Thursday, 29 August 2013

Hyper Focal Distance in Digital Photography - What is it? - How to Calculate? and How to Use it to Maximize Depth of Field

hyper focal distance
Photo by: Wynand

Factors that Influence Depth of Field

Depending upon the subject and situation; we might either wish for a shallow or a larger depth of field effect to our images. Certain genres like portrait photography go along with shallow depth of field effects to make their subject stand out from the rest of the elements. This is fairly easy to achieve, only requirement being a lens with a very wide aperture (get a 50mm f/1.8 for approx. $100 and you’re all set). But certain other genre of photography, like landscape photography often requires employing a very large depth of field to render objects across the frame, a few inches from the camera to infinity in sharp focus.

As photographers we are all familiar with the concept of depth of field and well aware of how it is influenced by the following four factors.

1. Focal Length
2. Aperture
3. Sensor Size and
4. Focusing Distance

Obtaining large depth of field is not as simple as getting it shallow, there are many factors that work against us when we try to do just that. Let us have a look at the four factors that we mentioned above that control depth of field.

1. Focal Length

The wider the angle of view of a lens is the greater the depth of field and vice versa. However the angle of view required for a frame is influenced by how we wish to capture the scene and it cannot be changed so as to increase depth of field.

2. Aperture

A narrower aperture generally yields greater depth of field; thus to a limit aperture can be used to increase depth of field in an image. There is a certain limit to it; every lens has a certain usable aperture beyond which if narrowed; it will adversely affect image quality as diffraction sets in. In most cases it is not recommendable to go beyond f/16. Moreover other factors like the amount of light available at the scene will also come into play when deciding aperture.

3. Sensor Size

A larger image sensor in the camera gives shallower depth of field. Even though a smaller sensor will produce images with slightly more depth of field than a larger one; but we do not have the flexibility to change sensor size as required. Using smaller sensor also causes a decrease in the value of the narrowest aperture before diffraction sets in. 
A bigger sensor can be stopped down a lens till f/22 without causing diffraction, whereas a camera with a smaller sensor can be stopped down the lens only to f/16; thus practically both cameras end up producing images with similar depth of field.

Now that we have discussed three of the factors that control depth of field and limitations on using them, let’s discuss the fourth factor - focusing distance. This is one element that we could make use of without any limitations and applied properly will help us get the maximum possible depth of field in our images.

4. Focusing Distance

The closer the focusing distance is the shallower the depth of field and vice versa. However when you are trying to shoot a landscape you have the flexibility to either focus up close on a foreground element, focus on something in the middle of the frame or on something that is far in the background. No matter where you set your focus to, it is nearly impossible to get the entire scene in sharp focus. When focus is set on a subject in the foreground elements in the background will be blurred, similarly if you focus on a background element, elements in the foreground will be blurred!

But there is one focusing distance determined by the size of your camera sensor, the focal length of the lens used and the aperture used, which will give you maximum depth of field. Welcome to hyper focal distance focusing.

What is Hyper-focal Distance in Digital Photography?

 Hyper-focal distance is the point which when focused at will maximize the depth of field. When focused at the Hyper-focal distance, everything from half the hyper-focal distance to infinity will be rendered acceptably sharp. This means that if you’re focused at a hyper-focal distance of 5 meters then, everything from 2.5 meters to infinity will be sharp.

How do you find the hyper-focal distance?

To set our lens to hyper-focal distance we should first find out what is the hyper-focal distance for that particular camera, lens, aperture and focal length. We’ll start with the formula for calculating hyper-focal distance. 

Hyper focal distance
Hyper focal distance

Hyper focal distance
Hyper focal distance

I know it looks scary, don’t worry; it’s much less complicated than it looks!

Let us consider the different variables first, we know the focal length we have set so that part is covered, next we know the aperture we wish to employ so that is also covered , what remains now is the circle of confusion.

Circle of confusion

Circle of confusion is based on the theory of what is considered as acceptably sharp in an 8” x10” print seen at normal viewing distance. It varies depending on the size of your camera’s image sensor.

For DSLR’s with Full Frame Sensor it is = 0.03 and
For DSLR’s with crop sensors it is = 0.02

Just remember the value for the camera you use and now you have all the variables required to calculate hyper-focal distance to be used in any situation you might ever encounter.

Choosing a Lens

Hyper-focal distance focusing is best done of normal to wide angle lenses. In terms of full frame cameras it is focal lengths of 50mm and shorter. When set to larger f numbers, they have relatively short hyper-focal distance. For example when a 24mm lens is set to f/16 the hyper-focal distance is 1.22 meters. Everything from half a meter to infinity will be rendered acceptably sharp.

In general telephoto lenses are rarely used for hyper-focal distance focusing. The main reason being the hyper-focal distance is quite distant with a long lens. For example, the hyper-focal distance for a 200mm lens set to f/16 on a 35mm camera is about 275 feet.

Rule of Thumb for Hyper-focal Distance

hyper focal distance
Photo by: Chaval Brasil

When photographing scenes that  either does not include the near foreground or do not extend all the way to the horizon, the rule of thumb states that you should focus roughly 1/3 of the way into your scene in order to achieve maximum depth of field.

Although this works reasonably well it is not the optimal solution as the hyper-focal distance is depended on many factors like subject distance, aperture set and focal length.

Smart Phone Apps

There are applets that can be downloaded and used with smart phones like DOF Master and Tack-Sharp which will tell you the precise hyper-focal distance for your camera, lens and aperture combination.

Using Hyper focal Distance in Practice

As a creative photographer one must master hyper-focal distance to get the most out of any situation. But blind adherence to any method regardless of the theme is not recommended. Focusing at hyper-focal distance is not for every situation; it works well when the subject matter extends far into the distance, and if no particular region in the frame requires more sharpness than others.

But when photographing scenes which has elements either in the front or back that require more sharpness; set focus accordingly. Over emphasis on hyper-focal method often neglects regions of a photo where sharpness is critical. For example, a finely detailed foreground element may demand more sharpness than a hazy background. Alternatively, a naturally soft foreground can often afford to sacrifice some sharpness for the background.

Some Practical Tips that are Useful in the Field

hyper focal distance
Photo by: Doug Wheeler

Focus a little beyond the hyper-focal distance

Remember that hyper-focal distance is just a reasonable estimate for actual photographic lenses.  . Make your best estimate of where the hyper-focal distance is from the camera position and focus a little beyond the hyper-focal distance and you'll know that everything from at least 1/2 the focus distance to infinity will be acceptably sharp in the photo.

Stop Down One More Stop

Once you have calculated the hyper-focal distance and set the focus, stop down your lens one more stop; this will give you some leeway and any slight mistakes will thus be covered.