A student will ask “How do I pick exposure settings?”
The question comes up during almost every class, whether it is milky way photography, bird photography, landscape photography or even a destination workshop. They don’t mean just choosing some “proper” exposure. Instead, they are asking how to pick one of the numerous correct equivalent exposure from the huge number of options. Even experienced photographers can struggle with this. The answer boils down to intention and re-framing how you think about the 3 settings that affect exposure: aperture, shutter speed and ISO.
Exposure – Just 3 Controls
When solving a problem, I try to simplify it, removing unneeded complexity. Often, I re-frame the issue to simplify it.
I usually say something like: “Ok, lets talk about cameras. See this?” I hold up my Nikon D810. “It has exactly three controls for determining exposure: shutter speed, aperture and ISO. Everything else, all these other buttons and menu options, are like cruise control and AC on your car. They are nice, but they are NOT a fundamental part of a camera. We can ignore them.”
Cameras have exactly three controls that determine exposure: shutter speed, aperture and ISO. Everything else is like cruise control on a car, nice, but not required.
The problem is already simpler. Cameras only have 3 controls that matter when you are talking about how bright an image is. And because each of the three controls makes the image brighter or darker, you can achieve the same exposure with different combinations.
There is no “right” combination if all you look at is how bright the photo is. ISO 100, 1/100s, f/8 is the same brightness as ISO 1600, 1/6400s, f/4. However, those 2 equivalent exposures are not at all the same in terms of the resulting image. To know what is “right” you have to consider your intent for the picture.
To know what the “right” combination of settings are, you have to consider the intent of your picture.
Re-Framing: Side Effects and Artistic Choices
We need to re-frame what the 3 exposure settings do.
Each of the three exposure settings: shutter speed, aperture and ISO, has a side effect. Choosing between the side effects is one of the places where art happens in photography. While equivalent exposures produce the same brightness, different equivalent exposures can produce very different looking results.
Lets start by talking about the side effects of each of the three controls.
ISO controls how sensitive your sensor is to light. You can think of it as a volume nob. In fact, this isn’t far from the underlying truth for digital sensors. Sensors have only 1 sensitivity. Higher ISOs are achieved by turning up the volume. When you turn the volume up, you amplify the noise as well.
ISO controls digital noise.
Digital noise is the only one of the three side effects that isn’t normally useful. In fact, as a rule we want to minimize digital noise. I personally would never shoot at super high ISO just to add noise to a photo. I can easily add it digitally after the fact. I do however shoot at high ISO when I’m forced too by choices in the other 2 settings.
Aperture controls how large the hole in the lens is. This affects how bright the photo is. A bigger hole in the lens lets in more light. It’s side effect is to control the depth of field.
Think of aperture as depth of field control.
Controlling the depth of field is highly useful to a photographer. Some situations call for a shallow depth of field (pretty blurry backgrounds in wedding photos), others call for a very deep depth of field (think Ansel Adams landscape photos).
In general, the smaller the aperture (bigger f-number = smaller aperture, less light), the more depth of field you have. The larger the aperture (smaller f-number = bigger aperture, more light), the less depth of field you have.
The key for me, is to think of aperture as depth of field control. If you care most about depth of field, you will pick aperture as the anchor for your exposure.
Shutter speed controls how long the sensor is exposed to the world. Longer shutter speeds let in more light and result in a brighter exposure. Shutter speed also controls how much motion blur you capture. A longer exposure will show more motion blur.
Shutter speed is motion blur control.
1/100s is some time, and it is a lot more time than 1/1000s and a lot less than 1/10s. 1/10s will show a lot more motion blur than 1/100s, which will show more blur than 1/1000s. Sometimes you want to show motion blur and other times you want to freeze the action.
Consider bird photography. Conventional wisdom says that you want to freeze the bird. Many birds move fast, especially their wing tips. To freeze them, you need a high shutter speed (1/1000s or faster normally). If your goal is to capture a tack sharp image of a bird in flight with no motion blur, you need to freeze action, and that means a high shutter speed.
