Today we'll be walking through the concept of exposure, right from the beginning. This article won't befuddle you with complicated numbers and jargon, but it will help you feel more confident with your photography and understand a core concept that's a vital piece of the photography jigsaw.
Every few weeks, we revisit some of our reader's favorite posts from throughout the history of the site. This tutorial was first published in June of 2010.
I believe there are three different types of photographers: technical, artistic, and those that have good artistic vision and combines technical knowledge to achieve that vision. None is right or wrong. All are just different in approach and execution.
The first photographer I ever mentored was an amazingly artistic woman. She had a knack for seeing things like no-one had ever seen. She was also a voracious learner and she asked me for help on the technical side of things to make herself better.
I started spouting all the numbers, all the theories, and all the science. The response? "Whoa, My mind doesn't think that way". And she was right. People who are very artistic think differently, their brains compute things differently than someone that thinks more analytically and scientifically.
I needed to change my teaching methods to fit the way she thinks. So that is what I will do with this article - explain the technology without the math and numbers and theories.
By the way, that student is now one of the top portrait photographers in her state (heck, I think everywhere) and I couldn't be more proud of her. She inspires me every day.
The Triangle of Exposure
There are three main ingredients to good exposure: Aperture, Shutter Speed, and ISO. Okay, yes I said no math, but I will use a bit of geometry to illustrate the effect that the three components of exposure have on each other. I am certainly not the first to think of this as a triangle but I do think it is the best representation
Think of a perfect exposure as a perfect triangle - all the angles are equal, all the sides are equal. Now if you change just one part of that exposure or triangle, it is no longer perfect so you will need to change another point of the exposure or triangle an equal but opposite amount to make that triangle and therefore the exposure perfect again
As you can see, all the elements of exposure have an effect on the others - so with that in mind we need to know the hows and whys of all the different elements to best understand how to get both a good exposure and the desired results in our photographs.
Now you may ask; "Why do we need to have all different settings for exposure. Why isn't there just one?" Well, in the last century on almost all of the point and shoot cameras, that's the way it was. The aperture was fixed, as was the shutter speed and - even though you could buy film that was different ISOs - there was usually only one recommended for that particular camera. But it was very limiting.
Since those camera were set up for an average scene, you either shot a normal daylight scene or (if you popped on a flash) an indoor scene. Forget about shooting a natural light sunset or night shot. Forget about stopping action of a race car. You were stuck with what you had.
Now we want to be more artistic in our photos and we want more control over what we shoot. So to achieve that artistic and technical control, we need to know about the different settings we can use and why we use them.
So let's start with Aperture
Aperture is a circular opening (somewhat) in our lens that is adjustable from a very small circle to almost as large as the lens itself. We adjust it to let more or less light hit the digital sensor or film. Think of window blinds as your aperture, and the wall in your room opposite the blinds is your sensor or film. As we open the blinds, more light comes through and we can see the wall behind us get brighter and brighter
Likewise, as we open up the aperture on our lens, we get more light on our sensor or film.
The opening of our lens or Aperture is expressed in f stops and here is a very typical range of f stops:
You may say; now wait a minute, why do the largest openings have the smallest numbers? Well, think of it this way; imagine the number as the bottom number of a fraction. So if we have f 4 that would be 1/4 and f8 would be 1/8 and 1/4 is larger than 1/8. Okay? Okay.
The Aperture numbers I have above represent "Whole Steps" of light from one to the next. What is a whole step? A whole step represents the doubling or halving of the light through the lens. So f1.4 will let in twice as much light as f2.0. f2.0 twice as much light as f2.8 or we can also say that f2.8 is half as much light as f2.0.
On your lens, you may see numbers in between the numbers above. Those represent either 1/3 or Ω stops (depending on your camera model) so we can fine tune beyond just whole stops.
So we have all these different apertures available to let in different amounts of light. Why should I care and why should choose one over the other? This is where the artist side comes in - to help make those decisions. The following are different artistic styles we can use and the different parts of an image that Aperture effects.
