Hyper focal Distance& Depth of Field

Hyper focal Distance and Depth Of Field Made Simple(er) by Sean Bagshaw

November 23rd, 2011 by photocascadia

One of the questions that I repeatedly get via email and at field workshops is about how to achieve the best focus in landscape images. This has proven to be one of the most difficult topics to explain and understand without advanced degrees in optical physics and mathematics (seriously…just look up hyperfocal distance on Wikipedia). Furthermore, even though it would be nice to have a simple set of rules or formulas to follow, as with most photography related topics, there is no single answer. Where to focus is largely determined by the characteristics of the scene, your specific camera settings and your own artistic intent as a photographer. Additionally, like most things in photography, focus is a game of trade offs. To improve the focus in one part of the image means giving up some focus somewhere else. Within a single frame it isn’t possible to have complete focus throughout the entire scene.

This image is acceptably sharp from front to back. Using hyperfocal focusing in this scene worked well because both near and far subjects need to be equally sharp. The hyperfocal distance can be determined from a chart or approximated by feel with practice.

Let’s begin by describing some basic focus terms in plain English:

  1. The focal plane or focal point is the actual point you have focused on in the scene.
  2. The only spot in the scene that can be in complete focus is at the focal plane. As you adjust focus, the point of complete focus either gets closer to you or further away.
  3. Focus decreases on either side of the focal plane.
  4. Depth of field is a measurement of how quickly the sharpness defocuses on either side of the focal plane.
  5. With a narrow depth of field the focus drops off quickly on either side of the focal plane.
  6. With a wide depth of field the focus drops more slowly on either side of the focal plane.
  7. Landscape photographers often like to use a wide depth of field so that as much as possible has “acceptably sharp” focus.
  8. The smaller the aperture, the greater the depth of field; in other words, the greater the area of the image that has acceptably sharp focus. I commonly use apertures between F/11 and f/18 to take landscape images that have the sharpness I want from front to back.
  9. Depending on the lens and camera combination, very small apertures can cause blurriness due to diffraction even though the depth of field is greater. Some lens/camera combinations have noticeable diffraction blur at f/14 or lower. However I often use apertures of f/18 to f/22 with my lenses with acceptable results.
  10. Hyperfocal distance is the distance from your camera at which you can focus to achieve maximum acceptable sharpness from about half the hyperfocal distance to infinity. For example, if the hyperfocal distance is 10 feet then focusing on an object 10 feet away will give you acceptable sharpness in the scene from five feet away to the most distant object in the scene.
  11. Acceptable sharpness is a relative term. In general the area within the depth of field is considered to be “acceptably sharp”. Acceptable sharpness depends on display or print size, viewing distance and your own expectations and standards as a photographer. Acceptable sharpness may only be barely acceptable to you at the outer edges of the depth of field.
  12. The hyperfocal distance changes depending on a variety of factors including the aperture, the lens focal length, the distance from the camera to the subject and the size that the image will be displayed or printed. Unless you want to do the calculations yourself it is helpful to use a chart to determine the hyperfocal distancefor any given situation.

 

Now let’s address some common questions and misconceptions about hyperfocal focusing:

Q: Does using the hyper focal distance mean that everything in the scene will be completely sharp?

A: No it doesn’t. Complete focus only occurs at the focal plane. Focusing at the hyper focal point only ensures that there will be acceptable sharpness from half the hyper focal distance to infinity. Objects closer than half the hyper focal distance will not be sharp and sharpness near the edges of the depth of field will be barely acceptable. How far the acceptable sharpness extends from the hyper focal point is determined by the aperture being used.

Q: I have heard that focusing one third of the distance into any scene will approximate hyperfocal focusing. Does that actually work?

A: Since I can’t say it better myself I’ll quote Sean McHugh from the Cambridge In Colour site. Sean says, “I encourage you to ignore [the 1/3 rule] since this distance is rarely optimal; the [hyper focal point] actually varies with subject distance, aperture and focal length. The fraction of the depth of field which is in front of the focal plane approaches 1/2 for the closest focus distances, and decreases all the way to zero by the time the focus distance reaches the hyperfocal distance.The “1/3 rule of thumb” is correct at just one focus distance in between these two, but nowhere else.”

Q: How do I determine where the hyperfocal point is in any given situation?

A: This involves some math that is beyond me to calculate quickly in the field with or without a calculator. THIS ARTICLE on the Cambridge In Colour site has a great tool for calculating hyperfocal distance. This works great if you are sitting in front of your computer. For finding hyperfocal distance in the field there are many charts on the web that you can print out and carry with you, such as THIS ONE.

Q: Is the hyperfocal point always the correct place to focus in a landscape image?

