The term exposure is a very literal term. What it refers to is really a measurement of how long the sensor (or film in the old days) is exposed to the light entering the camera.
Exposure reveals itself by how appropriately dark or bright an image is. An image image that is too dark is said to be under exposed, an image too bright is over exposed, meaning either too little or too much light was collected by the sensor.
Think of an image as a bucket that collects water. Ideally, we want to get as much light in the bucket as we can to maximize its efficiency at holding water. Too much water and it the bucket won’t hold anymore, too little and it won’t be as useful as it could be. When this is translated to photography, an image that has light “spilling over” will look too white and the information will be lost. Likewise with the empty bucket analogy, if there is not enough light, just like if there is not enough light in your house, everything will appear dark.
If you have to be off with your digital exposure, it is much better to err on the side of being underexposed because an underexposed image has more of an ability to be brightened than an overexposed image has to be darkened. Shooting to the left refers to your exposure meter inside your camera’s viewfinder. There is a little graph, pictured below, that will tell you how much light is coming into your lens. As you point your camera (in manual mode) at different objects, you should see that little graph bounce around from one side to the other – left for darker objects, right for lighter ones.
Exposure is affected by three controllable variables: aperture, shutter speed, and ISO.Aperture, as mentioned in the previous bit on lenses, is the size of the opening in the lens that light can pass through. Consider the bucket analogy. Which would fill up your bucket faster, a garden hose or a fire hose? Clearly the fire hose. In effect, it has a bigger “aperture” – an opening for something to pass through it.
Similarly, if your aperture is bigger, more light will pass through it in a given amount of time. Sometimes lenses with wider apertures are said to have “fast glass” because of the way they affect relative shutter speed. If an aperture lets through twice the amount of light, as you might expect, only half the amount of time is needed to capture the same amount of light – or make the same exposure.
Shutter speed is the amount of time the shutter is opened, allowing light to pass through to the sensor. Generally measured in fractions of a second, though multiple second exposures are equally possible, shutter speed is a little simpler by the numbers than aperture.
For example, if my shutter is set to 1/100th of a second (which would read out as 100 on your camera’s display) and I wanted to double the light, I would cut the speed in half to 1/50. If I wanted to reduce the amount of light by half, I would double it to 1/200.
Because shutter speeds are measured in fractions, the higher number is a smaller slice of time, and less light gets in as a result.
Back to the bucket, if you turn the water on a little, no matter how big around the hose is, a little water will come out. If you turn the hose a lot, a lot more water will come out.
This describes the relationship between aperture and shutter speed fairly well. If you want the lessen the water that comes through, using the smallest hose (small aperture) with the weakest flow (fast shutter speed) will result in a mostly empty bucket (darker image). If you want the most amount of water coming to the bucket, use the biggest hose (wide aperture) with the strongest flow (slow shutter) and your bucket will be filled quite quickly (brighter image).
Ultimately, what is desirable is the right amount. The goldilocks. Not too much, not too little, but juuuuust right.
Mentioned in the last post, changing apertures has a secondary impact on images, a changed depth of field. Shutter speed has a secondary impact on images as well. Blur. Ever not been able to take an image that’s sharp? Here’s what happened:
First, you were likely in auto mode. The green square of shame. Hear me now: auto is the worst thing you can do to your photography. They should make the green square a red square so people are at least subconsciously inclined to stop using it. If you want to take better images, pack it up with your pop-up flash and vow to never use it again.
Your camera is programmed to read that exposure meter a certain way and it will never deviate. What it cannot account for is what your current priorities are based on what your camera is pointed at. More on this later.
Your second problem likely was that you were in a place that didn’t have enough light. Most times in on-location photography (as opposed to in a studio) we get what we get with ambient light and there’s not a lot we can do to change it. What we can change, is how our camera is going about dealing with the light that’s available to us.
Because there wasn’t enough light, your camera said to itself “I’ve got to open up the aperture!” so it did. But that cheap-ish kit lens that came with your camera can only open so far, as we’ve already established in the previous post. So it opened it as much as the lens would allow and it moved onto the next variable, the shutter speed.
Then that little plastic box, in all its genius thought, “Dang, there’s still not enough light, I’ll slow the shutter speed down!” so it did. And your shutter was so slow, it instantly became humanly impossible for you and/or your subject to stay motionless long enough to be recorded by the sensor. Your camera doesn’t know what is situationally best. It never will.
For the sake of saying, let’s imagine you were in Manual mode, where you can pick all your exposure settings, what were you supposed to do in that situation? Even then, your aperture can’t open any wider and your shutter speed needs to be fast enough to freeze the image.
Hold that thought, there is a solid rule of thumb that fits right here: it is generally the case that while hand-holding your camera, your shutter speed should never be slower than your focal length. Translation: shooting at 50mm? Don’t shoot slower than 1/50. Shooting 200? Don’t shoot slower than 1/200. This rule can be pushed a little bit with the implementation of image stabilization if your lens has it, but even IS isn’t a silver bullet.
