Understanding Focus

Focus

Welcome to my comprehensive guide to understanding focus. Understanding focus is key to learning photography, and hopefully this article will clear things up for you (pun 100% intended!). In order to be able to control focus in our photos we must first define what it means to be in focus. When we say something is in focus, what's technically happening is that the position at which rays of light from a lens are converging to form a sharply defined image. Camera lenses can be configured to focus on a specific distance from the camera, where objects at this distance are in focus, and all objects exactly this distance from the camera form what is called the "focal plane". In addition to the focal plane, there is a range of distances in our images starting from a distance in front of the focal plane, and extending to a distance behind the focus point, where objects are considered acceptably sharp, this is called the depth of field.


Focus

The distance from the nearest acceptably sharp distance to the focal plane will always make up the first third of the depth of field, while the distance from the focal plane to the furthest edge of the depth of field will make up the remaining 2/3 of the depth of field.

In landscape photography, we often want to maximize our depth of field so that as much of our frame as possible is acceptably sharp. In portrait photography we often want to minimize our depth of field so that just a face, for example, is acceptably sharp, and isolated from the background which is out of focus, this is known as a bokeh and is discussed in my guide to understanding the exposure triangle article. There are 4 factors that determine the size of the depth of field for a given shot.

1)Focal Length: The longer the focal length, or more zoomed in our cameras are, the narrower the depth of field we'll have. While we'll typically set our composition and focal length based on other factors (see my article on composition), it can be useful to move further away from your subject and use a longer lens for portraits, for example, to create a narrower depth of field and more isolated subject.

2)Aperture: As discussed in my guide to understanding the exposure triangle, the narrower the aperture, or larger f-stop will result in a larger depth of field. So you may be thinking that for landscape photography we would want to set our aperture to the narrowest setting possible, such as f22, but that is not the case. Camera lenses tend to degrade in sharpness as the f-stop gets higher, a side effect called diffraction. Lenses typically have a sweet spot usually about a step above their maximum aperture, for example my 14-24mm f/2.8 lens has a sweet spot at f5. Since f5 usually gives me too narrow a depth of field particularly when shooting with close subjects in the foreground, I'll typically shoot landscapes between f8 and f11.

3)Distance to focal plane: For a given focal length and aperture, the closer the focal plane, the narrower the depth of field.

4)Camera sensor size: Depth of field is inversely proportional to the format size when the effective focal length is factored in. So the larger the sensor, the smaller the depth of field. While this is good to know, it is also a constant for us, unless we're going back and forth between a full frame and crop sensor.

How to Apply Focus in Landscape Photography

Particularly in landscape photography, we want to maximize our depth of field. Assuming we make our composition independent of focus, this leaves aperture and our focus distance as the only variables that impact the size of our depth of field. Assuming that we are shooting from a tripod at a perfectly still scene, we should be able to shoot at a pretty high aperture without having to worry about motion blur, and our only concern being the aforementioned loss of sharpness that comes along with shooting at high apertures. Said differently, when shooting landscapes, we want to shoot at the lowest f-stop possible while maintaining acceptable focus throughout the entire scene. This is where things get, for lack of a better term, fuzzy.

Hyperfocal Distance

Given all the factors that contribute to the depth of field, there is a distance at which when focused upon will yield the maximum depth of field where from 1/2 of this distance all the way to infinity, all objects appear acceptably sharp, this distance is known as the hyperfocal distance and generally speaking is where we'll want to focus when shooting landscapes. The hyperfocal distance is based on the following formula:

Hyperfocal Distance = (focal length)squared / (f-stop) * (acceptable circle of confusion) + focal length

Without getting too sidetracked, the acceptable circle of confusion is a constant related to the noticeability of blur in photographs. Depending on the size of a print, the viewing distance, and the quality of eyesight in any individual this constant can be adjusted but for the purposes of the below chart we'll use the value 0.03mm which is based on the assumption that a photograph is viewed in 8x10 print size from a distance of 10 inches by a person with 20/20 vision.

Let's take a look at a chart displaying the hyperfocal distances for a 14-24mm f/2.8 lens. You should generate a chart based on your favorite lens' aperture and focal lengths.

Focal
Length
Aperture
f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22
14mm 7.7 5.4 3.8 2.7 1.9 1.3 1
16mm 10 7 5 3.5 2.5 1.7 1.3
18mm 12.7 8.9 6.3 4.4 3.2 2.2 1.6
20mm 15.6 10.9 7.8 5.5 4 2.7 2
22mm 18.9 13.2 9.5 6.6 4.8 3.3 2.4
24mm 22.5 15.7 11.2 7.9 5.7 3.9 2.9

So this chart is saying that for an aperture of f/2.8 at 14mm, I need to focus on an object 7.7 feet away, and any object 3.85 (1/2 of 7.7) feet away or further will be in focus. As we increase our aperture, objects closer to us start to become more in focus, so for example, at f/16, still at 14mm, I can set my focus point to 1.3 feet from the camera, and objects .625 feet and further will be in focus.

One major caveat of the hyperfocal distance is that it requires a preselected aperture in order to determine the focus distance, but often, we want to choose the aperture based on what we want to be in focus. It's can be a sort of chicken or egg problem.

