Action photographers will often spend large sums of money, as much as they can afford, in order to purchase a camera body with the most advanced focusing system possible. They will tweak the focus point location, the number of auxiliary focus points, how fast or slow the focus changes from one object to another, the case of focus tracking and how the camera will react to acceleration. Sadly, many photogs will omit adjusting the adjustment available on almost all dslrs that assists your most important focus aid--Your Eyes!
It's vitally important to be able to discern when your camera's focus system is locked on the desired subject and when the camera has locked onto a subject close to the desired but not exactly the one intended. That can be the difference between a tack sharp image and one that is disappointingly soft. If you can't tell the difference in the viewfinder, don't be surprised if you camera's focus remains locked momentarily off target even if you've re-aimed the focus point properly. You can often adjust the length of the lag before the camera refocuses but there will always be a slight lag. You often hear action photogs complain that the focus is misplaced even though the focus point is properly located at the moment of exposure. More often than not, the focus point was misplaced earlier and the focus had not yet relocked on the target.
How can you improve your eyes ability to see proper focus? With the dioptric adjustment.
Many shooters have visual impairments that can be improved upon with an adjustment of the diopter of the viewfinder. That adjustment is usually located adjacent to the viewfinder. It may be to the left or right depending on the manufacturer. Check your owners manual. A Canon installation is shown below.
Adjust the wheel until the image appears as sharp as possible. Note the amount of adjustment and repeat this several times in order to get a consistent outcome. It should be that simple. For most folks, it is. However, some people have a discrepancy in excess of the maximum adjustment. This can occur when a photog uses glasses but wants to shoot without them for some reason. Be aware that some camera manufacturers offer adjustment lenses that dramatically extend the amount of adjustment available. You may want to consult your optometrist for a recommendation of how much diopter adjustment you will require.
You are ready to shoot. Now that you can tell when the focus has drifted off target and are using back button focus (yes, back button focus), you can use the "Bump Focus" technique to correct.
Bump focus refers to the practice of releasing and immediately re-pressing the back button for focus. This will expedite the acquisition of a new focus lock on the subject centered on the focus point. In fast moving sports with numerous targets such a basketball or football, it is easy for the focus system to lock on another player or a referee. Proper dioptric adjustment lets you recognize when you need to acquire a new lock with a bump focus. A higher keeper rate is your reward.
Can you tell in the viewfinder when you are focused on the player and when you are focused on the net? Not without a proper dioptric adjustment!
Philip S. Zivnuska