What stabilization is good for is fixing the movement of the person holding the camera. What it doesn’t fix is the movement in front of the camera. So if you go to a NASCAR event and shoot the cars going by at 1/50 on a 50mm lens, you’ll probably find the cars (but nothing else because you’ve followed the rule) blurry anyway. And in that case, blur might be desired to emphasize how fast they are moving. If you want to stop them in action, you’re going to have to speed that shutter up, which shouldn’t be that difficult at a NASCAR event anyway, with all the sunshine they usually entail. Case in point: knowing how your camera works will enable blur to be something you choose for effect, not something that you’re a victim of.
Back to being stuck at the dinner party and your aperture can’t open any wider and you’ve slowed down your aperture as slow as you can by that rule of thumb, and your image is still too dark. What then? Your second-to-last resort, the ISO.
ISO is the last exposure variable and the one I am most conservative with. ISO is basically a gain-knob on your sensor. It makes your sensor more sensitive to the light that is coming in the lens. “So crank it up! Right?”
Well, sort of. Ever turned the radio up in your car to the point where the sound quality started to degrade? The same happens in imaging.
Raising the ISO is a convenient fix, but it does come at the cost of image quality. At some point you will start to see what is called noise in the image. In fact – flash back a few pages to the image I took in my camera obscura. It’s not a good image. Given the circumstances it’s still a neat picture, but in terms of quality, I wouldn’t deliver an image like that to a paying client if my ability to feed myself depended on it. The variation in the colors when you look closely is what noise looks like, especially in the darker shadow areas.
Native ISO is 100 on most cameras, that’s where you want to be ideally, but honestly I rarely shoot that low. The truth is that the biggest selling point for me is how a camera handles noise at high ISO settings. Today’s cameras shoot astronomical ISOs, at the time of writing this, Canon recently created a sensor that could capture images at 4 million ISO. The images that come from it look awful, but no joke, you can take pictures in the dead of night and see what’s going on like it’s early evening. It’s amazing. So the effect does trickle down, which is the good news. My first camera was a Canon 30D. At 1600 ISO, it didn’t manage noise well at all so I usually shot it around 400 to get some of the benefits for my shutter speed without compromising image quality noticeably. My current camera, the 5D Mark IV handles noise fairly well even at 3200 and if I’m in a real bind, I can get away with 6400. I usually leave my Mark IV on 800 or 1600 when indoors. With technology’s advance, we will continue to see improvements in this department for sure. That said, you still need to know the limitations and let that information guide you as you make your decisions about the appropriate time to turn up the volume, so to speak.
I set myself up a little a few paragraphs back. I said that there are three variables that affect exposure, and then called the third one I mentioned the “second-to-last resort.”
Say you’ve maxed your aperture, reached the feasible limit of your handheld shutter speed and you’ve pushed your ISO to its reasonable maximum as well – WHAT THEN?
Unless you feel like laying some money down on a lens with a better aperture, the last resort is to bring your own light. Flash.
Pop-up flash not included for aforementioned reasons, every major brand of camera manufacturer makes flashes that will mount on your DSLR’s hot shoe. There’s also a growing number of off-brand flashes that have similar capabilities and are delightfully inexpensive. The big difference from the pop-up is that they have a head that can be swiveled and tipped, which gives you control over your light in key ways.
There is a post dedicated entirely to lighting, so we can leave it at that for now, but it is certainly worth mentioning in the conversation on exposure.
Ultimately, good exposure is a balance game, meaning you can get an identical exposure with a variety of settings. First I tend to what I have to tend to based on your surroundings. These rationales might sound like, “I need a fast shutter speed to stop the motion on this race car,” “I need a wide aperture so the ugly background will be out of focus,” or “I need to lower my ISO because it is too bright out” (note that the first two consider the secondary effects of shutter speed and aperture choices).
After I do what I have to do, the rest of the variables are mine to choose. Sometimes I don’t get to choose very much if the conditions are very adverse, other times I have plenty of light and there are multiple ways to get the very same exposure. Just like paying the bills, I have to settle my debts before I can splurge!
Though manual mode will offer the most control, its major shortcoming is that it can be slow or tedious to work with if your lighting situation is changing rapidly or frequently. Conversely, if your lighting situation isn’t fluctuating at all, its a godsend, you just set it, forget it, and focus your attention to other details like composition. For the times that it may be more of a burden than a blessing, your camera has other modes and settings that revolve around exposure. You know my feelings about the green square of shame, but a couple of the others can be useful to be familiar with as well.
Use the chart below as a quick reference to the effects of altering aperture, shutter speed, and ISO.
But wait, there’s more: Exposure & Metering Modes: the only way to trust your camera
**If you’re interested in treating these posts as a self-study with assignments, you can get our Assignment Series 1 question pack to help you learn these ideas through real, targeted, hands-on practice. The Assignment Series 1 question pack is included with enrollment in our beginner’s course.**
