With the advent of digital cameras people don’t understand exposure so well any more. It’s not like in the early SLR days where the cameras didn’t have built in light meters. You had to use a hand held light meter or, lacking that, just guess. There was no way you could get along without knowing ISO (ASA at the time), shutter speeds and f/stops. But that’s all changed now and probably not for the better, at least if you want to take photographs that make an impact.
Exposure is one of the fundamentals that you have to get right or, no matter how well you’ve done everything else, you won’t have an image. Yes, with RAW files we have some leeway and our sensors have increasingly impressive dynamic ranges. But a bad exposure will ruin an otherwise great photograph.
So where to start in mastering exposure? Well, let’s start in Norway and Italy. What? Yes, Norway and Italy. Put a Norwegian and an Italian on the beach right next to each other and see which one gets sunburned first. The Norwegian may start to burn in just an hour because of his fair skin while the Italian may take four hours to get the same level of sunburn because of the Italian’s olive complexion. Our Italian friend requires four times as much sunlight as our Norwegian friend to get the same amount of sunburn. To look at this the other way, fair skinned Norwegians are four times more sensitive to sunlight than Italians with their olive skin. (Please don’t take these numbers as medical fact because they’re not. I’m pulling them out of the air to illustrate a point.)
We’re going to see very soon that mastering exposure come down to a very simple concept – we use multiple of two (2) when determining the amount of light needed to make a correct exposure. Everything that we’ll be talking about comes down to doubling or halving light.
That said, we can control the sensitivity of the sensors in our digital cameras. We can make them very sensitive to light like the Norwegian or much less sensitive to light like the Italian. The level of sensitivity is set by the ISO control. Low numbers mean low sensitivity and high numbers mean high sensitivity. For many cameras the lowest ISO number is 100 or 200. Let’s use 100 as our lowest ISO setting. So if we change the ISO from 100 to 200 we doubled the ISO number and therefore doubled the sensitivity. And by double the sensitivity, we need half the amount of light to get the same exposure.
So to relate this idea to our Norwegian and Italian friends, the Italian could be said to have a skin ISO of 100 and the Norwegian, 400. This is because it took the Italian 4 times as long (4 times as much light) as the Norwegian to start to burn. Or, the Norwegian was four times more sensitive to light as the Italian – hence ISO 100 for the Italian and 400 for the Norwegian.
But there’s another aspect to exposure that we need to introduce at this point. Let’s move our Norwegian and Italian friends from the beach at sea level and put them on the shore of a lake in Switzerland at an elevation of 10,000 feet. Now, at 10,000 feet half the oxygen in the atmosphere is below you and the same goes for the sunlight blocking effect. In other words, sunlight is two times as bright at 10,000 feet as it is a sea level.
So our Norwegian will now get a sunburn in half an hour instead of one and our Italian will get the same amount of sunburn in two hours instead of four. The important thing to note here is when the intensity of the light doubles the length of time to get burned is halved – again, factors of two. And the take away for photography is, given the same ISO, we need to adjust the amount of light falling on the sensor to balance the intensity of the light in our scene, Basically, the sensor has an optimum amount of light necessary to give a correct exposure and that optimum amount of light is determined by the ISO.
By the way, this doubling or halving of the amount of light is called a “stop.’ In other words, if we double the amount of light we increase it by 1 stop. If we halve the amount of light we decrease it by 1 stop. The same goes with sensitivity or ISO. Doubling the ISO from 100 to 200 increases the sensitivity by 1 stop which then decreases the amount of light needed for a correct exposure by 1 spot. Double the ISO again (now 400) and we’ve increased the sensitivity by another stop for a total of two stops (two doublings). In fact, our Norwegian is 2 stops (4 times) more sensitive to light than our Italian. And at 10,000 feet the light is 1 stop more intense than at sea level.
So where are we? We can control the sensitivity of our camera’s sensor with the ISO setting. And we need to be aware of the amount of light in the scene so that we can provide the sensor with just the right amount to make a proper exposure. That’s what the camera’s light meter attempts to do – measure the amount of light in the scene, take into account the ISO setting of the sensor and set the shutter speed and f/stop to deliver the correct amount of light.
