Guides and information for photography enthusiasts, by photography enthusiasts

For the most part our digital SLR cameras handle the heavy lifting of determining our photo exposures these days. In a shocking number of day-to-day uses the camera’s auto exposure will handle things just fine. However, when you’re going into a shoot trying to get a very specific photo, it’s vital to understand the basics of exposure.

You know what, even if you’re not trying to do something specific, you should still understand exposure.

Once you know how it works and understand the concept of stops, you can easily adjust your settings to freeze motion, or increase or decrease your depth of field. This exposure guide is written specifically for beginning photographers — more experienced photographers are unlikely to learn anything new.

The Three On-Camera Elements of Exposure

There are three elements on your digital SLR that affect exposure. These are the shutter speed, aperture, and ISO. You also have an off-camera element, which is just the amount of light shining on your subject area. Here’s some brief info you should know about each of these elements:

• Shutter Speed: controls how long the shutter on your digital SLR camera is open, or how long a period it lets light in to the sensor. Your Shutter Speed also controls your ability to freeze motion. A slow shutter speed can make a moving target appear blurred, and slower still shutter speeds can make your whole picture blurred from the minute shaking of your hands. Shuter Speed is measured in seconds, or more often fractions of a second.
• Aperture: controls how wide the iris of your digital SLR camera opens, or how much light it lets in at once. Your Aperture also controls your depth of field, with lower apertures giving you a shallower depth of field. Extremely high or low apertures can result is a slightly less sharp photo (though these differences are often very difficult to see without very close inspection). Aperture is measured in f-stops.
• ISO: controls the sensitivity of the imaging sensor on your digital SLR camera, or how sensitive it is to the light hitting it. The higher the ISO, the more noise you will get in your photo, but the less light you need coming in your lens to get a proper exposure. See our Digital SLR ISO Guide for more details.

The Concept of a Stop in Exposure

We refer to stops often in photography, and you’ll hear things like, “I stopped up two stops to compensate for the sky.” This then encourages the sometimes frustrated question, “What is a stop in photography terms?”

In photography, a stop is a doubling or halving of the amount of light on your exposure.

As a simple example, let’s say I’m taking a picture at ISO 200, f16, with a shutter speed of 1 second. If I double the shutter speed to 2 seconds, that’s a 1 stop difference. If I double it again to 4 seconds, that is also a stop. Doubling it again to 8 seconds is again 1 stop. So going from 1 second to 8 seconds is a difference of 3 stops — I doubled the light going in three times.

Now more likely my shutter speed would be maybe 1/125th of a second. Let’s say I’m taking a picture of a dog running around — but at 1/125th the dog is blurry. So I want to get a faster shutter speed. My aperture is still f16 and my ISO is 200.

To get a faster shutter speed I can move 1 stop  to 1/250th of a second (halving the amount of light) and then again to 1/500th of a second (1 stop again). I now have a nice fast shutter speed, but I’m not letting enough light in to expose my shot! I’m 2 stops underexposed.

Since I know that stops are just a doubling or halving, I could move my ISO from 200 2 stops to ISO 800 (200 to 400 is one stop — 400 to 800 is a second stop). But perhaps I’m worried about the noise from shooting at ISO 800, so instead I just stop my ISO 1 stop to ISO 400, then I can stop down my f-stop from f16 down to f11.

I’ve now increased my shutter speed to let 2 stops less light in. I compensated by letting 1 stop more light in via my aperture, and 1 stop increase in my sensor sensitivity via ISO.

It’s worth noting that f-stops are the least intuitive — where ISO and aperture are just a matter of doubling or halving the value, aperture increment by the square root of two, so your stops are f1, f1.4, f2.8, f4, f5.6, f8, f11, f16, f22, etc.

Dynamic Range

The details of dynamic range is a whole separate post, but essentially the dynamic range refers to how many stops your camera is capable of displaying. A common dynamic range for a digital camera is 5 stops. So you start at the darkest shadows with detail in them, and you can double the amount of light coming in 5 times — then you have the brightest clouds with detail in them. If you go any further, everything is just pure white.

This is a particular problem on bright sunny days — it’s very easy to take a shot where the shadows under the trees are pure black, and the sky is pure white. This happens because the dynamic range is larger than your camera is capable of displaying.

At that point your only options are to find a way to brighten the shadows (like with a flash), find a way to darken the bright areas (standing in the shade), or more commonly, finding a better time and conditions in which to shoot.

In photography, ISO is a measure of how sensitive our film is to light. In digital photography we still use ISO (technically ISO Equivalent) to express how sensitive our imaging sensor is to light.