Depth of Field
When we look at an image, there is a part that is in perfect focus and then there are parts that begin to be out of the range of focus. You can have a small Depth of Field (DOF) where only your subject is in focus, or you can have a deep Depth of Field where practically everything is in focus - or really anywhere in between.
Depth of field is determined by three things; aperture (f stop), distance to your subject, and lens focal length (50mm, 200m etc), with Aperture having a profound effect on DOF. Let's see what an image looks like just varying the aperture and leaving alone the other two parts:
Photographers that shoot portraits usually use larger apertures (low numbers) for a shallow DOF to highlight and isolate their subjects. Landscape photographers usually use small apertures to have a very deep DOF, all the way from the foreground to the background.
As usual, there are exceptions to these rules and that is ruled by the photographer's artistic ideas and vision.
Shooting in Low Light
As we saw with the window blinds example, opening up our aperture allows more light to hit our sensor or film. So when shooting in low light, it may help to open up your aperture to make it easier on our other two sides of the exposure triangle which I will explain more later in those sections on Shutter speed and ISO.
Shutter speed controls how long the light comes through our aperture to our digital sensor or film. The longer the time, the more light will hit their surface. That is technically what shutter speed does.
Artistically, shutter speed controls motion. Whether we want to freeze motion or show motion, shutter speed is the portion of exposure that will control that aspect.
Shutter speeds are expressed in fraction of a second 1/8, 1/125, 1/1000 etc. but in most modern digital cameras you may not see the 1/ expressed (but they still are fraction). You most likely see the above expressed as 8, 125, 1000 etc.
The first consideration we have to think about is: can we hand-hold the camera at this shutter speed and not have the slight movement of the camera show up in our image making it blurry or less sharp? Most people are able to hand hold a camera in the range of 1/60th to 1/200th with a normal lens on. When you use telephoto lenses you may need to use a faster shutter speed.
A common rule of thumb these days is to take your focal length of your lens and shoot at least as fast as that. If you have a 300mm telephoto lens, your minimum shutter speed should be 300 (1/300 or 1/320 as is common) If you cannot hand-hold, you must use a tripod.
Beyond that we can now make an artistic judgment - do we want to stop action, or show movement? And this is a judgment you need to make. Sometimes we may want to freeze our subject and keep it sharp and clear. Other times we want some blur on the subject to give the viewer the impression that the object is moving or is at speed.
In this image of the trolley downtown, in the first example we used a high shutter speed to stop the trolley as it moved past the buildings (which may be what we want). But does the viewer really know if the trolley was moving past or just stopped at the station?
In the second example we slowed the shutter speed way down to .3 (3/10th) of a second. Now the viewer can tell that the trolley is moving at speed past the building.
Again this is the decision you have to make. What do you want to convey? Other examples of using a high shutter speed to stop action:
Stopping the flight of a jet as it flies by, or a car at high speed. Freezing the swing of the bat of a baseball player, a high diver in mid arc above the water. Again you would use a high shutter speed to freeze the movement or action.
In the image below, a slow shutter speed was used (on a tripod) to silken the water and show motion. The same can be done for a waterfall, an ocean or a fountain.
ISO is the sensitivity to light of the sensor or film. The higher the sensitivity, the less time (shutter speed) or the less amount of light (aperture) needs to hit the sensor for the correct exposure. It is adjustable in most cameras, in a range of about 200 to 1600. High end cameras can go way beyond that.
We use ISO to help us achieve what we want to do with the other two sides of exposure; Aperture and Shutter Speed. You may ask; Why don't we just use the most sensitive one and forget about it? Well because the downside of higher ISO is that it increases the noise or grain in our images. This can sometimes make the image look so bad that it becomes unusable (or at least un-printable at a decent size).
So our objective is to use the lowest ISO possible, but balancing that to what we want to achieve.
Shooting outdoors on sunny or even slightly overcast days we can use ISO 100 or 200 with ease. On heavy overcast days we may need to change our ISO to 400. Especially if we use a small aperture (letting in less light) for shooting a deep depth of field landscape shot, while still being able to maintain a shutter speed that we can safely handhold without a tripod.
