Lenses, Light, and Shutter Speed

Overview

Here’s a little background on shutter speed, aperture, and ISO sensitivity… and how focal length of the lens plays into that as well. Don’t let any of those terms concern you at this point; they’ll all be made clear.

You may have heard the phrase “cheap, fast, good: you can have any two”. It’s kind of like that with light and photographic images.

You can do four things to increase the light available for a photograph; each has tradeoffs, and numbers 2, 3, and 4 will be discussed in the following sections:

1. Turn up the lights, wait for the sun to come out, or use a flash
2. Decrease the shutter speed – which allows the light that is available to shine on the sensor in the camera for a longer time
3. Open the aperture of the lens more – which allows more light to pass through the lens and get to the sensor
4. Increase the sensitivity of the sensor itself

Shutter speed: How long the shutter is open, exposing the sensor to light.

The slower the shutter speed, the longer the shutter is open and the more light that is allowed through. 1/30 second is about the slowest you can shoot with a handheld camera and a 50mm lens without getting blur from camera shake. Shutter speeds can be as high as 1/16,000 second on some cameras (think “photo of polar bear in a snowy field on a cloudless, sunny day at noon”). Taking pictures of stars, etc. often means leaving the shutter open for minutes at a time … but it requires a tripod. Taking pictures of fast-moving subjects – sports, etc., - requires a faster shutter speed (at least 1/125 second; I prefer 1/250 second) so you freeze a subject in time.

Aperture: The opening of the lens that allows light through. It’s also called “f-stop”.

A smaller f-stop number (f1.4, f1.8, f2.8) means a wider lens opening and lets significantly more light through to the sensor or the film … a larger f-stop (f8, f11, f16, f22) means a smaller lens opening that lets less light through but provides a greater “depth of field” (area that will be in focus). This works the same way that our eye’s iris does, and relates to why we can see better in bright light (more things are in focus) than we can in the dark. At a setting of f22, a substantial area in front of and behind the item focused on will be in focus. For portrait work, where you might want the foreground and background blurred and only the subject in focus, you could use a smaller f-stop number (larger aperture opening) of f1.4 or f1.8.

The tradeoff works something like this

For the best possible image, allowing for a fast moving subject and the greatest depth of field (area that is focused), you would want:

• Low ISO setting (ISO 100)
• High shutter speed (1/500 second)
• Small aperture (high f-stop) setting – f16, f22

Settings such as those, though, require a LOT of light. It’s one of the reasons that movie sets and television studios are so bright.

Working in a “low light” situation requires adjustments. If you must have a fast shutter speed, you can have a wider aperture (f1.8, f2.8), but that gets you only so far.

So … you need a fast shutter speed, and you’d like a smaller lens opening (higher f-stop), and you can’t change the amount of light falling on the subject. What do you do? You increase the ISO sensitivity of the sensor. The tradeoff here becomes one of image quality.

ISO Sensitivity: The speed with which the sensor gathers light.

•       Lower ISO = slower light gathering (and better image quality).

•       Higher ISO = faster light gathering (and lower image quality).

ISO Sensitivity is the equivalent of the old ASA number for film … “slower” film had lower ASA numbers; “faster” film had higher ASA numbers. The speed – slow vs. fast – refers to how quickly the film can collect light. “Fast” film could collect light faster … you could have less light, or use faster shutter speeds (how long the film is exposed to light) with fast film (ASA 400, for example). The price paid, however, was one of image quality: you would see more graininess from a faster film.

In the digital world, instead of film, there’s a sensor. In the pocket cameras, it’s about the size of your little fingernail. In Digital SLRs, it can be as large as a piece of 35mm film. (In very high-end systems, if can be a few inches across … but you pay $30,000 for a system such as that.) The size of the sensor has a lot to do with the quality of the final image. Another factor is the ISO setting used. “100” is a typical setting, but that’s a (relatively) “slow” setting … remember, this is NOT shutter speed … it’s the speed with which the sensor collects light. Since this is digital technology, not film, you can change the ISO setting of the sensor from one picture to the next. The lower the setting, the better the quality of the saved image, but more light is required as well. “More light” can be obtained by brighter lights, having the lens aperture open wider, or having the sensor exposed to light for a longer time (e.g., a slower shutter speed).

Higher ISO settings in digital cameras are achieved by putting more energy into the sensor and actually making it hotter. The downside of that is that it adds an amount of noise to the image – the equivalent of “graininess” in a faster film.

Long Lenses

Adding a longer lens to the mix doesn’t complicate things that much, but there are some “givens”:

• The more elements in the lens (e.g., the more pieces of glass), the more light gets dispersed along the way. Zoom lenses typically have more lens elements.
• The cheaper / lower quality the lens, the less light transmission (e.g., the more light gets absorbed or scattered along the way by the lens material). Cost increases markedly for zoom lenses with small f-stop numbers (a 300mm lens that will do f1.8 at its widest opening will be substantially more expensive than a 300mm lens that will do only f3.5 at its widest opening).

The additional factor here is that your shutter speed is the only way to make sure you freeze the subject and don’t get affected by camera shake – which, unfortunately, is amplified in long and zoom lenses.

A good “rule of thumb” for handheld photographs with long lenses is “1 over the focal length”. For example, if you’re using a 300mm lens, the slowest shutter speed you can use without seeing blurring from camera shake is 1/300 second. If you’re using a 400mm lens, the slowest shutter speed is 1/400 second. (With a tripod, of course, it doesn’t really matter – except with a moving subject, you’ll still want a faster shutter speed.)

Summary

So, with the following conditions:

• You’re using a longer lens, and
• You want a fast shutter speed (to reduce lens shake and freeze a sports image), and
• Your aperture is already set as wide as it can be, and
• There’s no way to increase the amount of light shining on the subject (e.g, you’re too far away to use a flash), then…

the ONLY way to accomplish your goal of a properly-exposed image is to increase the ISO sensitivity of your image sensor. And you get some slight degradation of the image (compared to one taken with a lower ISO setting) in the process.

Experiment! See what works best for you. Unlike with film, you get to see your results right away.