On the other hand, if you want to do something else besides freeze the bird, a slower shutter speed can be desirable.
Regardless, your main goal is to control motion blur. You will choose shutter speed as the anchor for your exposure.
Choose Exposure Settings
The ultimate question is, if there are many different settings that will produce the “right” exposure, how do I pick? For me, I start with my artistic intent. What am I trying to show the viewer?
If that sounds like a BIG question, you’re right. It is. But, to keep it simple, lets consider a couple of real world scenarios.
A Wedding First Dance
The sun has set, the ceremony is over. The couple is ushered into the reception by the applause and cheers from their friends and family. They go right into their first dance. Behind them there are vintage lights hanging from the ceiling, and tables full of guests. The room is dim but there is a beautiful glow.
How do I decide what settings to use?
For this sort of photo, I want to isolate the couple. The background, in particular the guests who are flicking through facebook, the beer cans on the tables, and the caterer’s staff busing tables are all distractions from the real story: the couple’s first dance as husband and wife.
In this situation, I go straight for shallow depth of field. With a shallow depth of field the background is obscured. Since depth of field is my primary concern, I choose it FIRST by setting my aperture to a bright setting (something like f/2). The other settings I end up with are not that important provided they are “sane”.
Because the room is dim and I still want a reasonable shutter speed (it’s my secondary concern, say 1/250s) to freeze action, I will end up with a high ISO.
My settings for the above shot were ISO 400, 1/250s at f/2.0.
Automotive Panning Shot
I’m on the pit wall at a race, photographing race cars. I want to show the speed to the cars in a visceral way. What I care about is motion blur in the background while panning with the car.
In this type of scenario, I start with my shutter speed. What constitutes “slow” for a given subject depends. 1/1000s is really slow for a hummingbird, but plenty fast for a running human. For panning shots of cars, there are a lot of factors. To slow a shutter speed and you risk everything being blurry. To fast, and the background won’t be blurred out enough. I usually start at 1/100s and see what it looks like. It’s digital after all, so I can check and revise as needed.
Whatever shutter speed I end up at, I will set my aperture and ISO to achieve the right exposure (or let the camera handle them). If I’m outside on a bright day, I will probably need a low ISO (= low noise, that’s good!) and a small aperture ( = high depth of field, e.g. f/16).
For this photo, on a dim overcast day, I ended up with ISO400, 1/40s at f/6.3. I chose the aperture and the other setting were a result of that choice.
Freezing a Bird In Flight
When I’m photographing birds in flight, sometimes I want to freeze there motion as I did with this hunting northern harrier. I care most about motion blur. Specifically, I want to eliminate all motion blur.
To do that, I care most about shutter speed (motion blur control). For a bird like this 1/1000s is a safe minimum shutter speed. How do I know? Experience and rules of thumb… If you aren’t sure, but want to freeze motion, go as high as practical (i.e. that doesn’t push your ISO to high).
With the shutter speed locked in, the aperture and ISO can “float” to what was required to get the right exposure. Because I had bright light, I chose a low ISO and let the aperture open up producing a shallow depth of field. The depth of field wasn’t intentional, but a side effect of the shutter speed and ISO I used.
My setting for this photo were ISO360 (auto ISO), 1/1000s, f/5.6. I chose 1/1000s to freeze motion and let the camera manage the other 2 settings.
So far I’ve talked about exposure in manual terms as if I always shoot in manual and do everything myself. While there are time I do that, most of the time I let the camera handle the details. That’s right, I use automatic modes…
Remember all those other controls that I equated to cruise control and AC? Cruise control and AC are really nice when you are rolling down the highway.
Most cameras have at least 4 exposure modes, that is, ways the camera sets exposure: Manual (M), Aperture Priority (A), Shutter Speed Priority (S) and Program (P). Each mod allows the photographer and camera different amounts of control. Each is a good choice in certain situations.
