter apertures in 1/3 stop increments or decrements, so you’ll see many more f-stop settings than what I have written out above.

Sensor sensitivity

You can vary your camera sensor’s sensitivity to light by changing the ISO setting; in the old days, this was called the speed of the film that you had loaded. As with aperture, we’ll discuss this in terms of stops of light.

Going from ISO 100 to ISO 200 is one stop of light: you’re making the sensor more sensitive to light, and if you recall the definition of a stop of light, you now know that going from ISO 100 to ISO 200 doubles the amount of light captured by the sensor. As with the table in the aperture section, adjacent entries differ from each other by one stop of light:

50 100 200 400 800 1600 3200

Your camera will allow you to increment in 1/3 stops of light in ISO settings as well, but for now, it’s more important that you understand full stops of light, and not worry too much about 1/3 or 2/3 stops.

One crucial thing to remember is that the higher the ISO, the more noise your photo will have, so you must try to keep ISO as low as possible, preferably 100 or 200.

Measuring light

Your camera has a built-in light meter that measures how much light it detects coming in. This system is called TTL, or through-the-lens, metering. On Canon dSLRs, you’ll see on the rear LCD numbers marked -2 to +2, or -3 to +3 on newer models. You’ll see the same information when you’re looking through the viewfinder. If you half-press the shutter release button, you’ll see a line appear below one of the numbers, as in the following photos:

In the photo on the left, the line is under the 0 mark; in the other case, it’s under the -1 mark. In the 0 case, the light meter is telling you that it thinks that the photo is properly exposed; in the -1 case, the meter is telling you that it thinks that the photo is underexposed by 1 stop. Note that I said, “it’s telling you that it thinks the photo is properly exposed”. The reason is that your camera’s light meter is programmed to calculate exposures only for medium gray, which is the tone of the background color of this site. So unless your photo is of a medium gray sky, or of a medium gray tone wall, the light meter is wrong! It’s a good point to start at, but it’s going to need a bit of tweaking to expose the scene properly. Note that I’ve mentioned medium gray tone, not the color medium gray; between black and white is gray, which is both a tone and a color. But between dark maroon and bright red is “medium gray” tone red. It’s this tone that the meter is programmed for.

More specifically, if you’re taking a picture with a lot of white in it – the day after a snow storm, for example – if you take a photo with the meter saying 0, you will end up with an underexposed image in which the snow will be gray instead of white! Likewise, if you take a photo of a dark subject such as a forest with the meter reading 0, you will end up with an overexposed image.

The rule of thumb to remember is: if your subject is very bright, overexpose by 1 stop; i.e., make your meter read +1, not 0. If your subject is very dark, underexpose by 1 stop; i.e., make your meter ready -1, not 0. Other subjects will require a bit more tweaking, and may require a -1/3 or +2/3 meter reading, for example.

Tying it all together

Earlier, I mentioned that capturing light depends on three things:

- Time
- Aperture
- Sensor sensitivity

We’re now ready to tie these together using the concept of stops of light.

If the camera’s light meter shows you 0 for the settings 1/100s at f/11 and ISO 200, but you’re using a 400mm lens, you know that you’ve got a problem. The basic rule of thumb says that you must keep your exposure time at 1/400s or faster (1/500, 1/600 etc.). However, let’s also assume that you’re using the Canon 100-400mm IS lens, which has a built-in image stabilizer. This lens will give you approximately 3 stops of light advantage over a non-IS lens. Recall that one stop of light is the halving or doubling of light, so if our rule of thumb requires us to be no slower than 1/400s, this lens will now allow us to relax this requirement by 3 stops of light. Let’s use 2 stops instead, to be on the safe side; Canon’s made a great product, but we also want to be conservative to have a high probability of getting a sharp hand-held photo. Changing 1/400s to 1/200s is 1 stop of light: I’ve doubled the amount of time, and therefore, the amount of light captured by the sensor. Going from 1/200s to 1/100s is another stop of light. In this case, we know that 1/100s is guaranteed to get us a sharp photo at the settings that we’ve set.

