AUTOFOCUS AND METERING
A number of settings on your dSLR camera help it capture the exact image you are hoping for. Because the camera can only guess at the subject of your photo and what is most important, we use some of the camera’s features to ensure the camera is making the right choices when we press the shutter release.
KEY
TAKE-AWAYS
Autofocus area modes
Focus/frame/shoot
Metering modes: spot, partial and matrix
FOCUS
Focus modes, Autofocus modes and AF points
What are you looking at? The camera can't always tell.
Remember the photo we looked at for depth of field? The close kid, the middle kid and then that dude way in the back?
What should the camera focus on? All of them? One of them? and if so, which one?
If you're in aperture mode, you can set a small aperture (and thus a wide depth of field) and the camera will keep all of them in focus. But what if you want narrow depth of field? How does the camera know what to keep sharp and what to allow to be blurry?
To help get it right cameras use a combination of settings that tell the camera whether it is automatically focusing, and if it should be paying attention to anything in particular. This will be covered in class but the main points are as follows:
FOCUS MODES
There are 4 focus modes:
AF-A: Autofocus-Automatic: camera decides if it should use AF-S or AF-C
AF-S: Autofocus-Single: camera automatically find focus only once while the release button is half-pressed
AF-C: Autofocus-Continuous: camera automatically finds focus until the release button is fully pressed
MF: Manual focus: photographer decides on focus
AF AREA MODES & POINTS
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If your camera is using one of the autofocus functions, you can help ensure the right part of the frame is in focus by selecting the right autofocus mode.
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The available modes are:
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Single point (there are 11 available points on the D3400 (cameras and makes vary). Any one of these points can be given priority and the camera will focus on whatever is framed at that area.
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Dynamic area is similar, but allows the camera to search a small radius around the selected point
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3D autofocus mode allows the camera to shift attention around the frame and track a subject. This is useful if the subject is moving quickly and around the frame. An example might be a fast moving sporting event.
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Auto area mode allows the camera to switch between these modes as it sees fit.
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The Nikon D3400 has 11 AF points. In auto area mode, the camera automatically chooses between these points, searching for contrast differences to focus. More expensive cameras have more AF points.
In AF-S mode and AF-C mode the camera bases its focus on the user-selected AF point.
METERING
Metering is how the camera decides how much light is in a scene. Once the camera has a defined scene in-frame and as you are finalizing your composition, the camera is measuring the overall lights and darks of the scene and (if you are in anything but the manual priority mode) setting the shutter speed, aperture and/or ISO to achieve an even exposure (18% grey).
Without getting into an overly complicated explanation, sometimes the camera gets it wrong. For a couple of reasons (one being incident vs. reflected light, another being the actual reflectance in a scene) your camera may need help deciding on how much light is available. Enter metering modes.
Metering modes are basically a way to tell the camera to measure the light in a scene, but to give different levels of importance to different areas of the photo depending on the mode selected.
The three available modes are matrix (or evaluative) metering, central-weighted, and spot metering. Different cameras have different names, and each camera maker has their own algorithm for this, but here's what's happening:
AF single point, centre
metering: matrix mode
shutter speed: 1/6 sec
aperture: f/4.5
iso: 100
This added blur demonstrates what the camera sees as the available light. It is viewing the entire frame and making a decision based on how light or dark the resulting scene is.
In this situation, the focus is set to single-point and is at the centre of frame. The camera is in matrix metering mode, so it is looking at the entire scene and taking its reading from all of the light available, though it is more heavily weighting the area around the current location of the AF point, the centre.
AF single point, centre
metering: centre-weighted
shutter speed 1/8 sec
aperture: f/4.5
iso: 100
Here, the camera is looking at the current location of the AF point and taking in a little information from just around that point as well. The camera read the available light as too much and darkened the scene slightly.
AF single point, centre
metering: spot metering
shutter speed 1/4 sec
aperture: f/4.5
iso: 100
Here, the camera is looking directly and almost exclusively at the active autofocus point. In this case, the active autofocus point is dead-centre, and the camera read the scene as a little dark. The camera thus lightened up the scene.
Still not sure? Visual learner? Here's a cheat sheet (Note: the cheat sheet is based on Canon's terminology)
C/T
Completion Task: Bad Photo.
Read and complete the activity.
Submit links to Flickr and the Brightspace dropbox.
METERS
Light
In the 1930s, Kodak analyzed 1000s of photos. They discovered that the average photo taken by the average photographer, in average lighting conditions, ended up all having something in common. If you blended all the colours in the photo (like if you put them all in a blender) they mostly ended up at 18% grey.
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To solve this problem, we use a light meter.
