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The Exposure Triangle

COW Blogs : Richard Harrington's Blog : The Exposure Triangle
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exposuretriangle
An essential concept in photography is the exposure triangle. Three settings in your camera affect how your camera exposes an image: ISO, aperture, and shutter speed. If you’re used to shooting in Automatic mode, you may have never adjusted these properties. However, if you’ve used Aperture or Shutter Priority mode, you’ve started to dabble with manual control.
When shooting video, you’ll likely need to shoot entirely in Manual mode and take precise control over all three properties to get the exposure you need. Even if you think you’ve mastered exposure for your DSLR when shooting stills, keep reading. Getting the correct exposure for video is more complicated because of video’s limitations.

Shutter Speed

The first property you’ll set is the side of the triangle with the least flexibility. The shutter speed controls how long your camera stays open when you take a photo. It has a similar function in video because it greatly impacts how much light comes through. The shutter speed also controls the amount of motion blur in an image.

04fig02_alt
The camera was locked down on a tripod and properly exposed for this shot. When my son is moving quickly, the shutter speed emphasizes the motion blur; when he’s moving slowly or holding still, he’s much more in focus.

To simulate a filmic image, you need to use the optimum shutter angle to accompany the 24p frame rate in a DSLR. You can use this simple formula:

one second ÷ (frame rate x 2)

For example, when shooting 24 fps, you would set your light meter to a 1/48 second exposure time (you may only have 1/50 as a choice). At 30 fps, you would use 1/60 of a second. Following this guideline will help ensure that the motion blur created by the camera looks natural.
Can this rule be broken? Of course. There are two instances in which you will break this rule:

  • If you want to take on a more stylized approach to your video, you can change the shutter speed. A long shutter speed creates more motion blur and streaking. A shorter speed creates more of a hyper-action look with staccato movements.
  • If all else fails and you can’t get the exposure you need, you can change the shutter speed to let more (or even less) light into the camera. However, this change should only be made after you’ve exhausted the available aperture and ISO options.

Aperture

An easy way to think of aperture is as a window. The bigger the window, the more light you let into your camera. Easy enough, right? Of course, a lower number for the f-stop means a bigger opening (which can seem backwards at first).

04fig03_alt

The lower the f-stop, the wider the aperture. A wider opening lets more light into your camera. Image from Wikimedia Commons. Image by Cbuckley and Dicklyon.

The advantage of having a lower f-stop means that you have more control over how much light gets into the camera. This sounds easy; just use the lowest number, right? Well, it’s not that simple. Here are a few details to consider:

  • The more you open the aperture, the shallower the depth of field. When shooting with an f/1.4 lens, you can literally have a person’s nose in focus while the ears are out of focus.
  • A lens with a lower f-stop is often more expensive. Most kit lenses have f-stops that range from f/4 to f/6. On the other hand, professional zooms can get as fast as f/2.8 and professional prime lenses (fixed focal length) can get even faster.
  • Cheaper zoom lenses change their f-stop as you move through the zoom. This can lead to an exposure change in the middle of a video shot if you attempt to use the zoom options.
Typically, I’ll use aperture as my first control for exposure. After I’ve locked in my ISO, I then adjust my aperture to achieve a proper exposure. Often, aperture can be used to control the depth of field in an image (how soon the image starts to go out of focus). For many, this shallow depth of field is a desirable aspect to shooting on a DSLR.

ISO


Your camera has an ISO setting that controls how sensitive its sensor is to light. The lower the number, the less sensitive the sensor is. For most cameras, an ISO setting of 100 is considered the base setting. This ISO works well when shooting under bright lights or sunny days.
As lighting conditions change, you can bump up the ISO setting to 200 or 400 to deal with mixed lighting or overcast days. Higher ISO settings, like 800, 1250, and even 1600, can be used for nighttime and low-light shooting. Many newer DSLR cameras offer even higher ISO settings.
However, it’s important to remember that cranking up the ISO is literally like turning up the volume. As the signal is amplified, the amount of visible noise increases. For still workflows, this noise can often be cleaned up with filters. For video, you’re out of luck. Too much noise will result in a grainy image with dancing pixels. Be sure to test your camera and determine how high of an ISO setting you’re comfortable using.

04fig05_alt

The noise becomes very visible in this low-light image. In this case, an ISO of 6400 was used; however, it produces an unusable image. Be careful not to boost your ISO too high when shooting in low light, or visible noise will ruin the shot.

Be sure to check out the book — Creating DSLR Video: From Snapshots to Great Shots




The Exposure Triangle
Republished by Richard Harrington

Posted by: Richard Harrington on Nov 27, 2012 at 7:35:00 pm DSLR VideoPhotography
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