Program mode is the original full auto. Frankly, I don’t use it. That doesn’t mean you can’t. However, it is full auto so you don’t get to pick any settings (or, if you want to you have to “shift” the program around, etc). For me, it’s to complicated and just makes life hard.
I always want to make at least one choice, aperture or shutter speed intentionally so I don’t use program mode.
Aperture priority is an automatic mode where you set the aperture and the camera manages the shutter speed (and maybe the ISO).
Right off I’m going to tell you to rename “Aperture Priority” in your head. Call it “Depth of Field Priority” because that is how it should be used.
Aperture Priority = Depth of Field Priority
In aperture priority, you set the aperture (depth of field) and the camera worries about “everything else” to get the right exposure.
When you enter a situation where depth of field is your primary concern, switching to the “A” mode (aperture priority, NOT full auto), is a solid choice.
Shutter priority is an automatic mode where you set the shutter speed and the camera manages the aperture (and maybe the ISO).
Think of shutter priority as motion blur priority. Now the name tells you when to use it. If your primary artistic concern is showing a specific amount of motion blur (freezing the image or blurring it out), then shutter priority is a solid choice. After all, it lets you set the amount of motion blur. The camera handles the other settings for you.
Manual mode gives you complete control. The camera’s tiny brain won’t try to “fix” your exposure for you. You set everything.
Most situations, in particular, quickly changing situations, call for automation. In those I use Aperture Priority or Shutter Priority as the situation calls for. However, in some situations, the camera’s brain just gets it wrong, or isn’t consistent enough. Other times, the situation is not changing very fast (think landscape or studio photography).
In these situations, I fall back to manual control. In manual mode, you are in charge. You can make a totally black photos, or a totally white photo and anything in between.
ISO and Auto ISO
In the film days, changing ISO required changing film so it was handled separate from the 2 other settings. Film cameras only really had 2 settings you could easily change from shot to shot: aperture and shutter speed.
That isn’t true in the digital world. However, some of that legacy of “hard to change” has stuck with us in the digital world. The automatic modes (aperture priority, shutter priority, program mode) do not necessarily adjust the ISO.
In Aperture priority you set the aperture and the camera adjusts the shutter speed to achieve the right exposure. In shutter priority it is the other way around, you set shutter and the camera handles aperture. Unless you enable auto ISO, the camera doesn’t change ISO at all.
Most of the time there is no reason not to allow the camera to change the ISO from shot to shot. Enter Auto ISO.
Auto ISO in S and A Mode
Auto ISO varies from camera to camera. But, in general, when you enable it in an automatic mode, the camera will try to get to correct exposure using the available setting (aperture or priority) first. If it can, ISO stays at base or whatever you had it set to.
If it can’t get the right exposure (for example in shutter priority, the aperture is all the way open but the image is still to dark), the camera will raise the ISO until you reach the proper exposure.
In general, cameras try to keep the ISO (noise) as low as they can and get the right exposure. That is usually the proper strategy and it works well in a lot of situations. Auto ISO plus shutter or aperture priority can be a life saver in quickly changing situations like bird photography.
Auto ISO in Manual
Auto ISO in manual mode is a different story, and unfortunately it varies from camera to camera. For Nikon and many others, if you enable Auto ISO in manual mode, you are no longer in a manual mode. Instead you set Aperture and Shutter speed and the camera adjust the ISO to get the right exposure. The camera “fixes” the exposure using ISO.
This can be frustrating if you don’t realize what is going on. Say you try to make the exposure darker by speeding the shutter speed up and leaving the aperture the same. But, when you check the photo, it is just as bright. Auto ISO “fixed” it for you.
To have true manual mode, you have to disable auto ISO.
He is a self taught experiential learner who is addicted to the possibilities that new (to him) gear open up. He loves to share the things he has worked out. Andrew started with a passion for landscape and night photography and quickly branched out to work in just about every form of photography. He is an ex-software developer with extensive experience in the IT realm.