Friday, November 8, 2013

Understanding and Achieving Image Focus and Sharpness: Part II

Nikon 51 point Autofocus System layout in the Nikon D4 DSLR, photo courtesy of Nikon Corporation
Image 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, below, we'll discuss 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.

DSLRs and some digital compact cameras have both manual and autofocus. DSLRs with specific lenses can also use a combination of manual and autofocus, which for some kinds of photography, such as wildlife photography, can be very helpful.

Most of the time, using your camera's autofocus is the best focus method to use. It's normally fast and accurate, but in Part I we learned that low light levels, lack of contrast in our subject, and camera and/or subject motion can degrade autofocus. In fact, according to conditions, it may be best not to use autofocus, and instead manually focus your camera.

Here are some situations photographers encounter for which manual focus will likely be a better choice than autofocus:
  1. Night or low light photography — with little or nothing for your focus sensors to see, a switch to manual focus would be in order,
  2. Subject lacks contrast or texture — if there are no clear areas of contrast, texture, color or tonal differences in your subject, and no nearby object on which to focus, swap to manual focus,
  3. Subject in motion — fast moving objects can be difficult to get in focus, even if you have great panning skill, and sometimes, even with high quality continuous focus mode. Instead moving to manual focus, and pre-focussing on a particular location at which the moving object will pass may be your best bet.
  4. Macro/close-up photography — with the extreme short depth of field typically encountered in macro photography, focusing must be precise, and exactly on the specific spot on the subject desired. Therefore, most macro photography is accomplished with manual focus. (In addition to manual focus, many macro photographers additionally mount their camera/lens on focusing rails to improve the precision of their focus.)
  5. Shooting through glass, fences, cages, etc. — autofocus often gets fooled when trying to shoot through something else, as it's in the foreground, in front of your subject. Often when shooting an animal in the zoo, autofocus focuses on the cage, or when focusing on an object under glass, fingerprints, scratches and reflections can also fool autofocus. In wildlife photography, a small bird among twigs, leaves, and branches will be a tough autofocus.
  6. Fog — is another difficultly for autofocus, as there is generally a lack of any contrast when shooting through it.
  7. HDR Photography — When you're shooting HDR you're merging a series of shots together. To have the best final image, you need to ensure your camera's position, focus point and focus remains identical from shot to shot, which is best accomplished with manual focus.
Some lenses have the ability to be both manually focused and autofocused, when the camera is set to autofocus. Other lenses don't. Being able to zero in toward good focus, then finish with manual focus can be a major help in quickly obtaining good focus for such genres as wildlife photography, especially when shooting small birds perching in trees.

DSLRs offer both single focus and continuous focus modes, as do some compact cameras. Recently, a third focus mode, automatic, as been introduced.

Single focus (One-Shot AF (Canon) and AF-S (Nikon)) is a mode where when you activate autofocus, the camera focuses on the subject just once, and doesn't refocus unless autofocus is reactivated. This mode is ideal for subjects which aren't moving. The mode works best when a cross type focus point is chosen for single focus as it can detect contrast patterns in multiple directions to quickly focus on the subject.

DSLRs allow photographers to pre-choose a focus point and lock the choice so that it's preset. When using single focus, I normally preset the center focus point (cross type), focus on whatever point on my subject works best for the image, lock the focus, reframe my shot as needed, then make the photo. While you can choose a different focus point so you don't have to reframe your image after focusing, choosing a different focus point for each image is generally more time consuming than reframing.

Continuous focus (AI Servo AF (Canon) and AF-C (Nikon)) is a mode where when you activate autofocus, the camera focuses on the subject, and refocuses as necessary as it detects subject movement. In this mode, as long as autofocus is active, the camera will continuously refocus to keep the subject well focused, however, there are times the camera may not accurately predict the movement of the subject, and not be able to keep it in focus, especially if it's moving quickly and erratically.

Automatic focus (AI Focus AF (Canon) and AF-A (Nikon)), is a mode in which the camera decides which focusing mode, single or continuous, is appropriate for the image being photographed. On cameras which require accessing the menu system to change focus mode, this mode can be very helpful, but for cameras with easy to use external controls to change focus mode, I prefer to directly set my focus mode, as I've found under automatic, the camera's choice is not necessarily mine.

In Part III of the series, 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.

1 comment:

Dan-Dublin said...

Wow, I hadn't thought that there were so many situations where I should swap to manual focus, even though I was having trouble with some. Thanks.

Post a Comment