Hyperfocal Distance & Diffraction

Alpha Tutorial :27 - Hyperfocal Distance & Diffraction

One of the challenges of landscape photography is focusing so that everything in your photograph looks sharp. In order to do that, you have to understand the hyperfocal distance and the effects of diffraction.

Hyperfocal Distance

The hyperfocal distance is the distance at which to focus a lens, at a given aperture, so as to make everything from half of that distance (to you) and behind that distance acceptably sharp. Simply put, you focus at the hyperfocal distance to shoot with as deep a depth of field as possible to make the most detailed image you can.

Hyperfocal Distance

Acceptably Sharp

When talking about the hyperfocal distance and maximum sharpness, we have to understand what constitutes sharpness in an image.

Our eyes have a finite ability to see fine detail, so sharpness in a photograph is also a condition of just how much a person can discern fine detail at the image’s viewing size. A photo might look acceptably sharp when printed out to 4 x 6 inches, but out of focus areas will look more obvious if the same photo is printed five times larger.

So how do we define what is acceptably sharp? Lenses cannot focus on everything perfectly in an image, so the term ‘circle of confusion’ (COF) is used for the smallest blurred spot a lens can make which is indistinguishable from a point.

The COF is generally accepted to be 0.03mm in a 35mm or full-frame digital camera, with the tolerance that the image can by enlarged and viewed by up to five times and still look acceptably sharp. If you’re not confused yet, the COF will come up if you decide to calculate the hyperfocal distance using a formula.

Finding the Hyperfocal Distance

There are a few ways to discover the hyperfocal distance for a given aperture and focal length, ranging from guesswork to precise calculation. Some use a rule of thumb to focus roughly one-third of the way into your scene, but this hyperfocal trick depends mostly on luck.

Another way to find the hyperfocal distance using digital DSLR cameras is to focus your lens on the background (infinity) and take a shot. Then, using the enlarged preview on your LCD, find where your image transitions from looking sharp to looking blur – that distance in your image is roughly your hyperfocal distance.

This shortcut is also tricky, if you focus closer than the hyperfocal distance, your background will appear soft. If you focus further than your hyperfocal distance, then you end up with a shallower depth of field than best possible. But since you have a digital camera, you can test your estimation by shooting an image and previewing it at 100% to see if it’s acceptably sharp.

Finally, another way to find the hyperfocal distance is to calculate it using this equation:

Hyperfocal distance = (Focal length)2/Aperture x Circle of Confusion

For example, if we’re shooting a scene using a focal length of 50mm at f/8, the hyperfocal distance would be 10416mm, or 10.4m:

10416mm = (50mm)2/8 x 0.03

Focusing your lens at a distance of 10.4m should then make anything starting at a distance of 5.2m away from you to infinity be within reasonable sharpness. If you don’t feel like doing math in the field, there are hyperfocal charts which you can carry and smartphone apps to help you calculate the distance.

Although we use 0.03mm as the circle of confusion, it actually differs among different camera sizes due to different sensor sizes. With the exception of full-frame DSLR and medium-format cameras, smaller sensors will have a COF of about 0.02mm instead.

Breaking the Hyperfocal Distance

While the hyperfocal distance is a neat tool to help you get maximum sharpness, it doesn’t mean that all areas of your image will be equally sharp; instead there are varying shades of acceptable sharpness.

Compare this to when you shoot a single tack-sharp subject and have everything else be less sharp. Sometimes this is just what you want to do to emphasize your subject, as well as use the contrast between sharpness and blurriness to create the illusion of depth.

You don’t always want everything in your photo to be sharp and in focus. Photo by Shinya Morimoto

You don’t always want everything in your photo to be sharp and in focus. Photo by Shinya Morimoto.>

Diffraction

Since the narrower the aperture the deeper your depth of field becomes, it makes sense to simply increase the f-number to as high the lens goes in order to get as much of the scene into focus – right? Unfortunately, at the narrowest apertures something called diffraction kicks in, which softens the details in your images.

At the narrowest aperture settings, less details are actually captured due to diffraction.

At the narrowest aperture settings, less details are actually captured due to diffraction.

On the other hand, this doesn’t mean that larger apertures are better. Most lenses are soft in the corners when used wide open at the largest apertures available, and the sharpest images are available using f-numbers somewhere in-between the widest and narrowest settings.

Diffraction affects each lens and camera differently. Some diffraction is fine if a narrow aperture setting is unavoidable, but if it matters to you, you will have to test your lens to see at which point image softening becomes unacceptable.

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