Using Autofocus

A few recent questions on my favorite travel bulletin board and a chance encounter with someone new to DSLRs have convinced me that an article demystifying autofocus might be a good use of my time. So, here goes...

On all but the simplest point and shoot cameras, there are usually three (or more) autofocus modes: Wide, Center and Spot. Most cameras default to wide when dragged from the box and powered up. Wide is good for a wide range of general situations and many people will happily use this default for the life of the camera without ever using or even being aware of other modes. This article is not for them. If you want to skip the history and how-does-it-work section, click here

Origins

Autofocus was introduced to SLRs in the early ‘80s with a couple of models from Pentax and Nikon. These used focus sensors in the body and lenses with motors. Nikon released an 80mm f/2.8 and a 200mm f/3.5 while the Pentax was only available with a 35-70mm f/2.8 lens. Neither was a commercial success and it wasn’t until Minolta introduced the Maxxum 7000 in 1985 with autofocus sensors and motor integrated into the camera with a variety of new screw-drive lenses available that it saw wider acceptance. Canon released the first EOS autofocus camera in 1987 using lens-based focus motors. Advances in technology making the lens motors easier to manufacture and more practical than the earlier attempts. Nikon followed with a in-body motor system like Minolta but changed to lens-based motors in 1992. As of this writing, all manufacturers have moved to lens-based motors with varying support for older screw-drive lenses across their product lines. By the early ‘90s, autofocus was available from all the major players and models without it were rare.

Types of Autofocus

Active autofocus

This involves using a focusing system separate from the optical system that generates a signal, reads the results and adjusts focus. An example would be the ultrasonic sonar system used by Polaroid in it’s SX-70 instant camera. It projected a sound from the camera and measured the the amount of time between the sound and the echo off of the subject to focus the camera’s lens to the calculated distance. Another system used a pair of projected beams of infrared light to measure the distance using triangulation. These worked for point and shoot cameras that had a wide depth of field but didn’t lend itself to the kind of fine focus needed in an SLR or advanced video cameras.

Passive autofocus

This method analyzes the image that the camera sees through the lens and is the most common method in use today..  

Phase detection autofocus was adopted by SLR manufacturers and is still the predominant technology. It works by diverting (using a partially transparent section of the viewfinder mirror) and splitting a small portion of the image through a pair of micro-lenses and onto tiny CCD sensors behind each micro-lens. The autofocus system analyzes the signals from the CCDs as the lens adjusts and when they match, the section of the image in the sample area is in focus. If this sounds like a complicated setup that is difficult to calibrate, you’re right! Multiply by the number of focus points in your camera (dozens in some models) and thank the gods of technology that anything is ever in focus! Phase detection can be very fast because the processor can tell which direction the lens needs to move to bring the signals into phase and as they approach synchronization, it can start slowing the focus motor down when it is close to optimal focus to avoid overshooting and having to correct itself.

Contrast detection autofocus was used on video camera systems and later on non-SLR digital cameras and DSLRs when live-view on the rear LCD became popular. It is also the method of choice on ILC (Interchangeable Lens Compact)  mirrorless system cameras like the Sony NEX and Panasonic Lumix models. It works in a way similar to phase-detect by analyzing a small area of the image but no micro-lenses or beam-splitting is needed since the image is already falling on an electronic sensor. The section to be analyzed is measured for contrast and when the contrast reaches maximum, the point is in focus. This sounds simple and fast but in practice the only way to determine the maximum contrast of each sample point is to measure the increase until the maximum is reached, stop when the measurement starts to drop off and go back to where it reached the maximum. Depending on the speed and accuracy of the measurement and the reaction time of the control over the focus motor, the maximum point may be passed over back and forth several times before the focus locks on it. Because of this fine tuning and the ability to use multiple points at a time (there is no real limit other than processing power) contract-detect focus is quite accurate but the multiple passes needed to find the peak takes time. Needless to say, early contrast-detect systems were pretty pokey with the processors available back when it was introduced. That has changed a lot in the last few years with all the attention being focused (yes, it was a pun) on the mirrorless system cameras. Exponential increases in imaging sensitivity and available processing power have made the top-end contrast-detect systems very nearly as fast as phase-detect. 