A: No, it isn’t. While it may be the best focus point in some cases, every scene is different. It is up to the photographer to decide what part of the image takes priority for being sharp. In scenes with very close, large, detailed or visually important foreground subjects it may be critical to focus closer than the hyperfocal point so that the subject has maximum sharpness, even though it comes at the expense of some background sharpness. Scenes with softer or less important foregrounds may allow focusing beyond the hyperfocal point to achieve sharper objects in the distance. This is where a lot of practice and getting to know the performance of your equipment really pays off. More on that later.

Q: Does every composition have a hyperfocal distance that will allow for acceptable sharpness throughout the entire image?

A: Not always. In cases where objects are very close to the camera (inside 1/2 the hyperfocal distance), there is not a small enough aperture to allow everything in the scene to be acceptably sharp. This is true with wide angle landscapes with extremely close foregrounds as well as macro photography. In such cases parts of the image will not have acceptable sharpness no matter where you focus.

Q: How can I achieve a completely sharp image when the subject is close enough to the camera to be inside half the hyperfocal distance even at the very smallest apertures?

A: Large format cameras with very small apertures and the ability to tilt the focal plane of the lens can achieve extreme depth of field, but still not infinite depth of field. Tilt/shift lenses on 35mm cameras can also effectively increase depth of field in the right situations. Most recently, digital cameras and specialized software have spawned a new technique called focus stacking or focus blending. The best software I know for focus blending is Helicon Focus, although Photoshop CS has the ability to do limited focus blending as well. You can achieve virtually infinite depth of field by taking a sequence of focus “slices” (focusing at various points through the scene) and then using software to combine only the sharpest parts of each slice. This works great for static scenes but is challenging if anything in the scene moves between slices.

How I actually approach focus in the field:

While I find it useful to understand the concepts and underlying mechanics of depth of field and hyperfocal focusing, I rarely, if ever, compute or use actual hyperfocal measurements in practice. I’m usually working in rapidly changing light and recomposing my scene frequently. It isn’t practical to do the math or reference a chart repeatedly, especially when I can get excellent results by using my Jedi-like skills (and by that I mean learning how to focus by feel, which is somewhat like learning to ride a bike or play music by ear). The ability to focus intuitively is the result of practice. It comes from knowing intimately the characteristics of your equipment and spending time going through trial and error until you are able to visualize depth of field and where you need to focus within the scene to ensure sharpness where it is most needed.

I focused this image by feel. With the foreground very close to the camera I used an aperture of f/18 for a wide depth of field. At 19mm I knew the hyperfocal distance was about .5 meters. I focused just past this to ensure adequate sharpness in the rock towers.

If you would like to practice I would suggest taking series of test images. Try shooting different compositions (including very near, middle near and not so near foregrounds) at different apertures, focus points and focal lengths. Then view the images closely on screen to see where each image is sharp and where it isn’t. In time you will begin to know what aperture you need and where to focus to get the sharpness you want for any given type of scene. This process will also help you learn what meets your own personal standards of acceptable sharpness.

Allow me to attempt a simplified explanation of the basic approach I use for focusing in the field. First, I’ll point out that precise and deliberate placement of your focus point isn’t possible if you are using your camera’s auto focus mode. To place your focus point precisely where you want it you will need to use manual focus. I find that for most wide angle (16 to 30mm) landscapes I use an aperture between f/14 and f/18. The closer the camera is to a foreground object the smaller aperture I use. In extreme cases I will use f/20 and even f/22. Some lenses have a lot of diffraction blur at these apertures however, so do some tests with your own gear to find out what works.

In simple terms, as I composes a scene I assess what part of the image gets my highest priority for sharp detail. If it is the foreground, then I focus more towards the front. If it is the background then I shift focus towards the back. If all parts of the image require equal sharpness then I probably end up focusing somewhere near the actual hyperfocal point.

If you are shooting with enough light you can also use the depth of field preview button on your camera to view how much depth of field there is in your view finder. This can be very helpful in selecting your focus point. However, the depth of field preview stops down your aperture to whatever you have it set to. This make the view finder get darker. If you are shooting in low light you won’t be able to see anything making this approach useless.

If I wan’t to place my focus point as precisely as possible I use my camera’s live view mode and zoom in to 100% on the LCD to focus. In cases where I know that I want complete sharpness from front to back and even f/22 won’t give me enough depth of field, then I use f/16 and take several focus slices for later blending using Helicon Focus.

That’s a lot to chew on. Even still, this article is just a basic introduction to depth of field and hyperfocal focusing. You might start out hyperfocal focusing using a chart and then move to focusing more by feel as you gain practice. If you are interested in delving into these ideas even further I suggest the following links:

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