Monday, January 20, 2014

Understanding and Achieving Image Focus and Sharpness: Part III

Nikon 51 point Autofocus System layout in the Nikon D4 DSLR, photo courtesy of Nikon CorporationImage sharpness depends on multiple factors including focus.

While it's true an image can't be sharp unless the subject is in focus, the converse is not necessarily true, that an image which is in focus, will necessarily be sharp. Image sharpness is fundamentally controlled by focus, but there are other important factors which enable in-focus images to be sharp.

In Part I we looked at how modern digital cameras autofocus (AF), and what factors influence AF systems' ability to focus quickly and accurately. In Part II, we discussed choosing the best focus mode for a particular subject, manual or autofocus, or using them together, as well as single versus continuous focus, all to achieve a solid focus for a variety of situations common to travel photographers.

In Part III, below, we'll get to the nitty-gritty of using autofocus and manual focus, including looking at the different AF area and tracking modes, and focus locking methods.

In Part II we learned that autofocus focus has three basic modes: single focus, continuous focus, and automatic focus. As automatic focus mode merely swaps between the other two modes it won't be discussed here. When to use it, was discussed in Part II.

In single focus mode, ideal for static subjects, and even when there is subject motion, at times, the area upon which the camera focuses, is as the name implies, a single area (focus point). You can let the camera choose the area, “auto-area AF” (Nikon) or “automatic AF point selection” (Canon), or choose the single area yourself, manually.

If your subject is “front and center” or otherwise obvious, then “auto-area AF” will likely work perfectly for you. Unfortunately, for many photographers, where you want to focus is more often than not, ambiguous to the algorithm used by the camera to choose its focus point.

Two examples of this focus ambiguity problem travel photographers encounter are when you wish to photograph an interesting person on the street, or a when shooting a group of subjects at multiple distances. In each case, the focus point chosen may or may not be correct. That is all too often the problem with “auto-area AF,” unpredictable results. I don't use this method because if offers no speed of focus, or focus accuracy benefit compared to choosing my own focus point.

If you choose your focus point, you have the choice of every focus point your camera's autofocus system has, both linear and cross type. For example, if you want to focus on a subject seen on the left side of your viewfinder, using the “focus point selector” you can choose a focus point on the left side of your viewfinder directly on that subject. If in the next image, when framing your photo, the subject is to the right, you can change your focus point to one on the right.

Constantly choosing a new focus point to align with your image's subject is generally a waste of time as best, and can result in the loss of a photographic opportunity if while you're setting up your focus, the opportunity ceases.

I've found the best setting for single focus mode is to set the focus point manually to the center focus point, and lock it in place. This eliminates the time wasted to reset your focus point for each image, and ensures you always use a cross type focus point to achieve the fastest, and most accurate focus possible.

To use this setting effective is quick and easy. Frame your image, aim the center focus point at the subject on which you wish to focus, activate and lock your focus, reframe your image to the original and make the photo.

In continuous focus mode, ideal for subjects in motion, the camera has the ability to change the focus point automatically and continually, as the subject you're photographing moves. Unlike single focus where the focus is activated and locked unless reactivated, in continuous focus mode, autofocusing constantly continuous adjusting focus until the photograph is made.

Digital cameras generally give the photographer a choice of how many focus points can be dynamically utilized in this mode, and some have a special mode call 3D Tracking. Typically digital cameras allow the use of one point, all the focus points, and one or more settings of some number of points in between.

The effectiveness of the camera's continuous autofocus system depends on multiple factors:
  • The number total focus points available,
  • The number of cross-focus points available,
  • The speed of the autofocus processor,
  • The type and speed of the subject's movement,
  • The photographer's ability to move the camera with the subject.
I can't emphasize enough, how important the photographer's technique is to produce well focused images of moving subjects.

It's impossible to suggest to precise number of focus points to use for various photographic situations as every camera is different and behaves differently, but it's possible to suggest some “rules of thumb” based on my experience.
  • Despite the claims of camera manufacturers, while 3D tracking and dynamic area continuous focusing has dramatically improved, typically using the maximum number of focus points or 3D tracking is counter productive. I've found that too often, the camera's autofocus 3D tracking system can't keep up with the movement.
  • Start using the smallest dynamic area possible which should be centered around the center focus point. Especially if your subject is moving steadily, this will work especially well as it will likely include a reasonable number of cross type sensors.
  • If your subject is moving somewhat erratically, increase the size of your dynamic area, which increases the number of focus points used. Some will not be cross type, but you have a better opportunity to keep your subject shown on the viewfinder within the area of focus points used to maintain focus.
Focus lock comes into play when you use autofocus and need to ensure are carefully focused scene is not unfocused when the shutter release button is pushed to make the image. In manual focus, the shutter release is decoupled from focusing, so no focus lock is necessary.

I suggest two methods of focus lock.

Most of the time the standard method of focus lock will suffice. To focus your scene you will press the shutter release button halfway to activate autofocus, and lock the focus once the autofocus system has achieved focus. Then while holding the button halfway down you can reframe your image as desired, while maintaining focus. When you then press the shutter release button the rest of the way down to make your image, the focus will not change.

When autofocus is difficult to achieve, such as in wildlife photography when your subject is often obscured, and a combination of autofocus and manual focus is necessary, I decouple my shutter release button from autofocus, and use my AF-On button to activate autofocus as necessary. For DSLR cameras without an AF-On button, you would need to program another button to do the same. To focus, I begin by pressing the AF-On button to activate autofocus to get my focus as sharp as possible under the circumstances, as quickly as possible. I then finish focusing by manually focusing the lens. At this point, because my shutter release can't activate autofocus, the focus is essentially locked. Pressing the shutter release button will make the image, without altering the focus.

In Part IV, I will discuss photographer focus technique, and other factors which enable photographers to make sharp images.


Russ - Chicago said...

Great article. I just programmed in your advice on focus lock into my camera.

Wendy - Norman said...

Ned, do you ever use single point focusing with continuous focusing?

Ned S. Levi said...

Yes I do Wendy, especially if the moving subject is large.

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