A note on hybrid autofocus. Recently Canon and Sony patented slightly different variations of a method for performing phase-detection autofocus directly on the sensor. Using Sony’s A99 as an example, these on-sensor focus points combine with traditional reflective points to make the focus area larger, more detailed (over 100 points) and allow the camera to track moving objects in three dimensions more accurately than previously possible. On newer mirrorless compacts using this technology, the direction-aware phase-detect points are coordinated with contrast-detect areas to provide significantly reduced focus hunting and increased performance. One of the latest iterations of hybrid AF on my A6000 has a phase detect grid consisting of 179 phase-detect points that cover most of the sensor area and its AF is faster and more accurate than many DSLRs, including many above entry-level. I can see this technology eventually replacing the need for mirrors and traditional phase-detect systems as electronic viewfinders and sensors continue to improve in speed and resolution.
.

Using Autofocus

Using it? Doesn’t autofocus just work? The short answer is “yes”. The longer, article-prompting answer is “yes and no”.

Modes

Wide - Wide-area autofocus used the maximum number of phase-detect focus points available on a DSLR or the maximum contrast-detect focus area of an ILC or point & shoot camera. It will grab the first area that falls into focus and lock. In the simplest terms, it will make sure something is in focus before the shutter is released. This sounds rather random but for most scenery and casual group or people shots, the focus choice will tend towards the center (but not exclusively) and combined with scene analysis in full auto mode (also a common default), a pretty high percentage of usable shots will be captured.

Center - This mode limits the active phase-detect focus points or contrast-detect zone to a smaller area in the center of the frame. This allows you to choose the item to be focused on more carefully by centering the target in the frame. Some contrast-detect systems allow you to move this reduced area to anywhere on the image. Useful for off-center compositions, especially when using a tripod.

Spot - This mode limits phase-detect systems to a single point and contrast-detect to a very limited area. The center spot of the frame is usually the default and allows you to single out a fine detail for the point of focus. Almost all cameras with multiple phase-detect points allow the spot focus to be assigned to any of them. Contrast-detect systems are even more flexible and allow you to position the tiny spot virtually anywhere on the image. Again, great for off-center compositions using a tripod.

Now that we know what they are and where each mode starts, I will illustrate situations where each mode has worked well for me and why I chose that particular mode.

I almost always shoot in center-area mode. It allows me to point the camera at the chosen subject, lock on the focus by half-pressing the shutter, recomposing if needed and taking the shot. This method with the half-press focus lock is the same on all of my cameras and gives me the same type of control over focus on my point & shoot as I have with the DSLR in full manual.

In the photo below, the flower was isolated from the distant featureless background by locking the center focus area on the bloom and shifting the camera to the left before tripping the shutter.





In this Christmas scene the subject is much closer to the camera and I wanted to catch the area behind it as a soft background.  Again: select, lock, recompose and shoot.

See why I like this mode? It works well in virtually any static scene and gives you very flexible control over composition and where you want the viewer to be drawn to in the image.



A variation of center mode is available on more advanced DSLRs allowing you to shift from the group of focus points at the center to the right or left group as illustrated below.

For tripod-mounted still-life shooting, this gives you the ability to shift the focus over to a rule-of-thirds area without having to loosen the tripod focus and recompose. I seldom shift the points like this but when you really need it, it sure is handy.



When shooting sports, I usually use center area focus. It allows me to select the point of action that I want in focus (grandson clutching leg was centered in original and cropped later). At night under crappy lights with the lens wide open, shallow depth-of-field makes this even more important.

Under daylight with the lens stopped down, the extra depth-of-field makes it easier to get the subject right at the point of focus. I would suggest practicing following the action through the viewfinder. Football is usually easy to follow, but that isn’t the case when looking through a tube!



