(Craft of photography: part 4)
As I’ve said before, one of my objectives is to capture the rich texture and details in the landscape. I have an above average emphasis on resolution and image sharpness. I’ve previously described the importance of shooting technique, aperture selection to avoid resolution-robbing diffraction, and lens selection. All are important but we must insure the lens is critically focused if we are to achieve an acceptably sharp image that maximizes the detail rendered on the sensor.
In a modern DSLR, there are basically three approaches to focusing:
- Auto-focus using phase detection AF
- Auto-focus using contrast detection AF
- Manual-focus using live view with the camera’s display enlarged on the area of interest
Phase detection AF (PD-AF) is the classic SLR auto-focus method and has been around since the earliest days of auto-focus film cameras in the late-1980s. The principle advantage of PD-AF is speed: cameras focus rapidly and generally follow moving subjects quite well. The industry has continuously improved the method and the latest cameras are able to focus in exceptionally dim light, as low as -3 eV for the current generation.
The principle disadvantage of PD-AF is the location of the phase detection array: it’s normally located in the pentaprism, not on the image sensor. Focus is being controlled over a light path that is not the path of light that will be captured by the sensor. Manufacturing tolerances in the lens (e.g., the lens element to flange distances) and in the camera body (e.g., placement of the mirror and placement of the phase detection array) contribute to focus errors. The better consumer and virtually all professional DSLR bodies allow you to compensate for these errors on a per-lens basis (that is by calibrating a specific lens to a specific body).
The second disadvantage of PD-AF is the location and number of AF “points.” Today’s better DSLRs provide 50 to 60 AF detection points – some working horizontally, some working vertically, some working in both horizontal and vertical modes. AF points are of necessity clustered within roughly the central half of the sensor area. Want to set the focus point outside the area? Aim the camera, lock focus and then recompose with attendant errors in focus as you recompose.
The last disadvantage of phase detection AF: the focus sensors are not necessarily exactly located where the display shows them to be. You may think you are focusing on a given point but the camera could be locking on a high contrast area in the image a bit away from the displayed point.
If you are a nature photographer shooting moving subjects such as a bird in flight, today’s phase detection auto-focus is a godsend, providing a much higher “keeper” rate than most people ever thought possible with manual focus or with earlier phase detect AF. If you are a landscape photographer dealing with a static landscape, you can do better.
Contrast detection auto-focus (CD-AF) has been in DSLRs since the advent of “live-view” circa 2007-2008 and is the principal method of auto-focus in mirrorless cameras. Contrast detection uses the photo-sites on the image sensor itself to drive focus. The contrast across adjacent pixels is maximized when a subject is in focus. Contrast detection eliminates the errors that accrue from out of path focusing in phase detection. By design, contrast detection automatically compensates for manufacturing tolerances in the lens and camera body. Lastly, we can set the focus point to be any place in the image; no longer are we limited to using phase detection points in the central part of the image.
The first drawback of CD-AF is focus speed: it frankly is much slower than phase detection. This is true both in terms of the way you place the AF point (on the rear display) and in terms of the amount of hunting the AF system does searching for the point of maximum contrast. The nature photographer capturing moving birds would be hampered; focus speed is not an issue for the landscape photographer. In my experience, the second drawback of CD-AF is required minimum light levels: it frankly needs a bit more light than the best phase detection systems. This can be an important consideration in dim dawn and dusk shooting conditions – some of the prime shooting times for my practice of landscape photography.
Lastly, we have my preferred focus method: manual focus using live-view. With the camera’s display zoomed into the part of the image I want to use to set focus, I manually rotate the focus ring until I achieve optimal focus. This takes practice and patience: there is a (small) time lag between moving the focus ring and changes to the display. While this lag has become very short in the latest cameras, it is still not zero. With practice, you learn to compensate and can generally nail focus. This method shares some of the advantages of CD-AF: we are using image information from the sensor itself to judge focus. Some cameras use the lens’s “taking” aperture to drive live-view. To the extent this is true, you can inherently check for depth of field on the display. If your “taking” aperture provides considerable depth of field, you may find it hard to focus accurately. Open the lens to its widest aperture, set the focus manually, then set the lens to your desired shooting aperture.
Make no doubt, manual focusing with live-view is much slower than auto-focusing and suitable only for static subjects and frankly only when you have the camera mounted on a tripod. Too, like CD-AF, it does not work well in very low-light conditions such as those at the time of civil sunrise and sunset (generally 30 minutes before sunrise and after sunset). But as the sky starts to brighten as we near sunrise, the method is my preferred approach to achieving spot-on focus accuracy in my images.