We’re going to get into some interesting details of ISO below, but first we’re going to hit just the basics, the Cliff’s Notes version for those who just need the answers, and need ‘em now.

The ISO Digital Photography Basics

In digital photography ISO refers to how sensitive your imaging sensor is to light. However, the more sensitive it is to light, the more noise your picture will have.

• ISO and ASA are the same thing
• A lower ISO value is generally a higher quality, with less noise
• A lower ISO value requires more light
• A higher ISO value is generally lower quality, with more noise
• A higher ISO value needs less light
• In bright sunlight you’ll probably want to shoot at ISO 100 and enjoy excellent quality photos
• In dark or indoor environments you’ll need a higher ISO, ISO 800, or 1600, or higher. These photos will have more noise, thus a lower quality.

ISO vs ASA

ASA refers to the American Standards Association and is now a defunct and outdated measurement of film speed. It’s still often used on film packages though. ISO refers to the International Standardization Organization and is the accepted standard for film speed. Whenever something refers to the ASA of film, it’s the same thing as the ISO.

But use the term ISO: it’s more accurate. ASA is what your grandfather used.

Film Grain and Digital Gain

Back in the film days a higher ISO value meant larger grain on your film. The larger grain was more light sensitive, so it exposed faster. So when you shot with higher ISO film — like ISO 800 — you ended up with a grainer picture. But grainy wasn’t all bad. Grain gave pictures an interesting quality that some photographers deliberately sought out.

Now that we’re in the digital photography age, our ISO Equivalent is increasing the sensitivity of our photo sensor. This means that the photo sensor is now able to pick up a fainter light signal — but the flipside is that it’s also going to have more noise. By cranking up the sensitivity of the sensor, it is actually generating noise. Think of it like a speaker with that background hiss — when you crank it up, that faint hiss gets louder and louder. The higher your ISO is, the more noise you have.

Unlike film grain, noise is just plain ugly, and you really don’t want it there. Noise on a digital photo is colored specs or a speckled image, and in a very noisy photo they are immediately obvious, distracting, and unattractive.

Unfortunately, there is no “right” ISO, or “highest” ISO for a decent picture. The reason is that different cameras (or different sensors) have a different sensitivity to noise. Every camera has a range of ISOs that give acceptable image quality, and that range is different from camera to camera, from sensor to sensor.

Size Matters: Why Megapixels Ain’t Everything

In most things, a bigger sensor is better. Larger sensors tend also to have larger pixels. These larger pixels are less sensitive to noise. So a low-end consumer camera with a 1/1.8 in sensor might have incredibly noisy shots at ISO 400. But a standard SLR with a APS sensor can shoot ISO 400 with no noticeable noise, and may not get that bad until ISO 1600. Move up a step again to the full-sized sensor and you may be able to shoot at ISO 3200 with less noise than that cheap point & shoot was at ISO 400.

But the size of the sensor isn’t really the factor here — it’s really the size of those pixels that matters. This is where the constant obsession with more megapixels! starts to get us in trouble. You can fit more megapixels on the same-sized sensor by making those pixels smaller. But make them too small and you’re going to increase their sensitivity to noise. So you can easily have a camera with more megapixels that takes worse shots, even when blown up to the same size, because of the noise.

So megapixels ain’t everything. In fact, a 12 megapixel full-frame camera will have better shots, and better blow-ups than a 16 megapixel APS sensor camera. But then, a full frame sensor with 16 megapixels will be better still.

ISO & Stops

ISO is a very convenient and logical measurement of sensor sensitivity because it’s beautifully set up to use our stop system. If you double the ISO, you are doubling the amount of light (or sensitivity to light). If you halve the ISO, you are halving the amount of light in your exposure.

Thus if your exposure is correct at ISO 400, but you want better quality, you can stop down 1 stop to ISO 200, but you’ll have to lower your shutter speed or open your apeture by one stop to compensate.

Digital Camera Native ISO

Every digital camera has a “native” ISO setting, this is the setting that gives the camera it’s highest possible quality shot (lowest noise). In most cases this is the lowest ISO setting that the camera offers, often ISO 100.

However, some cameras have a native setting higher than their lowest possible ISO — most Nikon cameras have a native ISO of 200. The camera still offers ISO 100 (or lower), but in actuality ISO 200 will have the best signal to noise ratio. Be sure to take a moment and find out what your camera’s native ISO is, so you know where to set it for the best quality shots (assuming the lighting conditions will allow for it).