The exposure triangle and exposure compensation

What if I wanted to set my Panasonic Lumix DMC-FZ150 superzoom bridge camera for less light sensitivity (low ISO) with a relatively fast shutter speed and the smallest possible aperture — impossible, right? After all, the lens diameter is smaller than many DSLR telephoto zoom lenses (therefore it has less light-gathering power) and there’s a lot of glass inside the 600mm equivalent lens.

If you understand the “exposure triangle” and how “exposure compensation” affects exposure, then you know it is do-able. Here’s how it works.

Background

Every time ISO is doubled (or halved), that’s one full “stop” of exposure. For example, changing ISO from 100 to 200 is plus one stop. The same is true for both aperture and shutter speed. For example, decreasing the aperture from f/5.6 to f/8 is minus one stop, and halving the shutter speed from 1/250s to 1/500s is minus one stop. A list of full stops for ISO, aperture, and shutter speed is shown below.

  • ISO: 50, 100, 200, 400, 800, 1600, 3200, 6400, etc.
  • Aperture: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64, etc.
  • Shutter speed: 1/1000s, 1/500s, 1/250s, 1/125s, 1/60s, 1/30s, 1/15s, 1/8s, 1/4s, 1/2s, 1s, etc.

Exposure Triangle

If you set your camera for one of the auto modes (e.g., Program Mode) and Auto ISO, then every setting in the “exposure triangle” is a wildcard and anything can happen.

If you want more control over how a picture turns out, then the goal is to control two of the three variables in the exposure triangle. For example, when using my Panasonic superzoom camera I prefer to shoot in shutter priority auto-exposure mode in order to avoid “camera shake.” Typically the camera is set for ISO 100 at a shutter speed of 1/800s — the camera decides which aperture to use. But wait, you still have some control over aperture!

Exposure Compensation

Suppose I use the same camera settings listed above, but also set the camera for minus one “exposure compensation” (-1 ev). This reduces the amount of light reaching the sensor by one full stop of exposure. The camera can’t change either the ISO or shutter speed, so what’s left to change? The only thing the camera can do is decrease the aperture (which means the f-stop number gets bigger) so the aperture will decrease by one full stop. For example, if the camera selected f/5.6 and I selected -1 ev, then the aperture will be f/8, resulting in more depth of field (something I usually want). How cool is that?

Sample Photos

I shot photo sets of eight different male Blue-faced Meadowhawk dragonflies (Sympetrum ambiguum) during a photowalk at Huntley Meadows Park on 06 September 2015.

Male 1: Spot metering with a single focus point on lower half of thorax.

A Blue-faced Meadowhawk dragonfly (Sympetrum ambiguum) spotted at Huntley Meadows Park, Fairfax County, Virginia USA. This individual is a male.

ISO 100 | 108mm | 0 ev | f/5.2 | 1/800s

The preceding photo looks great, right? Look closely at the full-size version of the photo. Notice the focus is tack-sharp for the dragonfly’s head and thorax, but softer for the terminal appendages at the tip of its abdomen. That’s a no-no for someone like me with a fixation on terminal appendages! Usually I wouldn’t publish a photo like this one, but it clearly illustrates what can happen when the depth-of-field is too shallow for a given point of view.

Male 8: Spot metering with a single focus point on upper half of thorax.

A Blue-faced Meadowhawk dragonfly (Sympetrum ambiguum) spotted at Huntley Meadows Park, Fairfax County, Virginia USA. This individual is a male.

ISO 100 | 108mm | -1.66 ev | f/8 | 1/800s

In contrast with the photo of Male 1, notice the photo of Male 8 is tack-sharp from head-to-tail. The camera settings for ISO and shutter speed are the same for both photos, so what caused the latter photo to turn out better than the former? Notice the setting for exposure compensation is nearly minus two full stops, resulting in a decrease in the aperture of more than one full stop (f/8 is the maximum aperture for the DMC-FZ150).

Low ISO with a relatively fast shutter speed at the smallest possible aperture — just what the doctor ordered and all made possible by understanding the exposure triangle and the effect of exposure compensation on exposure. I’ll say it again. How cool is that?

Afterthoughts

Realize that some of the “magic” happens because I almost always use an external flash unit. And I always shoot “RAW” photos because that format enables maximum flexibility during post-processing.

And by now you should realize that knowing about the one-stop rule and the exposure triangle has wide-ranging application, e.g., how to set your camera using Manual Mode.

Plus it’s worth noting this is how program shift works when using “Program Mode.” After metering a shot, program shift can be used to select equivalent combinations of aperture and shutter speed. For example, if you increase the aperture by one full stop (the f-number get smaller) then the shutter speed will decrease by one full stop (the denominator gets larger and the time value decreases). Make sense?

Remember, experimentation is the best teacher — that’s how I learned almost everything I know about photography!

Related Resources:

Copyright © 2015 Walter Sanford. All rights reserved.

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4 Responses to “The exposure triangle and exposure compensation”

  1. Mike Powell Says:

    Great explanation, Walter, of the different settings and how they affect the shot. In addition to the settings, the angle of view helps to determine how much depth of field you get. It looks like the plane of your camera was more parallel to the dragonfly’s body in the second shot than the first, increasing the likelihood of getting more of the subject in good focus. The distance from your subject also can be a huge factor–your recent head-on shot of a Halloween Pennant is a great illustration that when you are close to your subject and shooting perpendicular to its body, it’s virtually impossible to have it all in focus.

  2. Christy Says:

    Spot on metering and exposure Walter. The flash has enabled you to get really tack sharp focus too.

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