workshop assignment 4 – exposure and shutter speed
In session four we looked at shutter speed as a component of exposure and using shutter speed creatively. Before we get into the assignment for this session I want to revisit the discussion on exposure and ISO and to introduce exposure compensation and histograms.
Exposure and ISO
We have learned that exposure value (Ev) is dependent on aperture size and shutter speed and effectively determines the amount of light hitting the image sensor.
We control how much sensitivity the sensor has to that light by setting the ISO value. Briefly, the higher the ISO the more sensitive the sensor. ISO is a whole number and ranges between 100 and 64,000 to 128,000.
Higher ISO values allow us to capture images under poor lighting conditions often with great success. But there is a practical limit to how high an ISO value we can successfully use.
Think of ISO as the volume control on an audio amplifier. When the volume is low the amount of noise (hum or hiss for example) we hear is negligible. But in the quieter passages many of the subtler nuances often aren't audible. By turning up the volume these nuances become clearer. However, as we turn up the volume, we amplify both the signal and the noise, and at higher volumes noise can become very evident and is usually most audible in those quieter passages. The same is true for ISO.
The higher the ISO "volume" setting, the more "digital noise" becomes apparent in the image, particularly in the shadows and dark areas. If we go too high this noise can become overwhelming and distracting.
What's too high? I find that anything above ISO 800 can be problematic, but this will vary widely from camera to camera, even cameras from the same manufacturer. Pick a dimly lit scene and take several shots, bumping the ISO up one notch for each image. Then, on your computer screen, have a close look at the darker portions of the image; magnify the image up to 200% or more.
If you see "dirt" or blocks of different colour in an area that should be one solid colour, too much chroma noise has been introduced. Likewise, if you see blocks of the same colour but of different brightness levels too much luminance noise has been introduced. By today's "digitally perfect" world, any visible noise is unacceptable.
ISO is combined with aperture ƒ-stop and shutter speed so that Ev reflects the amount of light recorded by the sensor. By adjusting any one of these variables we can compensate for any given Ev, deliberately underexposing or overexposing an image.
All digital cameras – dSLRs and point-and-shoots – offer exposure compensation. Look for a button or a menu selection with an icon that looks like this.
Typically, exposure compensation is done in either ±1/3 or ±1/2 steps with ranges from ±2 Ev to ±3 Ev. The camera will adjust one of the Ev variables; which one is changed will depend on the camera's priority or scene mode.
If the mode is aperture priority – Av – or either portrait or landscape scene mode, exposure compensation changes the exposure time; if the mode is shutter priority – Tv – or either action or kids scene mode, the ƒ-stop is changed.
On point-and-shoot cameras with the flash operating the brightness of the flash output may also adjusted. On digital SLRs, flash compensation is controlled through a menu setting with this icon.
Over or under exposing can be useful in some situations; there are two special techniques ...
- "shoot left" underexposes to preserves highlights. By underexposing we prevent very bright detail from being pushed too far to the right in the histogram and therefore lost, and
- "shoot right" overexposes to preserve details. By overexposing we again prevent very dark details from being squeezed too far to the left and being lost.
But how do we know when to use these tools?
Enter the histogram!
All digital cameras can display a histogram on the back LCD display panel. A histogram graphs the amount of light in the images versus the brightness of that light. Take a moment and find out how to show the histogram on your camera. Most often it can be displayed by pressing the display button more than once, or by pressing an INFO button when the camera is in playback or display mode (check the index in your camera's manual).
Most seasoned photographers will check their composition (no missing heads for example) and exposure right after taking a shot, make any adjustments needed and take another shot. While we can do some correcting in post-processing, it’s always better to do it “in-camera”.
The "ideal" normal exposure histogram looks like this ...
Auto-exposure mechanisms try to create this type of histogram where most of the images information is centred in the graph. This type of balanced histogram is not always possible, but we want to avoid if pushing the graph too far left (under exposed) or too far right (over exposed).
For example ...
One of the advantages of digital cameras is that it doesn't cost anything to try another setting. We can make an adjustment, take another shot, and keep doing this until it "looks right".
I strongly recommend that you don't delete any of your shots after viewing them in the LCD display. The LCD display is should only be used for checking composition and histograms. They aren't of high enough quality to reveal much else in the image. Wait until you upload them to your computer where your monitor will give you a far better feel for what worked and what didn't. Once you delete an image "in-camera" it's usually gone forever.
By making good use of exposure compensation and histograms we help ensure our image files capture the maximal amount of light for any given lighting condition.
Explore shutter speeds and motion blur, motion freeze and motion panning. Bring in two each of
- Motion freeze using a shutter speed of 1/800" of faster. On point-and shoot cameras, use sports or action scene mode, and
- Motion blur using a shutter speed of 1/60" or slower. Nighttime or fireworks on point-and-shoot cameras is the best choice, and
- Motion panning using a shutter speed between 1/60" and 1/250". For point-and-shoot cameras, modes like museum or candlelight should work; you will have to experiment.
And that wraps up session four. In two weeks we will look at meta-data and copyrights as well as explore some of my favourite photographers.