As a software engineer, I like to think about things algorithmically. Having a chart of hyperfocal distance calculator can be helpful in choosing both our focus distance and aperture. Let's take a look at a practical example. Suppose that I was shooting at 14mm, and the nearest foreground object that I wanted in focus was 1.5 feet away, what aperture would I choose, and what would be my focus distance?

Ideally, I would like to shoot at f/5.6 since that is the sweet spot of my lens. At 14mm though, my hyperfocal distance would be 3.8 feet, and objects would be in focus starting at half that distance, or 1.9 feet away, to infinity, not quite what I am looking for. If I bump up my f-stop to f/8 though, that gives me a hyperfocal distance of 2.7 feet, with objects in focus starting at 1.35 feet away, perfect!

So you may be asking yourself, do I go through this exercise for every single landscape photograph that I take? Of course not. In practice, you'll start to get a better idea of what f-stop you need to choose, and where to set your focus based on the proximity to foreground objects in your frame. You can use the viewfinder, or better yet, the live view on your camera to zoom in and decide if everything is acceptably sharp. There will be times when shooting landscapes with objects extremely close, where it is impossible to capture both the foreground and the background in an acceptably sharp manner. Though I usually advocate for getting it right in the camera, there is a technique called focus-stacking, where images with foreground elements in focus are merged with images where background elements are in focus to form a single image with both foreground and background in focus.

Focus In this image the foreground was captured in focus.
Focus In this image the background was captured in focus.
Focus These two images are blended together to form a single image with everything in focus.

Focus Settings on the Camera

Our camera's sensor is equipped with focus points which can automatically adjust our lenses to set the focus point at a specific object, and therefore our focal plane at the distance to that object. There are two ways which our cameras our able to determine the proper focus in a given scene. The first method, passive autofocus, works by finding small lines of sharp contrast in our frame, and adjusting the focus until it detects the sharpest contrast. The second method, active autofocus, works by sending a beam of red light at the subject, and using the response to determine the distance of the focal plane. For either one of these methods, we are required to put our focus point(s) on objects with detail as opposed to a uniform object such as a clear blue sky.

Now that we understand what a focus point is, and how to optimize the position of our focal plane, let's discuss the various options our camera has for determining our focus point:

Camera Focus Modes

1)Single Shot Autofocus: Our camera sets the focus for every image after we press the shutter button halfway. This mode is great for getting the focus exactly where we want it but it tends to fall short when we are shooting a moving target.

2)Continuous Autofocus: Like single shot autofocus, our camera will begin to set the focus when the shutter button is pressed halfway, but will continue to focus on the subject while the shutter is fully pressed. This can be especially useful when used in conjunction with burst mode so that we can hold down the shutter button and get in-focus shots of a moving subject.

3)Manual Focus: The camera will not perform any autofocus.



The aforementioned settings are independent of autofocus area modes. Remember our cameras are equipped with many autofocus points, the autofocus area modes determine how these focus points are used together or independently to determine the proper focus.


Autofocus Area Modes

1)Single Point Autofocus: In single point autofocus, just one autofocus point is used to determine the focus, the point can be changed by using the "d-pad" on our camera.

2)Dynamic Area Autofocus: Similar to single point focus, in dynamic focus, the photographer selects a single autofocus point to use and focuses on the subject, the camera will use nearby focus points when necessary to track and maintain focus on the subject while it is moving. The idea here is that the subject of the photo is composed to a specific place on the frame and as long as that subject is kept in that relative area, it will remain in focus using the nearby autofocus points. The range of nearby autofocus points can be configured in the camera.

3)Auto-area Autofocus: In auto area autofocus, the camera will automatically pick which focus point to use for you. While the camera's ability to select this, has and will continue to improve with time, this mode gives no control to the photographer and really isn't recommended for practical use.

Ok, so now that we understand how to take control of our cameras autofocus, let's talk about the various shooting scenarios that would require different combinations of autofocus settings.

1)Landscapes and architecture: Mode: Single Shot, Area: Single Point. Generally, this is how I apply focus when shooting landscapes and architecture. These settings allow me to take my composition, and set my focus precisely on the object that I want to serve as the focal plane. If that object doesn't happen to be on a focus point, i'll apply a "focus and recompose" method, where i'll compose the shot, then turn the camera so that the subject is on a focus point, set the focus, then recompose to the original composition and take my shot. This can be accomplished either by holding down the shutter button halfway after obtaining focus, switching the focus mode to manual after setting the focus, or using the autofocus lock button on your camera. Manual is another option for shooting landscapes and architecture. Manual focus can be used by adjusting the focus either in the viewfinder or live view mode to get proper focus, or, if you are really trying to optimize the depth of field, you can use a distance calculator in combination with a depth of field optimizer to determine the ideal distance of focus.

2)Photographing still people: Mode: Single Shot, Area: Single Point. Similar to shooting landscapes and architecture, it's best to use a single focus point that you can control. The rule of thumb for photographing people and wildlife is to set the focus point on the subject's eye.

3)Photographing Sports: Mode: Continuous, Area: Dynamic. With fast moving people we want the camera constantly focusing. Furthermore, while we can typically control where the subject of our photos fits in terms of the composition, we don't always have the ability to keep them at an exact focus point which is where dynamic autofocus comes in handy. It can be "forgiving" in that if we miss the exact focus point we are aiming for, one of the nearby points can come to the rescue.