Spot mode was used on this tripod-steadied macro. I wanted the stigma pads in sharp focus with the rest of the flower as a soft backdrop. This was taken with my old Minolta 7i Digicam which used contrast-detect focusing and the positionable focusing spot was placed on one of the pads.



Wide area is usually too wide. It will focus on whatever the camera deems to be the best target. On full auto modes in advanced point & shoot cameras and DSLRs, the scene analysis algorithms can do a decent job of picking what to focus on but when the scent becomes too busy with a lot of elements, the camera’s little brain reaches its limit pretty quickly. In bright light with small apertures and a wide depth of field, wide works well enough.

Wide = BAD here!

Wide = GOOD here!


When tracking a moving subject against a featureless background is where wide is most useful for me. It will pick the subject up anywhere in the focus area and lock on. It’s possible to use center focus to track the birds in a shot like this but wide just makes it easier. If you’re a fan of air shows and telephotos, wide mode may get a lot of use.



Autofocus Without Autofocus

 The three main modes cover just about any situation that may come up but sometimes you just have to flip the switch to manual and hand-crank your creative vision. Still life and macro shots are often best served by focusing manually but that doesn’t necessarily leave your camera’s electronics out in the cold. On DSLRs, ILCs and advanced compacts that allow manual focus, the autofocus sensing mechanism is still active and when you manually adjust focus, the focus lock indicator will still light up when focus is achieved.

 Another tool that appeared when live-view became available on almost everything that could capture an image is focus assist magnification. This little gem has made fine focusing easier than ever. No more peeling off the eyecup on your viewfinder (or carrying a pair of extras for the inevitable loss) and fumbling with mounting a magnifier which only magnified the center of the frame. Now you can just move the focus area to where you want it and blow the section up 5x, 10x or more to fine-tune the image manually.

It is very easy to fine-tune the point of focus at 11.7x!

Peaking
- Recently Sony migrated a feature from professional video to it’s NEX and SLT cameras. It is called focus peaking (as a fan of the Lensbaby, I was pretty sure it was done solely for my benefit!) When focus peaking is on and the camera is focused manually, the areas of highest contrast are given a halo of color to indicate where the camera is focused. In video, this allows the user to easily determine what the camera is focused on and use the distinct outline surrounding the focused area to follow-focus. The same is true while shooting stills. I mentioned the Lensbaby and if you have shot with one, you know how difficult it can be to locate the “sweet spot” of sharp focus. With focus peaking, you just move the Lensbaby’s sharp-focus area to where you want it in the frame and adjust focus until you see the peaking halo form around the subject. The Lensbaby is an extreme case where spotting the point of focus is a real challenge, but a similar challenge exists  when using any lens wide open with shallow depth-of-field.

You can see how the yellow-peaked area shifts with the changing focus.


Supercomputer Perks

One of the benefits from having in-camera computer systems that exceed the power of 20th century supercomputers is the ability to use some of that power for “gimmicks” like face recognition, detecting smiles or creating panoramas from multiple shots right in the camera. I put the word gimmick in quotes because gimmicks like in-camera metering, auto-exposure and autofocus were “gimmicks” at some point early in their lives. Most of the features mentioned first appeared in advanced point and shoot cameras but are finding their way into ILCs and DSLRs. Focusing on faces can be a real help if you have people against a busy background. The camera will identify features that computes to a “face” and focus on it, ignoring the otherwise attractive focus candidates in the background. I find the ability of my Sony A77 to “register” a particular face and then find it in a crowd of people slightly creepy but when it picks the granddaughter out of the line at a parade or school play and focuses on her before the shutter releases, I find I can forgive a little creepiness. The smile, blink and face modes in your camera shouldn’t be a factor in you decision to buy but you shouldn’t just dismiss them as “gimmicks” without investigating their potential usefulness.

Closing Points

Autofocus has come a long way since the mid-eighties. It is faster, smarter and more accurate by an order of magnitude. Experiment and learn to use the mode that works best for you in a given situation because while it is faster and often more accurate, it is still not smarter than its operator. Autofocus is a great tool but like any tool, the skill to use it to its full potential comes with practice.
 

Happy Shooting!