As we move indoors to a brightly lit room we may need to move up to ISO 800 - 1600 to take natural light photos without the need for using our flash. As we move to dimly lit rooms or street scenes, we may need to move up to ISO 3200 or higher (remembering again that not all cameras can shoot at these higher ISO without excessive noise).
Here are some close-ups crops to see the effects of ISO noise in an Image.
Putting It All Together for the Perfect Exposure
What is a perfect exposure?
Well technically, every scene we shoot has a dynamic range. The dynamic range is the difference between the brightest part of that scene and the darkest part. Our sensor or film has a dynamic range too (technically it's exposure latitude - but we're picking hairs).
What we hope to accomplish is to capture that dynamic range of the scene into our image. So that the brightest part of the image (say the sky) is not blown out and devoid of detail, and the darkest part (shadows or foreground dark areas) are not lost into noise.
Sometimes the dynamic range of a scene can exceed the dynamic range of our camera, so we have to make a choice of what part we want to have the best exposure. Usually with digital it is best to have the brightest areas in good exposure and not blown out. But it can depend on the situation.
If we are shooting a portrait, we want our subject to be perfectly exposed even if that mean that another part of the image my not be. Some times that is a sacrifice we have to make, if we cannot change the conditions of the shoot nor have the option of supplementing the lighting.
This first image is overexposed. There is good light on the rocks but the sky and clouds are missing detail because of overexposure.
This image is underexposed. Great detail in the sky but the foreground is dark and all the detail is lost to noise there in the rocks.
This image is exposed pretty well in a difficult high dynamic range lighting situation. There is great detail and color in the sky and you can make out all the rocks and foreground areas completely.
The truth is, while there may be technically a good exposure, you may use exposure artistically too and purposely blow out areas or choose a darker mood to an image. You can let your artistic eye lead the way when you need to.
The Exposure Triangle in Practice
Now that we have a basic understanding of the three elements of exposure. Let's examine how we might use them and see the interaction between them.
Let's say we wanted to shoot an automobile race and we want to stop the action. It's a bright sunny day so we are going to use ISO100. We want to stop the action of the car going by us very fast, so we choose 1/1000 of a second shutter speed - but with that shutter speed according to our meter in our camera that would give us an aperture of 5.6.
So we know from what we learned about aperture, that may give us a shallow depth of field and there are other race cars we want to be within reasonable focus. So how could we fix this? We can't change our shutter speed, so we turn to the other part of our trio; ISO.
If we move our ISO up two stops to 400, we can then make out aperture two stops smaller and get the depth of field we need plus the shutter speed we need to stop the action.
We still have our camera set the same, but while we are at the races we spot a beautiful classic car in the parking lot. We want to isolate it from the ugly background so we decide that we now need a shallow depth of field. So we open our lens up to f4 on our 200mm lens.
This gives us great separation and DOF but that drives our shutter speed up to 1/6000th of a second! This would be OK - we are hand holding our camera, not stopping any action - but we want this to be a great shot worthy of a very large print.
So why don't we lower our ISO two steps to ISO 100, That brings our shutter speed down two stops to a still fast 1/1600 but because we lowered our ISO we will have much less noise in our image to be printed large.
Remember back in the section on aperture I posted a picture shot at night on the street. I had my aperture at f8. Now I knew that it would be low light so I boosted my ISO to 3200 but I then had a shutter speed of 1/8th of a second - too slow to hand hold. But, I was also without a tripod.
Since the scene was flat and no real depth to it (and I didn't need to worry about DOF), I opened my aperture to f2.8 that allowed me to put my shutter speed at 1/60 at which I could very easily hand hold.
Notice something? For every action we take one way in stops, we make an equal but opposite adjustment in stops on another part of the exposure triangle. When we let more light in one place, we control it in another place to give us the perfect exposure.
Exposure and the three elements of it; Aperture, Shutter Speed and ISO, can be a very complicated and full of science subject. But I hope I have given you some of the basics and in a way that is easy to understand so that you may use your camera more efficiently. Better still, I hope I have allowed you to take your art to the next level because of the technical understanding.
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