So that brings us to two more variables we can control – shutter speed (the duration of the light passing through the lens) and f/stop (the intensity of the light). Let’s look at shutter speed first, keeping in mind that we are still dealing with factors of 2. Shutter speeds are measured in seconds or fractions thereof with the longest being 30 sec. and the shortest as short as 1/4000 sec. Now, you probably think that it’s pretty obvious that if I change the shutter speed from a half a second to a full second I just doubled the amount of light passing through the lens. And that’s what we do with shutter speed – increase or decrease the amount of light by increasing or decreasing the length of time the shutter is open. Now here are the landmark shutter speeds that we need to keep in mind, realizing that each varies by 1 stop. Beginning at 1/1000 and working down, the shutter speeds are
1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, 1/4, 1/2 and 1.
By the way, the corresponding numbers you’ll see on your camera are
1000, 500, 250, 125, 60, 30, 15, 8, 4, 0”5, 1”
We should note here that most digital cameras allow you to adjust ISO, shutter speeds and f/stops by fractions of a stop – usually 1/3rd stop but sometimes 1/2 stop. So between each ISO number, shutter speed or f/stop mentioned here there are two additional settings. But the ones we’re talking about here are all one-stop differences and that’s the important thing to understand.
So that leaves f/stop. Changing the f/stop is like changing the intensity of the light passing through the lens. Shutter speed changes the duration and f/stop changes the intensity. In our example, changing the f/stop is like our friends putting on sun screen. If a coating of sunscreen doubles the length of time it takes to get a sunburn, then in photographic terms that’s like decreasing the intensity of the light by 1 stop. If we use a stronger sun screen and it quadruples the length of time it takes to get a burn the intensity of the light was decreased by 2 stops. And now, back to f/stop.
So far ISO and shutter speed make sense; they’re logical. Double the ISO and you double the sensitivity. Double the shutter speed (the length of time the shutter is open) and you double the mount of light. But f/stop appears at first to be totally illogical. Well, it’s only logical if you have a flair for mathematics. It turns out f/stop is defined as the focal length of the lens divided by the diameter of the opening of the aperture And the aperture is that adjustable ‘pupil’ in the leans that can be opened and closed, just like the pupil of your eyes, to increase or decrease the intensity of the light. The result of this definition is that it makes f/stops appear to work backwards. In other words, an f/1.4 admits more light through the lens (has a wider opening) than f/16 – yea, backwards. The other thing that makes f/stops difficult to master is the fact that, even though we’re still changing the amount of light by a factors of 2, the numbers don’t reflect that. For example, when shooting at f/1.4, to cut the amount of light in half means you would select an aperture of f/2, not f/2.8. So the sequence of f/stops from the smallest aperture to the largest at 1 stop intervals looks something like this…
f/22, f/16, f/11, f/8, f/5.6, f/4.0, f/2.8, f/2.0, f/1.4 and f/1.0
Is it important to know these numbers? Yes, I think so. But they’re not as hard to learn as you might expect. If you look at the numbers you’ll see a pattern. So, just remember two adjacent numbers like 1.0 and 1.4 and you can go from there. Do you see the pattern?
One more thing to note about f/stops is that your lenses all have a maximum aperture. Many lenses have a maximum aperture of around f/4.0 or f/5.6. But some more expensive lenses have a maximum apertures of f/2.8 or even f/2.0, f/1.4 or in a few cases f/1.0. Lenses with larger maximum apertures are referred to as ‘fast’ lenses because they work better in low light situation. But fast lenses are more expensive and a lot heaver because they require a lot more glass.
Putting it all together we start with the brightness of the scene we’re photographing. Next we set the ISO to select the sensitivity of our camera’s sensor. At that point the camera’s light meter tries to determine the correct shutter speed and f/stop. If you have your camera’s mode set to aperture priority (Av) then you specify the f/stop and the camera selects the shutter speed. If the camera’s mode is shutter priority (Tv) then you specify the shutter speed and the camera selects the f/stop.
Most landscape photographers shoot aperture priority (Av) at the lowest possible ISO. Why? Because the low ISO gives the highest quality image and the f/stop controls depth of field. But there are times when shooting in shutter priority or even manual makes mode makes the most sense so you need to be familiar with all the modes and know when and how to use them.
I hope this ‘sunburn’ analogy helps explain the concepts of exposure in terms that we’ve all been painfully familiar with at one time or another.
I hope you’re finding good light.
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Right here is the type of information I have been in search of. Thanks for posting this information.
Regene, I’m delighted you find this helpful. There’s a lot that goes into taking a fine photograph and it takes a while to figure it all out. I’m happy to share what I’ve stumbled across and even happier when others fine it useful.