Autofocus is far more complicated and involved than most people understand. I’m going to try and keep things simple for the purposes of this series though.
Essentially, Autofocus allows the camera to make use of a number of Autofocus (AF) Sensors to shift a series of elements inside the lens in such a way the the subject which you are wanting to photograph appears sharp and in focus. When focus is achieved, a little red square lights up indicating that the subject over which that AF point rests is now sharp and in focus.
The location of this AF point is vital because your depth of field (linked to aperture value) revolves around this focal point. If the camera does not achieve focus in the correct place, your subject will not appear sharp an in focus.
In my experience, there are two main reasons for images not being sharp and in focus.
- You or the camera have achieved focus in the wrong place
- Your shutter speed is to slow
I’ll leave the second point for next weeks post.
Deciding Where to Achieve Focus
The AF sensors on cameras generally look for contrast and edges and through a series of very fast adjustments, shifts the elements inside the lens in order to bring the contrast and edges into alignment on the AF sensor. Contrast is the key variable here. Most people aren’t manually selecting their focus point and are simply allowing the camera to make use of all of the available focus points to decide where IT wants to focus. Based on how the camera achieves focus, it will simply focus in the region where there is greatest contrast.
This is not necessarily your subject though and this is where your subject can be rendered soft and out of focus.
Here’s a great example of where the cameras AF system picked up on the contrast of the grass against the white belly of the lion cub and chose to focus on that instead of the lions.
In the following example, the camera would very easily achieve AF on the rope upon which the dragonfly is perched. The contrast of the textured rope against a dark background provides a very clean cut edge of contrast for the AF sensors to achieve focus.
If the camera was left to automatically decide which of the many focus points available to it to use, it would more than likely focus on the edge of the rope. However, if you were to manually select just a single AF point out of those available to you, you may even be able to achieve focus on the contrast of the black and yellow bands on the tail of the dragonfly. Given our understanding of aperture and depth of field, we would far rather use a single AF point and achieve focus on the subject.
Even if you are making use of a manually selected AF point, simply placing the AF point onto your subject wont always do the trick.
Again, it comes back to contrast.
You need to try and achieve focus using the greatest point of contrast in the scene.
This second example is a more difficult situation both for the photographer and the camera’s AF system. The scene is pretty uniform in terms of colour and contrast and with so much grass in the foreground and in front of the subject, if left to its own devices the camera will almost certainly focus on the grasses and not the lion.
Using a single, manually selected AF point and placing it on the lions eye (which is usually the most important feature that we want to be in focus) would also present some difficult for the AF sensor. There is very little contrast around the various shades of browns, yellows and reds in that region. This may result in the camera “hunting for focus”. This hunting is the constant shifting of the elements inside the lens to try and achieve some sort of focus and is common in low light, low contrast situations where the camera simply cannot achieve focus.
So where would one focus in this scene?
I chose to focus on the white of the muzzle which is nicely contrasted against the black lip line of the mouth. Even in low light like this the contrast of black against white provides an easier target for the cameras AF system to lock on and achieve focus. Had the light been better I would have, and usually prefer to, focus on the contrast of the white patch of skin beneath the eye of the lion.
Because I wasn’t able to achieve focus in the eye region, I needed to compensate slightly for the shift in my focal plane (or depth of field) which, by focusing on the nose of the lion with a shallow depth of field (small number) may have rendered the eyes and mane soft and slightly blurred. In order to compensate for this, I used a slightly larger F stop value to increase the depth of field in the scene.
Does it matter which AF point I Choose?
Without going into too much detail, yes. all AF points are not created equal and some will have a much better sensitivity to picking up edges and contrast in a scene. This is also linked to your specific camera model with the more advanced cameras having a greater number of far more advanced AF points than the entry level cameras.
Typically the most advanced and sensitive AF point is the central AF point.
The peripheral AF points are however quite capable of achieving focus – in good light.
A combination of subtle vignetting, distortion and the fact that these AF points are less advanced than the central AF points means that the camera will really struggle to achieve focus using one of these AF points in low light.
So, feel free to select any AF point in order to achieve focus and your ideal composition in good light but switch back to your central AF point when light (and therefore contrast) become less intense.
Does the Lens I have have an Impact on the Cameras Ability to Achieve Autofocus?
Most definitely. Remember those numbers on the front of your lens? They tell you what the maximum aperture of your lens is at any given focal length.
The camera will always achieve focus using the maximum aperture (smallest F Number, largest opening).
The smaller the number the larger the opening and therefore the more light that enters through the lens and onto the sensor.
The more light that enters through the lens the greater the contrast and the easier it is for the AF sensors to achieve focus. A lens with a maximum aperture of F2.8 will be able to achieve focus much faster than a lens with a maximum aperture of F5.6. Think about squinting your eyes and looking at an object in the distance, when your eyes are wide open its easier to make out and define the edges of the object.
A simple analogy but hpoefully one which makes sense.
For those more advanced readers, this is why the use of teleconverters will always have some sort of impact on the speed with which the camera is able to achieve AF as the maximum aperture is reduced.
How Do I Manually Select An AF Point?
Set the Mode dial to P, Tv, Av, M, B, or C – You can only specify an autofocus point in these exposure modes.
For Nikon, press the sunken button on the AF Area Selector and using the front dial, change your AF Mode from Auto to S (for single).
For Canon, you’ll need to hit the top right AF Area Selector and then use the front dial to change the AF Area Mode from Auto to just a Single point.
I’ve tried to keep these examples as generic as possible but each camera model has its own subtle differences. This should get you to a single AF point though.
You may notice that there are a number of other AF Areas that you can select – park those for now. If you’re able to select a single AF point you’re already on the way to improving your hit rate in terms of images that are sharp and in focus.
How do I Move the AF Point around?
You’re not limited to using the central AF point in Manual Select mode. After switching to single-point AF, you can use the arrow keys or, with Canon, the joystick to switch to any of the other eight AF points. To return to the central AF point, press the Set button again.
So, you are now able to be precise in terms of the area of the frame where the camera will achieve focus (keeping points of greatest contrast in mind) which combined with your aperture value (depth of field) will help you ensure that your subject appears sharp and in focus.
If you’re still not getting good results, the source of your issues may not be with the autofocus but rather with Shutter Speed.
And that will be the feature of next weeks post.
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