To make sure you fully understand what our IS lens allows us to do, and doesn’t allow us to do, let’s consider another case: our camera currently shows 0 on the light meter for the settings 1/50s at f/11 and ISO 200, at the same focal length of 400mm. Even with IS on, we’re in trouble. In the previous paragraph, we saw that conservative calculations with IS will allow us to use 1/100s instead of the rule of thumb 1/400s, but we’ll likely get a blurred photo at 1/50s. What can we do?

There’re two options: change the aperture, or change the ISO. If you increase the ISO, your problem will be solved, but at the expense of increased noise in the picture. If, on the other hand, you open up the lens a little more by changing the aperture, the creative aspect of the picture will change: we learnt in the section on aperture that the smaller the aperture, the greater the section of the picture that will be in sharp focus.

In our current example, if you change ISO from 200 to 400, you have increased the exposure by 1 stop, and the light meter will now read +1 instead of 0, which is where we need to be. To bring the meter back to 0, we can change the time from 1/50s to 1/100s, which is also 1 stop.

Alternately, we could’ve kept our ISO at 200 to ensure that we don’t get a noisy picture, and instead changed the aperture from f/11 to f/8, based on the table in the aperture section. At this point, the light meter is again reading +1; to bring it back to 0, we can change the time from 1/50s to 1/100s, which is within the range of the IS in this example.

Technically correct exposure vs. creatively correct exposure

Based on the above discussion, the following settings will all show the same light meter reading:

1/100s, f/11, ISO 200
1/100s, f/16, ISO 400
1/50s, f/11, ISO 100

What is important, then, is to set the correct creative exposure for your picture. Here’s the order in which you should proceed:

- Set ISO first; your aim is to keep this as low as possible to keep the noise low. For general photography, I set this to 200, but whenever I have a tripod with me, I lower this to 50 or 100.
- Next, set the aperture based on what creative effect you’re looking for. For most landscape photos, you’ll automatically set this to f/13 or f/16, but other subjects you’ll need a different aperture.
- Finally, set your exposure time to bring the light meter to 0, or some other value, as discussed in the “measuring light” section above.

Manual vs. semi-manual mode

Your camera has at least four settings in which you have varying amounts of manual control:

- M: this is the fully manual mode, in which you can adjust time, aperture, and ISO. Most cameras will allow a maximum of 30 seconds in this mode.

- B: this mode is the same as M, but the shutter will remain open for as long as you’ve got the shutter release button pressed, helping to overcome the 30 second limitation in M. How do you calculate time in this case? The camera won’t show you a time above 30 seconds, so here’s an example: if, at 20 seconds, f/13, ISO 100, the light meter reads -2, you know that you’re 2 stops underexposed. To bring the meter to -1, you’ll double the exposure time, so you’re now at 40 seconds. To move from -1 to 0, you again need to double the exposure time, so what you need is 80 seconds, f/13, ISO 100. Remember to bring a stopwatch to count accurately! A word of caution: my Canon 5D Mark II’s light meter shows me readings from -2 to +2. If my settings will cause the photo to be exactly 2 stops underexposed, I’ll see a solid marker under -2, but if my settings will cause the photo to be, for example, 3 stops underexposed, I’ll see a blinking marker under -2. In this case, it’s not possible to figure out the time required to bring the marker to 0 without first changing ISO or aperture as well.

 

For example, if, at 30 seconds, f/11, ISO 50, the light meter is blinking at -2, I can try changing the aperture to f/8. If this causes the blinking bar under -2 to become solid, I know that this means that if 30 seconds, f/8, ISO 50 is 2 stops underexposed, then so is 60 seconds, f/11, ISO 50. Now, I can easily multiple 60 by 4, and trip the shutter for 240 seconds at f/11 and ISO 50 to get an exposure that the camera’s light meter calculated as being at 0.

- Av: the aperture priority mode lets you select the desired aperture and ISO, and then automatically sets the time so that the meter reads 0.
- Tv: the shutter priority mode lets you select the desired exposure time and ISO, and then automatically sets the aperture for you to bring the meter reading to 0.

I always use M or B to have complete control of the picture.

ID: basic exposure