Best Way to Learn About Exposure: Exposure Modes and Metering

Today, we'll build on out knowledge of histograms and exposure compensation and continue our exploration of exposure. You now may be wondering why your camera has so many exposure and metering modes, and which ones you should use. This article will help you understand the difference and select the best modes for you.

There are two more steps to taking control over your camera's exposure controls:

Choosing Exposure Mode for More Control

Most cameras (with the exception of some semi-professional and professional models) have a range of fully automatic exposure modes. These are indicated by various icons and have names such as portrait, landscape and night.

If you use any fully automatic exposure mode, now is the time to stop. These modes are quite restricted and don't give you much, if any, control over the shutter speed, aperture and ISO settings.

Why does your camera have so many exposure modes? The fully automatic modes are designed to help people who don't know much about photography to use the camera right away. They are not aimed at photographers who can decided for themselves which aperture, shutter speed or ISO to use.

In my view, the fully automatic exposure modes clutter up the dial and create confusion. In practice, you only need four exposure modes. They are Aperture Priority, Shutter Priority, Program and Manual. Here's how they work:

Aperture Priority

You set the aperture and ISO and the camera sets the shutter speed accordingly. I use this when I'm taking landscapes, as I normally set a small aperture to ensure front to back sharpness, or when I'm taking portraits as I often set a wide aperture (around f1.4 to f2.8) for a narrow depth-of-field.

I used an aperture of f/2.5 to take the photo of a flower above. My priority was to use a wide aperture to throw the background out of focus.

Shutter Priority

You set the shutter speed and ISO and the camera sets the shutter speed. I use this when I'm hand-holding the camera to set a shutter speed to prevent camera shake. Sports photographers may use this mode to set a shutter speed that freezes action.

Though in the landscape above, I used shutter priority to insure that the wave blurred. I set the shutter speed to 1/2 second and asked my model to stay still throughout the exposure.

Program AE (auto-exposure)

You set the ISO and the camera selects both aperture and shutter speed. Some cameras have an Auto ISO setting that lets you tell the camera that it can adjust ISO between two settings (say ISO 100 and ISO 400). This is a good general mode to use that lets you concentrate on composition while the camera takes care of aperture and shutter speed.

The photo above is the sort of scene you could use Program AE for. Neither the aperture or shutter speed is important for creative reasons, and it is be fine for the camera to select both.

Creative Exercise One:

Put your camera in aperture priority mode and take a series of photos where you decide which aperture to use, and let the camera take care of the other settings. If the shutter speed is dropping too low to hand-hold the camera safely, raise the ISO to compensate. This exercise works well if you use the widest aperture settings of your lens to take photos with a blurred background.

Portraits, like this one taken at f1.8 , are a good example. The out of focus background adds atmosphere and directs your attention to the girl.

Now put your camera in shutter priority mode and take some photos where you decide which shutter speed to use and let the camera take care of the other settings. This is a good chance to experiment with using slow shutter speeds to blur motion. Put your camera on a tripod to keep it steady while you do this.

I used a shutter speed of 30 seconds to take this photo. The long exposure has blurred the motion of the sea and the clouds.

Learning the Difference Between Metering Modes

Most digital SLRs have the following exposure modes:

Evaluative Metering

The camera's most advanced metering system. The camera divides the viewfinder up into zones and compares exposure readings from each zone to come up with a suggested exposure setting. Gives good results most of the time. Exposure is weighted towards the active autofocus point (or points) as they are likely to be covering the main subject.

This diagram shows how Canon's iFCL (intelligent focus, colour and luminance) evaluative metering sensor works. The sensor is divided into two colour sensitive layers and 63 zones. It is complex and sophisticated, but will still get the exposure wrong if the subject is lighter or darker than average.

This diagram shows Canon cameras with iFCL metering split the frame into 63 zones. Each zone is metered individually, with the overall exposure reading weighted towards the in-focus AF points.

Note: Evaluative metering is Canon's term; Nikon uses "matrix metering," Pentax and Sony "multi-segment metering."

Center-Weighted Metering

Weights exposure towards the centre of the viewfinder. Works well if your subject is in the centre of the frame. If not, you have to point the centre of the viewfinder at your subject, hold the shutter button half-way down to lock in the exposure, then reframe. Evaluative/matrix/multi-segment metering were developed to make it easier to measure exposure with off-center subjects.

Spot Metering

Takes an exposure reading from where your focus point is. Some people use this if they base exposure on the Zone System. When I find it really helpful is when you have small bright subject against a black background. You'll find this situation a lot during theater performances and the like.

Creative Exercise Two:

Try photographing the same scene with the different exposure modes. Do they give you different exposure readings? Is it easiest to get the correct exposure with evaluative or centre-weighted metering? What does spot metering tell you about the brightness range within the scene?

Results will vary according to what you're photographing, so try this exercise with several types of subject matter.

Personally, I use evaluative metering the mowt, but it's useful to know how the other metering modes work in case one of them suits your way of working better.

Using Manual Mode

One potential problem with evaluative metering is that the camera readings can change according to how you frame the subject, even though the ambient light levels haven't changed. This is down to the balance of light and dark tones within the frame, and is quite common.

The above photo shows a situation where this happened to me. The girl is dressed in white and the camera's suggested exposure settings changed as I framed the scene in different ways. Switching to manual mode stopped that from happening.

The advantage of manual mode is that, once you've established the optimum exposure setting, that the camera settings are locked in and won't change if you reframe the subject. This works well as long as the ambient light is steady. If the ambient light is changing (for instance, if the sun is going in and out behind clouds), then you are better off using an automatic exposure mode.

I start off by taking an exposure reading in aperture priority mode. I transfer the settings to manual mode then take a photo and check the histogram. I make any adjustments required then take another test photo. Once the histogram is where I want it I can continue with the shoot.

Creative Exercise Three:

Go out and take some photos in manual mode, adjusting the ISO, aperture and shutter speed individually and checking the histogram to arrive at the correct exposure. This is a good way to learn about the relationship between these settings.

Additional Resources Here at Phototuts+

Phototuts+ Exposure Session - A series of articles about exposure.
Exposing to the Right - Learn about exposing to the right area of the histogram, what it means and if it works.
18% Grey Cards - Grey cards are a useful accessory for helping to determine both exposure and white balance.
Phototuts+ Quiz: Exposure - You should be an expert by now, so why not take the exposure quiz?

Books

These books (linked to on Amazon) explore the topic of exposure in more depth:

eBooks

These short eBooks will also help you get to grips with exposure:

Conclusion

Exposure is a surprisingly complex topic, but once you understand the underlying principles you should be able to cope with just about any lighting situation. The lessons and exercises in these articles are a good start, and the articles and books listed will help you gain an deeper understanding.