Understanding Shutter Speed, Aperture, and ISO
Your Camera Needs Light
Like that prissy little Goldilocks and her porridge, your camera needs just the right amount of light to capture a good exposure. Too much light and your photos will be washed out, overly bright, and lacking detail. Too little light and your photos will be too dark to see anything.
Between pure darkness and staring at the sun (don’t do that), our world presents a near infinite range of brights and darks, also called dynamic range. Our eyes adjust to this on their own, helping our brains form a complete image of what we see. Our cameras can only see a small fraction of that light information, so you need to know how to set up your camera to capture what you see.
I’m going to throw some numbers at you for the curious (if you hate numbers, feel free to skip this paragraph): Our eyes+brain combo can see details in objects through a dynamic range of about 22 stops. A “stop” means a doubling of light, so this means we can see objects that are about 4 million times brighter than the darkest objects we can see. Today’s best cameras can see details through a relatively paltry dynamic range of about 13 stops (8000 times brighter), with the highest quality data falling in a tiny range of only about 7 stops (128 times brighter). This is why it’s so important to nail the exposure.
There are three built-in ways to capture the light going into your camera.
- Shutter Speed – duration of the sensor’s exposure to light
- Aperture – size of your lens’ opening
- ISO – sensitivity of your sensor to light
Shutter speed can be set between 1/8000 second to 30 seconds on new and high end digital cameras. Older and lower end models are limited to 1/4000 or 1/2000 second. Exposures that last more than 30 seconds require a remote shutter release on many cameras.
f/ This: All About Aperture
Aperture is the size of your lens’ opening. It’s controlled by you, and it’s measured in “f-stops”. An f-stop is the ratio of your lens’ focal length to its diameter, but you don’t need to know this to take good pictures. Aperture affects two things:
- The amount of light going into your camera
- The depth of focus (also called depth of field)
Large apertures (low f/numbers) mean large lens openings, a lot of light, and a shallow depth of field. Portraits are typically shot at large apertures between f/1.2 and f/2.8. This helps blur the background, remove distractions, and isolate the subject as demonstrated in the image below.
Small apertures (high f/numbers) mean small lens openings, less light, and a large depth of field. Landscapes are commonly shot at small apertures of at least f/8. This helps keep everything in focus as seen in the shot below.
A Closer Look
There are two optical qualities which are always present at low f/numbers:
- Depth of field is always shallow
- Light always falls off in the corners
Light falloff, or vignetting, causes the corners to appear dark. This is rarely a problem in practice because dark corners help to further isolate your subject against the background. Vignetting can also be used artistically to add a dreamlike feel as demonstrated in this scene from Istanbul.
Let’s take a closer look at some different aperture settings. The following 5 images show examples at f/1.4, 2.8, 4, 8, and 16 on a 50mm lens. Pay particular attention to the size of the lens opening as the aperture changes.
There is some confusing terminology in use today (particularly online). Remember that apertures are either large or small, and f/numbers are either low or high. The confusion occurs because the terms aperture and f/number are often casually and incorrectly interchanged. To be clear, low f/numbers mean large apertures, and high f/numbers mean small apertures. A lens set to its largest (maximum) aperture is set to its lowest f/number. Whew, enough of that.
ISO is your sensor’s sensitivity to light. The higher the ISO the more sensitive your sensor, and the more light it is able to record. High ISOs allow you to use fast shutter speeds in dim light. The byproduct of high ISOs is digital grain, or noise. A high ISO of 12,800 was used to capture the nighttime image below from Siberia in February.
Noise is usually considered a bad thing because it reduces sharpness, but it can also be used intentionally to add a moody, rough feel to your images. This high noise effect is demonstrated nicely in this picture Tallinn, Estonia, shot at an extremely high ISO of 25,600.
Low ISOs offer the best image quality. ISOs of between 100-400 are commonly used in landscape and daytime photography.
I hope this helps you get to know your camera better. For more posts like this, see the rest of my Visual Guide to Travel Photography Series.